This document combines several documents related to dry swale (grass swale). Individual documents can be viewed by clicking on the appropriate link below. Fact sheets are not included in this combined document.

Dry swale (grass swale)

# Overview

Photo of a dry swale. Courtesy of Limnotech.
Green Infrastructure: Swales can be an important tool for retention and detention of stormwater runoff. Because they utilize vegetation, swales provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

Dry swales, sometimes called grass swales, are similar to bioretention cells but are configured as shallow, linear channels. Dry swales function primarily as a conveyance BMP, but provide treatment of stormwater runoff, particularly when used in tandem with check dams that temporarily retain water in a series of cells. Dry swales with an underdrain and engineered soil media are considered a filtration practice. Dry swales with in-situ soils capable of infiltration, (A or B soils) are considered infiltration practices. Dry swales are designed to prevent standing water. Dry swales typically have vegetative cover such as turf or native perennial grasses.

## Function within stormwater treatment train

Dry swales may be located throughout the treatment train, including the main form of conveyance between or out of BMPs, at the end of the treatment train, or designed as off-line configurations where the water quality volume is diverted to the filtration or infiltration practice. In any case, the practice may be applied as part of a stormwater management system to achieve one or more of the following objectives:

• reduce stormwater pollutants (filtration or infiltration practices)
• increase groundwater recharge (infiltration practices)
• decrease runoff peak flow rates (filtration or infiltration practices)
• decrease the volume of stormwater runoff (infiltration practices)
• preserve base flow in streams (infiltration practices)
• reduce thermal impacts of runoff (filtration or infiltration practices)

## Typical applications

Applications of dry swales with or without underdrains can vary extensively. Typical applications include

• individual lots for rooftop, driveway, and other on-lot impervious surface;
• shared facilities in common areas for individual lots;
• areas within loop roads or cul-de-sacs;
• landscaped parking lot islands;
• common landscaped areas in apartment complexes or other multifamily housing designs;
• between buildings in industrial and commercial developments; and
• conveyance between detention structures and receiving waters.

## Infeasibility criteria

Schematic showing some horizontal and vertical separation distances from an infiltration BMP. A separation distance may be required, such as with a drinking water well, or recommended, as with an underground tank. (Source: CDM Smith) Not to scale.

Certain site-specific conditions may make use of dry swales without underdrains infeasible. Examples include sites where

• infiltrating water would threaten drinking water sources (e.g., in karst areas);
• ordinances established by the local government with jurisdiction, such as setbacks from structures, conflict with the proposed location;
• infiltrating water would threaten existing below grade basements;
• in situ soil infiltration capacity is too low or too high;
• high levels of contaminants in soil or groundwater exist;
• the only area available for siting does not allow for a safe overflow pathway to the municipal separate storm sewer system or private storm sewer system; or
• reasonable concerns about erosion, slope failure, or down gradient flooding exist and cannot be overcome by swale design modifications.

## MPCA permit applicability

One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least 1 acre of land is disturbed and1 acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.

For regulatory purposes, dry swales fall under the Infiltration / Filtration category described in Part III.D.1. of the MPCA CGP. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Although it is expected that in many cases the dry swale will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.

The following terms are thus used in the text to distinguish various levels of dry swale (grass swale) design guidance:

• REQUIRED: Indicates design standards stipulated by the MPCA CGP (or other consistently applicable regulations).
• HIGHLY RECOMMENDED: Indicates design guidance that is extremely beneficial or necessary for proper functioning of the dry swale, but not specifically required by the MPCA CGP.
• RECOMMENDED: Indicates design guidance that is helpful for dry swale performance but not critical to the design.

There are situations, particularly retrofit projects, in which a dry swale is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is important to note that additional and potentially more stringent design requirements may apply for a particular dry swale, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Warning: Permit requirements are highlighted in red text boxes

## Retrofit suitability

The use of dry swales as a retrofit practice primarily depends on existing infrastructure and the compatibility of existing storm drain inverts that need to connect to the dry swale outflow. In general, four to six feet of elevation above the existing collection system invert is needed for dry swale retrofits (2 to 3 feet is needed for perimeter filters).

## Special receiving waters suitability

The following table provides guidance regarding the use of dry swales in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. Note that the suitability of a dry swale depends on whether the practice has an underdrain (i.e. filtration vs. infiltration practice).

Summary of design restrictions for special waters.

BMP Group receiving water
A Lakes B Trout Waters C Drinking Water D Wetlands E Impaired Waters
Filtration Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments RECOMMENDED RECOMMENDED ACCEPTABLE RECOMMENDED for non-nutrient impairments

Infiltration BMP design restrictions for special watersheds. This information applies to all infiltration practices.

BMP Group Receiving water
A Lakes B Trout Waters C Drinking Water1 D Wetlands E Impaired Waters
Infiltration RECOMMENDED RECOMMENDED NOT RECOMMENDED if potential stormwater pollution sources evident RECOMMENDED RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride

1 Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies

## Cold climate suitability

Snow plowed and piled in parking lot. Consideration should be given in locating these "snow dumps", since they will contribute a significant amount of stormwater runoff.

Dry swales should remain effective water quality improvement systems for many years, even during winter conditions, if designed and constructed properly. It has been shown that hydraulic efficiency and infiltration rates can remain at levels used for design sizing. However, in cold climates, some special considerations are HIGHLY RECOMMENDED for surface systems like dry swales to ensure sustained functionality and limit the damage freezing temperatures and snow and ice removal may cause. One concern with dry swales used for filtration in cold weather is the ice that forms both over the top of the facility and within the soil interstices. To avoid these problems to the extent possible, it is HIGHLY RECOMMENDED that the facility be actively managed to keep it dry before it freezes in the late fall. This can be done by various methods, including limiting inflow and ensuring the underdrain is functional.

Even if the infiltration properties of a dry swale are marginal for snowmelt runoff during the period of deep frost in the winter, the storage available in the facility will provide water quality benefit if the facility is dry entering the melt season. However, flow originating in an industrial area, a high traffic area where large amounts of salt are added, or another potential stormwater hotspot (PSH) should be diverted away from dry swales if pretreatment features have not been properly designed to handle such an increase in loading.

For all BMPs it is HIGHLY RECOMMENDED that snow and ice removal plans, including predetermined locations for stockpiling, be determined prior to or during the design process. Dry swales cannot be used for significant snow storage areas as debris build-up, plant damage, and lower infiltration rates are likely to occur. Some snow storage is unavoidable when BMPs are adjacent to areas where snow removal is required. It is critical that the property owner and snow and ice removal contractor have identified other areas for large scale snow storage.

Excessive deicing agents have the potential could lead to reduced soil infiltration rates (from excess sodium) or concentrations that exceed surface water or groundwater standards (from excess chloride). Locations such as busy intersections on slopes, parking garage ramps, or walkways near the entrances of commercial buildings are likely to be heavily treated with deicing agents. This should be taken into consideration when siting any dry swale.

Plant selection is critical to ensure that the damaging effects of snow and ice removal do not severely impact plantings or seedings. Even a small amount of snow storage can break and uproot plants requiring additional maintenance in the spring. Woody trees and shrubs should be selected that can tolerate some salt spray from plowing operations.

For more information on cold climate effects, see Cold climate impact on runoff management.

## Water quantity treatment

Where a project’s ultimate development replaces vegetation and/or other pervious surfaces with one (1) or more acres of cumulative impervious surface, the Permittee(s) must design the project so that the water quality volume (Vwq) of one (1) inch of runoff from the new impervious surfaces created by the project is retained on site (i.e. infiltration or other volume reduction practices) and not discharged to a surface water. If the water quality volume cannot be retained due to site constraints, a portion of the water quality volume should be retained on site to the extent that site conditions allow.

The amount of stormwater volume infiltrated depends on the design variant selected. Smaller swales should either be designed off-line using a flow diversion, or designed to safely pass large storm flows while still protecting the infiltration area or filtration media. In limited cases (e.g. extremely permeable soils), these dry swales can accommodate the channel protection volume, Vcp, in either an off- or on-line configuration.

In general, supplemental stormwater practices (e.g. detention ponds) will be necessary to satisfy channel and flood protection requirements when dry swales are used. However, these practices can help reduce detention requirements for a site through volume reduction.

## Water quality treatment

Dry swales can remove a wide variety of stormwater pollutants through chemical and bacterial degradation, sorption, and filtering. Surface water load reductions are also realized by virtue of the reduction in runoff volume. Properly designed infiltration systems discussed later in this section will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design infiltration occurs within 48 hours. In no case should the by-passed volume be included in the pollutant removal calculation. No pollutant removal occurs for runoff water that bypasses the practice.

## Limitations

The following general limitations should be recognized when considering installation of dry swales without underdrains (infiltration).

• Limited monitoring data are available and field longevity is not well documented.
• Failure can occur due to improper siting, design, construction and maintenance.
• Systems are susceptible to clogging by sediment and organic debris.
• When used as an infiltration practice, there is a risk of groundwater contamination depending on subsurface conditions, land use and aquifer susceptibility.
• They are not ideal for stormwater runoff from land uses or activities with the potential for high sediment or pollutant loads.
• They are not recommended for areas with steep slopes. Use step pools for these situations.
• Swales are difficult to protect from sediment-laden runoff during construction.

The following general limitations should be recognized when considering installation of dry swales with underdrains (filtration):

• Limited monitoring data are available and field longevity is not well documented.
• Failure can occur due to improper siting, design, construction and maintenance.
• Systems are susceptible to clogging by sediment and organic debris.
• They are not ideal for stormwater runoff from land uses or activities with the potential for high sediment or pollutant loads.
• They are not recommended for areas with steep slopes.
• Nitrification of water in dry swale media filters may occur where aerobic conditions exist.
• They offer limited or no water quantity control.
• The potential to create odors exists.

# Terminology for swales

Photos, from left to right, of dry swale, wet swale, and step pool. Images courtesy of Limnotech.
Flowchart used to determine type of swale. Click on image to enlarge.
Green Infrastructure: Swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value. See the section Green Infrastructure for stormwater management.

In the following discussion, swales are identified as filtration or infiltration practices. Swales are considered filtration practices when an underdrain is employed and infiltration practices when an underdrain is absent. Impermeable check dams may be utilized for either configuration. Engineered media is typically utilized in filtration swales and not utilized for infiltration swales.

The flowchart to the right provides a way to differentiate between types of swales used as stormwater best management practices (BMPs). Section drawings are included on this page. For images of various swale details, link here.

## Design variants

This section describes the design variants used in this manual. Other names occur in the literature but they can be fit into one of these variants, as described below.

• wetland channel: wet swale
• grass swale: dry swale
• grass channel: dry swale
• regenerative stormwater conveyance: step pool swale
• bioswale: can be used for any of the five variants discussed in this section
• vegetated filter or filter strip: pretreatment practices
• vegetated swale: can be used for pretreatment or any of the five variants discussed in this section
• biofiltration: a swale designed for filtration (dry swale or step pool with underdrain, or wet swale)
• bioinfiltration: a swale designed for infiltration (dry swale or step pool with no underdrain, typically having check dams)

### Dry swales with no underdrain (bioinfiltration)

Typical dry swale profiles and sections. Diagrams include dry swales without underdrain (infiltration) and with underdrain (filtration). Click on image to enlarge.
Step pool detail.

A dry swale without an underdrain is a relatively shallow, gently sloping, vegetated conveyance channel that prevents standing water via infiltration into in situ soils. This type of practice may be used in tandem with check dams to increase water quality volume retention. Infiltration is the dominant unit process for stormwater control.

### Dry swales with underdrain (biofiltration)

A dry swale with an underdrain is a relatively shallow, gently sloping, vegetated conveyance channel that has an engineered soil media mix and is designed with an underdrain to prevent standing water due to insufficient native soil infiltration rates. This type of practice may be used in tandem with check dams and/or impermeable liners. Filtration is the dominant unit process for stormwater control.

### Wet swales (biofiltration)

A wet swale is a relatively shallow, gently sloping, vegetated conveyance channel that holds water. A wet swale acts as a relatively long and linear shallow biofiltration treatment system, and may be used in tandem with check dams to increase water quality volume retention. Wet swales may be planted with emergent wetland plant species to improve pollutant removal. Settling and nutrient uptake by plants are the dominant unit processes for stormwater control.

### Step pools with no underdrain (bioinfiltration)

A step pool with no underdrain is a type of conveyance channel used in areas with steep terrain that uses a series of pools, conveyance channels, and checks to slow down water and increase infiltration into in situ soils. A step pool with no underdrain is similar to a dry swale with no underdrain, with the main difference being the step pool is employed on relatively steep longitudinal slopes. Infiltration is the dominant unit process for stormwater control.

### Step pools with underdrain (biofiltration)

A step pool with an underdrain is a type of vegetated conveyance channel used in areas with steep terrain that uses a series of pools, conveyance channels, and checks to slow down water and is designed with an underdrain to prevent standing water due to insufficient native soil infiltration rates. A step pool with an underdrain is similar to a dry swale with an underdrain, with the main difference being the step pool is employed on relatively steep longitudinal slopes. Filtration is the dominant unit process for stormwater control.

## Specific design applications for various land uses

A bioretention parking lot island. Note the use of other BMPs, including permeable pavement and tree trenches. (Source: Minnehaha Creek Watershed District)

It should be noted that the layout of the swale area will vary according to individual sites, and to specific site constraints such as underlying soils, existing vegetation, drainage, location of utilities, sight distances for traffic, and aesthetics. Designers are encouraged to be creative in determining how to integrate swales into their respective site designs. With this in mind, the following are presented as alternative options.

### On-lot systems

On-lot systems are designed to receive flows from gutters, and/or other impervious surfaces. These applications of swales tend to be simpler in design and relatively smaller in size.

### Parking lot islands (curbless)

In a paved area with no curb, pre-cast car-stops or a “ribbon curb” can be installed along the pavement perimeter to protect the swale area. This application of swale should only be attempted where shallow grades allow for sheet flow conditions over level entrance areas. Water may be pooled into the parking area where parking spaces are rarely used to achieve an element of stormwater quantity control beyond the confines of the swale surface area.

### Parking lot islands (curb-cut)

For curb-cut entrance approaches, the water is diverted into the swale through the use of an inlet deflector block, which has ridges that channel the runoff into the swale. Special attention to erosion control and pre-treatment should be given to the concentrated flow produced by curb-cuts.

### Road medians and traffic islands

A multifunctional landscape can be created by utilizing road medians and islands for swales. There is no minimum width recommended for traffic islands from street edge to edge. A buffer may be necessary along the outside curb perimeter to minimize the possibility of drainage seeping under the pavement section, and creating “frost heave” during winter months. Alternately, the installation of a geotextile filter fabric “curtain wall” along the perimeter of the swale will accomplish the same effect.

# Design criteria for dry swale (grass swale)

Photo of a dry swale. Courtesy of Limnotech.
Photo of a dry swale with an underdrain. Courtesy H.R. Green

This page provides a discussion of design elements and design steps for dry swales, which are often called grass swales. The following discussion includes dry swales used as filtration or infiltration practices, with the distinction being the presence of an underdrain for filtration practices.

Green Infrastructure: Swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value. See the section Green Infrastructure for stormwater management.

## Terminology

The following terminology is used throughout this design page.

Warning: REQUIRED - Indicates design standards stipulated by the MPCA Construction General Permit (CGP) or other consistently applicable regulations

HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the practice, but not specifically required by the MPCA CGP.

RECOMMENDED - Indicates design guidance that is helpful for practice performance but not critical to the design.

Information: To obtain appropriate resolution on details, you must open the .pdf links below and increase the size as necessary to obtain the desired resolution

## Design phase maintenance considerations

Caution: Maintenance considerations are an important component of design

Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include

## Major design elements - Physical feasibility initial check

Before deciding to use a dry swale practice for stormwater management, it is helpful to consider several items that bear on the feasibility of using such a device at a given location. This section describes considerations in making an initial judgment as to whether or not a dry swale is the appropriate BMP for the site.

### Infiltration constraints

If a dry swale is being considered for infiltration, the following links provide additional information on specific constraints to infiltration (applicable to dry swales without an underdrain). The Construction Stormwater General Permit prohibits infiltration under certain conditions, which are summarized and discussed in detail at this link.

### Contributing drainage area

The RECOMMENDED maximum drainage area is typically 5 acres. Dry swales can be designed to convey runoff from larger drainage areas. However, volume reduction, water quality function, and ability to meet the MPCA CGP requirements is diminished.

### Site topography and slopes

Unless slope stability calculations demonstrate otherwise (for guidance on calculating slope stability, see [1], [2], [3]), it is HIGHLY RECOMMENDED that swales be located a minimum horizontal distance of 200 feet from down-gradient slopes greater than 20 percent, and that slopes in contributing drainage areas be limited to 15 percent.

### Site location/minimum setback

Schematic showing some horizontal and vertical separation distances from an infiltration BMP. A separation distance may be required, such as with a drinking water well, or recommended, as with an underground tank. (Source: CDM Smith) Not to scale.

If the swale is constructed as an infiltration practice, the following table summarizes required and recommended minimum horizontal and vertical setback distances from an infiltration practice to an above-ground or underground structure. It will be necessary to consult local ordinances for further guidance on siting infiltration practices.

Warning: A minimum setback of 50 feet between a dry swale without an underdrain (infiltration practice) and a water supply well is REQUIRED by the Minnesota Department of Health Rule 4725.4350

Required and recommended minimum vertical and horizontal separation distances. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is above-ground, the distance is measured from the edge of the BMP to the structure. If the structure is underground, the vertical separation distance represents the distance from the point of infiltration through the bottom of the system to the structure, while the horizontal separation (often called setback) distance is the shortest distance from the edge of the system to the structure.

Structure Distance (feet) Requirement or recommendation Note(s)
Vertical Saturated soil 3 Requirement1
Bedrock 3 Requirement1
Horizontal Public supply well 100 for sensitive wells; 50 for others Requirement
Building/structure/property line2 10 Recommended
Surface water none unless local requirements exist If nearby stream is impaired for chloride, see [4]
Septic system 35 Recommended
Contaminated soil/groundwater No specific distance. Infiltration must not mobilize contaminants.
Slope 200 Recommended from toe of slope >= 20%

1 Required under the Construction Stormwater General Permit
2 Minimum with slopes directed away from the building

### Depth to groundwater and bedrock

Schematic illustrating separation distance from bottom of infiltration BMP to water table or top of bedrock. This diagram includes a modified subsoil zone in which the subsoil has been ripped to alleviate compaction.

A separation distance of at least 3 feet is REQUIRED under the MPCA CGP between the bottom elevation of infiltration swales and the elevation of the seasonally high water table. Shallow bedrock areas should be avoided for dry swales with a minimum separation distance of 3 feet.

A field soil properties investigation is HIGHLY RECOMMENDED.

Warning: Infiltration is prohibited when the infiltration system will be constructed in areas with less than three (3) feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock

### Karst topography

It is HIGHLY RECOMMENDED that underdrains and an impermeable liner be used for dry swales with filter media in active karst terrain because infiltration is typically not allowed in karst areas.. Geotechnical investigations are HIGHLY RECOMMENDED in karst areas.

Warning: The CSW permit prohibits infiltration when the infiltration system will be constructed in areas within 1,000 feet up‐gradient, or 100 feet down‐gradient of active karst features

See stormwater and wellhead protection for guidance and recommendations for determining the appropriateness of infiltrating stormwater in a Drinking Water Supply Management Area (DWSMA). For more information on source water protection see Minnesota Department of Health.

Warning: Infiltration is prohibited in areas within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, subp. 13., if the system will be located:
• in an Emergency Response Area (ERA) within a DWSMA classified as having high or very high vulnerability as defined by the Minnesota Department of Health; or
• in an ERA within a DWSMA classified as moderate vulnerability unless a regulated MS4 Permittee performed or approved a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater; or
• outside of an ERA within a DWSMA classified as having high or very high vulnerability, unless a regulated MS4 Permittee performed or approved a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater

See NRCS Web Soil Survey for hydrologic soil descriptions for the swale location. A and B soils are potentially suitable for a dry swale without an underdrain (infiltration swale). C and D soils are potentially suitable for a dry swale with an underdrain (filtration practice). The maximum allowed field-measured infiltration rate shall not exceed 8.3 inches per hour for an infiltration swale.

Warning: Infiltration is prohibited when the infiltration system will be constructed in areas of predominately Hydrologic Soil Group D (clay) soils, and in areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour.

## Major design elements - Practice and site considerations

Several considerations are made in this section for the conceptual design of dry swales. Further design guidance and specifications are made in the following sections.

### Conveyance

It is HIGHLY RECOMMENDED that the design provide non-erosive flow velocities within the swale and at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for established grassed channels. Erosion control matting or rock should be specified if higher velocities are expected. See Erosion prevention practices for more information on erosion prevention practices.

### Pretreatment

Pretreatment prior to the dry swale such as vegetated filter strips or side slopes, small sedimentation basins, water quality inlets, or other pretreatment BMPs should be evaluated. If the dry swale is being used to meet the Construction Stormwater General Permit, pretreatment is required.

Warning: To prevent clogging of the infiltration or filtration system, the Permittee(s) must use a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system.

### Anticipated flow

Although local drainage criteria may require a certain frequency event be used in the design, it is HIGHLY RECOMMENDED that larger events be considered depending on the adjacent property and associated risks.

• Slope of swale: The longitudinal slope of a dry swale may vary from 0.5 to 2 percent and will affect the selection of swale type. It is HIGHLY RECOMMENDED that the design engineer consider the expected watershed flow to be conveyed by the swale in making the preliminary determination of design type.
• Swale bottom: It is HIGHLY RECOMMENDED that the swale bottom be no less than 3 feet wide and should be adjusted with the cross-sectional area to be able to contain the expected range of flows within the swale. See additional design information on the cross-sectional area under the Swale Depth section.
• Side slopes: It is RECOMMENDED that the maximum side slopes within a swale do not exceed 3H:1V and be designed based on the relative stage-dependent flow driven cross-sectional area.
Hydraulic parameters of a channel section. Click on image to enlarge.
• Swale depth: Swale depth (pooled water depth) will be estimated based on the relative stage-dependent flow driven cross-sectional area to keep the swale from over topping.

The schematic at the right illustrates hydraulic parameters of a channel section. The area (A) is given by

$A = ((b + d/tan(θ))d$

the wetted perimeter (P) is given by

$P = b + 2 (d / sin(θ))$

the hydraulic radius (R) is given by

$R = (bd sin(θ) + d^2 cos(θ)) / (b sin(θ) + 2d)$

and the flow quantity is given by Manning's Formula for swale sizing

$Q = vA = 1.49/n AR^{0.67} S^{0.5}$

where

Q = Flow Quantity (ft3/sec),
v = Velocity (ft/sec),
n = manning’s coefficient,
S = Slope (ft/ft), and
the other dimensions are shown in the schematic to the right.
• Infiltration and filtration considerations: The design engineer should review the results of the feasibility check to assist in the selection of swale type. An additional consideration includes watershed soil transport to the site. Watersheds with unstable soils or lack of vegetative cover (e.g., construction, farmland and highly impervious surfaces) can generate and transport excessive sediments to the swale that may affect both infiltration and filtration capacity. In these situations, pretreatment via sedimentation processes is REQUIRED. Another consideration is the level of compaction and structure of in-situ soils, when considering dry swales. Construction of developments and roads, for example, significantly alter the parent state of native soils and therefore their hydrologic soil classification should be downgraded for feasibility study purposes.

### Filter media

Swales designed for filtration (i.e. swales that have an underdrain) typically have bioretention engineered media. The media is comprised of a combination of sand and organic material on top of a pea gravel bed that encases a perforated drain pipe. The media assists in the removal of fine particulate and dissolved pollutants, improving on the overall performance of swale systems. See design specifications for media. If the filtered water is eventually discharged to a receiving water impaired for phosphorus, the practice should be designed to minimize phosphorus loss.

Soils with high infiltration rates (A and B soils) typically do not utilize engineered media. Swales constructed on these soils are suitable for infiltration and underdrains are not needed.

### Underdrains

Underdrains are used when drawdown requirements cannot be me (e.g. C and D soils) or when there are other constraints to infiltration (see constraints to infiltration). Underdrains are comprised of a perforated, PVC pipe laid within filter media to convey runoff to either a stable day-lit area, a second form of treatment, or the storm sewer. A solid-walled PVC section of piping should be connected to the perforated drain pipe with a “tee” junction piece and extended to the swale’s surface to serve as an inspection and cleanout access point. These observation/maintenance ports are spaced throughout the system. See specifications for underdrains.

### Treatment

Stormwater treatment in dry swales varies by design. For swales designed as infiltration practices, pollutants are attenuated through settling of sediment and adsorption of pollutants on soil media. Pollutants not attenuated by these processes will infiltrate deeply into the vadose zone, where they may be adsorbed, undergo chemical change, or leach to groundwater.

For swales designed as filtration practices, pollutants are attenuated through settling of sediment and adsorption of pollutants. Engineered media, which typically has a relatively organic matter content, is effective in attenuating metals, most organics, and bacteria. Soluble pollutants, such as nitrate, dissolved phosphorus, and chloride, may be taken up by vegetation but will largely be captured by the underdrain and returned to the stormwater drainage system. Unless lined, some infiltration will occur below the underdrain in filtration systems.

The use of impermeable check dams or weirs can enhance treatment by increasing the volume of water retained and increasing the contact time between soil or media and runoff water.

### Vegetation

Vegetation plays a crucial role in dry swale treatment capacity, flow attenuation and stabilization of the swale itself (i.e., erosion control). It is HIGHLY RECEOMMENDED that preference is given to robust native, non-clump forming grasses as the predominant plant type within the swale flow area. Care must also be taken to consider species selection in light of sun exposure duration/timing as well as soil moisture, ponding depth and ponding duration.

### Landscaping

Swales can be effectively integrated into the site planning process and aesthetically designed as attractive green spaces planted with native vegetation. Because vegetation is fundamental to the performance and function of the swale, aesthetically chosen vegetation may only be possible on the surface of the swales.

### Snow considerations

Considering management of snow, the following are recommended

• Plan a plow path during design phase and tell snowplow operators where to push the snow. Plan trees around (not in) plow path, with a 16 foot minimum between trees.
• Plan for snow storage (both temporary during construction and permanent). Don’t plow into dry swales routinely. Dry swales should be a last resort for snow storage (i.e. only for very large snow events as “emergency overflow”.
• Snow storage could be, for example, a designed pretreatment area.

For more information and example photos, see the section on snow and ice management.

### Safety

Swales do not pose any major safety hazards. Potential hazards could occur from the steep side slope and checks of the swales if they are close to pedestrian traffic or roadways with no shoulders.

## Materials specification

### Erosion control (MNDOT Standard Specifications 2575, 3861-3898)

The use of temporary erosion control materials is REQUIRED in the design and construction of all swale types to allow for the establishment of firmly-rooted, dense vegetative cover. The dry swale bottom and side slopes up to the 10 year event should use robust erosion control matting that can resist the expected shear stresses associated with channelized flows. The matting should have a minimum life expectancy of three years. Upper banks of the swale slope should be protected by either similar matting or a straw/coconut blend erosion control blanket. See MNDOT specifications for guidance on selection of erosion control products.

### Filter media

Filter media used in dry swale designs should follow guidance on material specifications within the bioretention section of the MN Stormwater Manual.

### Underdrains (MNDOT Specifications 3245, 3247, 3248, 3278)

The following are RECOMMENDED for infiltration practices with underdrains.

• The minimum pipe diameter is 4 inches.
• Install 2 or more underdrains for each infiltration system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area.
• Include at least 2 observation /cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inches diameter vertical non-perforated schedule 40 PVC pipe, and extend to the surface. Cap cleanouts with a watertight removable cap.
• Construct underdrains with Schedule 40 or SDR 35 smooth wall PVC pipe.
• Install underdrains with a minimum slope of 0.5 percent, particularly in HSG D soils (Note: to utilize Manning’s equation the slope must be greater than 0).
• Include a utility trace wire for all buried piping.
• For underdrains that daylight on grade, include a marking stake and animal guard;
• For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating the system as either an infiltration system, filtration system, or both. The valve should enable the ability to make adjustments to the discharge flow so the sum of the infiltration rate plus the under-drain discharge rate equal a 48 hour draw-down time.
• Perforations should be 3/8 inches. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below berms, down steep slopes, makes a connection to a drainage structure, or daylights on grade.
• Spacing of collection laterals should be less than 25 feet.
• Underdrain pipes should have a minimum of 3 inches of washed #57 stone above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage.
• Avoid filter fabric.
• Pipe socks may be needed for underdrains imbedded in sand. If pipe socks are used, then use circular knit fabric.

The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual. Underdrain spacing can be calculated using the following spreadsheet, which utilizes the vanSchilfgaarde Equation. The spradsheet includes an example calculation. File:Underdrain spacing calculation.xlsx

### Weir

See MNDOT Standard Specifications 2461, 2573, 3137, 3301, 3491, 3601.

### Plants

See MNDOT Standard Specifications 2571, 2574, 2575, 3861, 3876, 3878.

Refer to the swales plant list section of the manual for selection of Minnesota native plants to be used in swales. Care must be taken to specify plants for their position in the system (swale bottom, side slopes and buffer). For the bottom of the swale, preference should be given to robust non-clump forming grasses or sedges that can withstand flow forces as well as provide adequate filtration functions. It is also important to understand draw-down time not only within the channel itself, but in either in-situ soils or the filter media, as plants have variable tolerance to the depth and duration of inundation as well as soil moisture period. Lastly, care should be taken to understand sun exposure and salt tolerance requirements of various plants to ensure a robust, dense establishment of vegetative cover.

Dry swale materials specifications

Parameter Specification Size Note
Topsoil Topsoil per MNDOT 3877 n/a Characteristic of local site soils
Media sand ASTM C-33 fine aggregate concrete sand 0.02” to 0.04”
Check dam (pressure treated) AWPA Standard C6 6” by 6” or 8” by 8” Do not coat with creosote; embed at least 3’ into side slopes
Check Dam (natural wood) Black Locust, Red Mulberry, Cedars, Catalpa, White Oak, Chestnut Oak, Black Walnut 6” to 12” diameter; notch as necessary Do not use the following, as these species have a predisposition towards rot: Ash, Beech, Birch, Elm, Hackberry, Hemlock, Hickories, Maples, Red and Black Oak, Pines, Poplar, Spruce, Sweetgum, Willow
Check dam (rock, rip rap) Per local criteria Size per requirements based on 10-year design flow Cannot get water quality volume credit when using a permeable check dam
Check dam (earth) Per local criteria Size per requirements based on 10-year design flow Use clayey soils with low permeability
Check dam (precast concrete) Per pre-cast manufacturer Size per requirements based on 10-year design flow Testing of pre-cast concrete required: 28 day strength and slump test; all concrete design (cast-in-place or pre-cast) not using previously approved State or local standards requires design drawings sealed and approved by a licensed professional structural engineer.

Embed at least 3’ into side slopes

Filter Strip sand/gravel pervious berm sand: per dry swale sand gravel; AASHTO M-43 sand: 0.02” to 0.04” gravel: 1/2” to 1” Mix with approximately 25% loam soil to support grass cover crop; see Bioretention planting soil notes for more detail
Pea gravel diaphragm and curtain drain ASTM D 448 varies (No. 6) or (1/8” to 3/8”) Use clean bank-run gravel
Underdrain gravel per pre-cast manufacturer 1.5” to 3.5”
Underdrain ASTM D-1785 or AASHTO M-278 6” rigid Schedule 40 PVC 3/8” perf. @ 6” o.c.; 4 holes per row

## Design procedure – design steps

It is important to acknowledge that each site has unique and defining features that require site-specific design and analysis. The guidance provided below is intended to provide the fundamentals for designing dry swale systems to meet regulatory requirements but is not intended to substitute engineering judgment regarding the validity and feasibility associated with site-specific implementation. Designers need to be familiar with the hydrologic and hydraulic engineering principles that are the foundation of the design and they should also enlist the expertise of qualified individuals in stormwater management and stream restoration plantings with respect to developing appropriate planting plans and habitat improvement features.

### Step 1. Make a preliminary judgment - consider basic issues for initial suitability screening

Make a preliminary judgment as to whether site conditions are appropriate for the use of a dry swale, and identify its function (filtration or infiltration) in the overall treatment system.

A. Consider basic issues for initial suitability screening, including:

• Site drainage area
• Site topography and slopes
• Soil types
• Regional or local depth to ground water and bedrock
• Bottom of facility to be at least three feet above the seasonably high water table
• Site location/minimum setbacks
• Presence of active karst

B. Determine how the swale will fit into the overall stormwater treatment system, including:

• Decide whether the swale is the only BMP to be employed, or if are there other BMPs addressing some of the treatment requirements.
• Decide where on the site the swale will most likely be located.

### Step 2. Confirm design criteria and applicability

A. Determine whether the swale must comply with the MPCA CGP. To determine if permit compliance is required, see Permit Coverage and Limitations.

B. Check with local officials, watershed organizations, and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

### Step 3. Perform field verification of site suitability

Consider the following when determining if a swale is suitable:

• The drainage area and area for swale
• The slope to get the drainage to the swale in addition to the swale slope to fully drain to a discharge point
• Any utility conflicts or roadway crossings that will need to be addressed in design
• MnDOT standards for roadway channel slopes and adequate depth for the 10-year drainage without overtopping the roadway

Consider the following when infiltration is desired in the swale design.

Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.

Surface area of stormwater control measure (BMP)(ft2) Borings Pits Permeameter tests
< 1000 1 1 5
1000 to 5000 2 2 10
5000 to 10000 3 3 15
>10000 41 41 202

1an additional soil boring or pit should be completed for each additional 2,500 ft2 above 12,500 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2

Groundwater mounding, the process by which a mound of water forms on the water table as a result of recharge at the surface, can be a limiting factor in the design and performance of infiltration practices. A groundwater mounding analysis is RECOMMENDED to verify separation distances required for infiltration practices. For more information on groundwater mounding, see the following sections in this manual.

### Step 4. Select design variant based on physical suitability evaluation

Once the physical suitability evaluation is complete, it is HIGHLY RECOMMENDED that the better site design principles be applied in sizing and locating the filtration practice(s) on the development site. Given the drainage area, select the appropriate swale practice for the first iteration of the design process. Note: Information collected during the physical suitability evaluation (see Step 1) should be used to explore the potential for multiple swale practices versus relying on a single facility.

### Step 5. Compute runoff control volumes and other key design parameters

1. Calculate the following runoff control volumes.

Vwq = 1 inch x Areaimpervious surface

To calculate the volume behind a check dam, see item 3 below.

Vcp = 24 hour extended detention of post-development 1-yr 24-hr storm event
Vp10 = peak discharge from the 10-yr storm to 10-yr predevelopment rates
Vp100 = peak discharge from the 100-yr storm to 100-yr predevelopment rates

2. Once the runoff control volume is determined for design, compute the following design parameters to determine the swale size required.

$q = (0.00236/n) · Y · 1.67 · S · 0.5$

Where:

q = discharge per foot of length of the swale, from Manning’s equation (cfs/ft);
Y = allowable depth of flow (inches);
S = slope of swale (percent) (0.5 to 2 percent); and
n = Manning’s “n” roughness coefficient (use 0.15 for short prairie grass, 0.25 for dense grasses such as bluegrass, buffalo grass, blue grama grass and other native grass mixtures).
B. Use a recommended hydrologic model to compute Qwq
C. Minimum swale length (in feet) = Qwq / q

Where:

Qwq = the water quality peak discharge (cubic feet per second)

3.The water quality volume (Vwq) achieved behind each check dam (instantaneous volume) is given by

$V_{wq} = h^2 * (h * H + B_w)]/(2S)$

where

h = check dam height (inches)
H = horizontal component of the swale side slope (1 vertical : H horizontal)(inches)
S = slope (unitless); and
Bw = channel bottom width (inches)

Add the Vwq for each check dam together to obtain the cumulative water quality volume for the swale.

For information on check dams, link here. For an example calculation of volume, link here.

### Step 6. Compute number of check dams

Profile of swale with structural check dams (not to scale). Source: Virginia DOT BMP Design Manual, Chapter 6. Click on image to enlarge.
Space check dams in a channel so the crest of the downstream dam is at the elevation of the toe of the upstream dam. Click on image to enlarge.
Profile of Swale with earthern check dams (not to scale). Source: Oregon Department of Environmental Quality Erosion and Sediment Control Manual.

The number of check dams should be computed based on swale slope, length, and treatment objectives. For example, a swale designed to contain the entire Vwq may require more check dams than a swale that only contains a portion of the Vwq.

Channel slopes between 0.5 and 2 percent are recommended unless topography necessitates a steeper slope, in which case 6- to 12-inch drop structures can be placed to limit the energy slope to within the recommended 0.5 to 2 percent range. Energy dissipation will be required below the drops. Spacing between the drops should not be closer than 50 feet. Depth of the Vwq at the downstream end should not exceed 18 inches.

For an example calculation of number of check dams to employ, link here.

### Step 7. Calculate drawdown time

Filtration swales (swales with an underdrain) include a bed consisting of a permeable soil layer at least 30 inches in depth, above a 6-inch diameter perforated PVC pipe (AASHTO M 252) longitudinal underdrain in a 12-inch gravel layer. The soil media should have an infiltration rate of at least 0.25 inches per hours with a maximum of 1.5 inches per hour and contain organic material to facilitate pollutant removal but not contribute to phosphorus leaching. A permeable filter fabric is placed between the gravel layer and the overlying soil. Dry swale channels are sized to store and filter the entire Vwq and allow for full filtering through the permeable soil layer.

Dry swales with no underdrain will have infiltration occurring. The drawdown time will be equal to the maximum pool depth behind a check dam divided by the soil infiltration rate. See design infiltration rates for different soil groups. For example, considering a swale with a 1 foot pool depth behind a swale, the drawdown time for a B(SM) soil with an infiltration rate of 0.45 inches per hour will be 26.7 hours, while the drawdown time for an A(SP) soil with an infiltration rate of 0.8 inches per hour will be 15 hours.

### Step 8. Check 2-year and 10-year velocity erosion potential and freeboard

Check for erosive velocities and modify design as appropriate based on local conveyance regulations. Provide a minimum of 6 inches of freeboard.

### Step 9. Design low flow control at downstream headwalls and checkdams

Design control to pass Vwq in 48 hours.

Inlets to swales must be provided with energy dissipaters such as riprap or geotextile reinforcement. Pretreatment of runoff is typically provided by a sediment forebay located at the inlet. Enhanced swale systems that receive direct concentrated runoff may have a 6-inch drop to a pea gravel diaphragm flow spreader at the upstream end of the control. A pea gravel diaphragm and gentle side slopes should be provided along the top of channels to provide pretreatment for lateral sheet flows. The underdrain system should discharge to the storm drainage infrastructure or a stable outfall.

### Step 11. Check volume, peak discharge rates and drawdown time against state, local, and watershed organization requirements (NOTE: steps are iterative)

Follow the design procedures identified in the Unified Sizing Criteria section of the Manual to determine the volume control and peak discharge requirements for water quality, recharge (not required), channel protection, overbank flood and extreme storm.

Model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site. This includes defining the parameters of the swale practice defined above: ponding elevation and area (defines the ponding volume), infiltration rate and method of application (effective filtration area), and outlet structure and/or flow diversion information. The results of this analysis can be used to determine whether or not the proposed design meets the applicable requirements. If not, the design will have to be re-evaluated.

Warning: The following items are specifically REQUIRED by the MPCA Permit:

A. Volume: Swale systems shall be sufficient to infiltrate a water quality volume of 1 inch of runoff from the new impervious surfaces created by the project. If this criterion is not met, increase the storage volume of the practice or treat excess water quality volume (Vwq) in an upstream or downstream BMP (see Step 5). If this requirement cannot be met, infiltrate to the extent possible and ensure that the remaining volume is treated through filtration.
B. Drawdown: Dry swales shall discharge through the soil or filter media in 48 hours or less. Additional flows that cannot be infiltrated or filtered in 48 hours should be routed to bypass the system through a stabilized discharge point.

Experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore defined as the time from the high water level in the practice to 1 to 2 inches above the bottom of the facility. This criterion was established to provide the following: wet-dry cycling between rainfall events; unsuitable mosquito breeding habitat; suitable habitat for vegetation; aerobic conditions; and storage for back-to-back precipitation events. This time period has also been called the period of inundation.

### Step 12. Finalize the cross-section and profile design for the project

• Grading plan: Develop a grading plan based on the preliminary profile and cross-section typical design.
• Dimensions: Adjust the preliminary profile dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic, water quality, and quantity management criteria should be rechecked based on adjustments to the dry swale to ensure that safe and adequate conveyance is still maintained.
• Check dams: Adjust the preliminary check dam dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic and water quality criteria should be rechecked based on adjustments to the check dams to ensure that safe and adequate conveyance is still maintained.
• Site stabilization: Course woodchips and compost should be used throughout the limit of disturbance for site stabilization. All areas should be seeded and planted as well as blanketed/matted. An appropriate erosion control blanket with biodegradable neeting should be used within the swale bottom and side slopes.
• Excess materials: It is advisable that excess materials, i.e., cobbles and boulders, be placed at the edge of the cross-section for use during the maintenance phase to correct any physical instability.

### Step 13. Prepare vegetation and landscaping plan

A landscaping plan for a swale should be prepared to indicate how the enhanced swale system will be stabilized and established with vegetation. Landscape design should specify proper species and be based on specific site, soils, sun exposure and hydric conditions present along the swale. Further information on plant selection and use can be found in the swale plant list section.

### Step 14. Prepare operation and maintenance plan

See Operation and Maintenance section for guidance on preparing an O&M plan.

### Step 15. Prepare cost estimate

See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.

# Construction specifications for dry swale (grass swale)

Green Infrastructure: Dry swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value. See the section Green Infrastructure for stormwater management.

This page provides a discussion of construction specifications for dry swales (infiltration and filtration-media systems).

## Access agreements

An easement is a legally binding agreement between two parties, and is defined as “a non-possessory right to use and/or enter onto the real property of another without possessing it. “An easement is required for one party to access, construct, or maintain any feature or infrastructure on the property of another. Easements can be temporary or permanent. For example, temporary easements can be used if limits needed for construction are larger than the permanent easement footprint of constructed features. Having an easement provides a mechanism for enforcement of maintenance agreements to help ensure dry swales are maintained and functioning. See an example access agreement.

## Construction specifications for swale practices

Construction of swale practices incorporates techniques and steps that may be considered nonstandard. It is recommended that construction specifications include project pretreatment devices, construction sequencing, temporary and permanent erosion control measures, excavation and fill, grading, soil decompaction, material specifications, and final stabilization. All of these topics are addressed in further detail below.

Additional specifications for items applicable to swale practices can be found in the Minnesota Department of Transportation’s (MnDOT) Specifications for Construction. The current version of this resource was completed in 2018. Below is a list of MnDOT sections that may be helpful when writing project specifications for dry swales.

### Pre-construction meeting

A pre-construction meeting is recommended and should include a walkthrough of the site with the builder/contractor/subcontractor to identify important features of the work and to review and discuss the plans. This is the best time to identify potential issues related to construction methods and sequencing that will affect site protection, erosion and sediment control, and proper installation of the work.

### Site protection

#### Pretreatment

Pretreatment is a required part of infiltration and filtration practices. Pretreatment is needed to protect BMPs from the build-up of trash, gross solids, and particulate matter. When the velocity of stormwater decreases, sediment and solids drop out. If pretreatment is not provided, this process will occur in the BMP, resulting in long-term clogging and poor aesthetics.

Warning: The Construction Stormwater general permit states: To prevent clogging of the infiltration or filtration system, the Permittee(s) must use a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system.

#### Temporary erosion and sediment control

Photo of silt fence used to protect swale infiltration area during construction. Courtesy of HR Green.
Photo of erosion control mat used to protect swale side slopes from erosion. Courtesy of HR Green.

During construction, it is critical to keep sediment out of the swale device as much as practicable. Utilizing sediment and erosion control measures will help to keep swale areas from clogging. As soon as grading is complete, stabilize slopes to reduce erosion of soils. Protect temporary soil stockpiles from run-on and run-off from adjacent areas and from erosion by wind. Sweep as often as required if sediment is on paved surfaces to prevent transport offsite by tracking and airborne dust. All sediment and erosion control measures must be properly installed and maintained. When sediment build up reaches 1/2 the height of the practice, action is required, such as removing the accumulated sediment or installing additional sediment controls downgradient of the original device. Link here for more information.

Potential techniques that could be used to divert runoff and isolate the dry swale may include one or more of the following:

• Cofferdam and bypass pump
• Bypass channel
• Place plastic sheeting over swale when rain in forecasted – anchor with staples, sand bags, etc.
• Isolate BMP from runoff using sand bags, fiber logs, coir logs, silt fence, or other barrier
• Temporary pond and bypass pumping
Warning:
• It is REQUIRED that future infiltration swale locations not be used as temporary sedimentation basins unless 3 feet of cover is left in place during construction.
• If an infiltration swale area is excavated to final grade (or within 3 feet of) it is REQUIRED that rigorous erosion prevention and sediment controls (e.g., diversion berms) are used to keep sediment and runoff infiltration away from the infiltration area until the contributing watershed is stabilized.

#### Compaction prevention

Preventing and alleviating compaction are crucial during construction of infiltration and filtration swale practices, as compaction can reduce infiltration rates by increasing bulk density of the soil. The infiltration or filtration area should be marked with paint and/or stakes to keep construction traffic from traveling in the area.

### Inspection and documentation

Inspections before, during, and after construction are needed to ensure swale practices are built in accordance with the plans and specifications. It is recommended that onsite inspectors are familiar with project plans and specifications to ensure the contractor’s interpretation of the plans are consistent with the designer’s intent. The inspectors should take frequent photos and notes of construction activities and features as work progresses and at all critical points (such as immediately prior to backfilling). They should check dimensions and depths of all installed materials. All materials and products should be verified or tested for conformance with the specifications.

## Construction sequence

### Step 1 – Site examination and preparation

It is the responsibility of the contractor to

• examine the areas for performing earthwork and determine that conditions are satisfactory to proceed, or to correct all unsatisfactory conditions prior to starting work;
• arrange to locate, mark, and protect all existing utilities and underground facilities in the areas of work; and
• remove all existing features marked for removal and required earthwork.

### Step 2 – Excavation

For in-situ soil infiltration dry swales, cut the swale and infiltration area as shown on the plans. Where possible, excavation should be performed with a backhoe and work should be done from the sides and outside the footprint of the infiltration area to avoid soil compaction. If it is necessary to work in the infiltration area, only low ground pressure tracked equipment should be allowed to complete the work. Rubber tire equipment should be strictly prohibited within the infiltration area, unless working from pavement outside of the basin or trench. The contractor should start the work at the far side of the trench or basin and work their way out.

The contractor is to ensure all laws and regulations are followed regarding stability of excavations. This may require shoring, bracing, sloping, or benching. Materials should not be stockpiled near the edge of the excavation. Drainage and control of water in the excavation must also be considered.

Warning: It is REQUIRED that infiltration systems not be excavated to final grade until the contributing drainage area has been constructed and fully stabilized.

For filter media dry swales, cut the swale area and sub-cut the filtration area as shown on the plans. Where possible, excavation should be performed with a backhoe and work should be done from the sides and outside the footprint of the filtration area. The contractor should start the work at the far side of the trench or basin and work their way out.

The contractor is to ensure all laws and regulations are followed regarding stability of excavations. This may require shoring, bracing, sloping, or benching. Materials should not be stockpiled near the edge of the excavation. Drainage and control of water in the excavation must also be considered.

### Step 3 – Decompaction

Soil decompaction is required in all swale bottom areas with the exception of the bottom of the sub-cut for filter media dry swales. Decompact subsoil with a backhoe ripper attachment or other approved method to a depth of at least 18 inches below subgrade in all locations indicated on the drawings. Also known as soil loosening or soil ripping, this technique has been shown to increase infiltration and reduce compaction from construction activities. For more information on alleviating compaction, link here.

### Step 4 – Subsoil Infiltration Testing for Dry Swales (in-situ soil infiltration swales)

Subsoil infiltration testing is HIGHLY RECOMMENDED for dry swales depending on in-situ infiltration. After the subsoil is decompacted, test the infiltration area to verify the assumed infiltration rate and that the infiltration area will drain dry within 48 hours.

This can be accomplished by performing double ring infiltrometer tests (ASTM D3385) in the bottom of the basin, or by filling the infiltration basin and timing how long it takes to drain from maximum water depth to dry bottom. The measured infiltration rate should equate to double the designed infiltration rate. If the basin is filled with water to perform this check, be sure sediments are not being washed into the basin during filling. If sediments are washed into the basin, they need to be removed prior to placing infiltration media.

If the basin does not drain dry within 48 hours (24 hours is recommended for trout waters), or the infiltration rate is slower than twice what was assumed in the design, additional soil loosening or modification may be necessary. Information on soil testing can be found here.

### Step 5 – Installation of materials - filter media (if applicable)

Soil test results should be provided to the designer a minimum of two weeks prior to delivery of filter media to the site. Submitted test results should include gradation and USDA soil texture classification or certification that the soil mix meets MnDOT specifications or other requirements. Samples of the mixed product should be also provided to the designer two weeks prior to delivery of media to the site. The designer should review the materials as soon as possible to avoid any potential delays in the procurement and review of another media source should the initial submittal not meet specifications.

All accumulated sediment and silt from the bottom of the facility should be removed prior to the placement of filtration media. The contractor should make every effort possible to place the filtration media in a way to minimize compaction of the subgrade and the filtration media itself. No construction vehicles are allowed in the filtration area after the media is placed unless approved by designer. Loose placement of filtration media shall be accomplished by dumping from the edges and spreading with the bucket of a backhoe, which is outside of the filtration area, or some other acceptable means determined by the designer. If spreading with a backhoe is not possible for the entire area of the filtration area, only tracked skid steers or other low ground pressure equipment should be permitted in the basin to spread the filtration media. This method should be minimized as much as possible. Travel over placed filtration media should be strictly prohibited.

The contractor should overfill the filtration media areas approximately 20 percent to account for consolidation of the loose soil once wetting occurs. Any small irregularities at the designed finished grade should be worked out with hand tools.

The contractor should contact the designer upon final placement of media for a final inspection prior to planting and mulching. At this inspection, the designer should check thickness and grades after soil wetting occurs and notify the contractor of areas that do not meet the tolerances specified. Tolerances in final grade are commonly vertically +/- 0.1 foot and horizontally +/- 0.5 foot.

If time goes by between the initial placement of infiltration media and planting, the contractor should be required to remove accumulated silt. This work is also a chance to perform any final subgrade grading adjustments required to obtain the finished grades as shown on the drawing.

### Step 6 – Restoration and plantings

After final placement of grading and filtration media (if applicable) has been approved, planting or seeding should happen as soon as possible to avoid erosion, sedimentation, and the establishment of weeds. The contractor should notify the designer at least four days in advance of when planting or seeding will occur in advance of delivery of materials to the site to allow for scheduling of site inspections. At least two weeks prior to the planting or seeding dates, any existing weeds should be thoroughly eradicated mechanically or with herbicide within the project area. Follow herbicide recommendations regarding duration to wait between application and seeding/planting.

Warning: It is REQUIRED that the planting or seeding contractor have proven successful experience installing and maintaining projects of similar scope and scale and provide a superintendent that will be onsite during the entire seeding or planting process.

All seed and plants should be shipped and stored with protection from weather or other conditions that would damage the product. All plants and seeds will be inspected by the designer and items that have become wet, moldy, or otherwise damaged in transit or in storage should be rejected. Plants and seed should arrive within 24 hours of delivery. Plants and seed needs to be protected against drying and damage prior to planting.

It is typical for the plant or seeding contractor to guarantee the work for some length of time. The common minimum for herbaceous plantings or sod is 60 days during the growing season. The growing season in central Minnesota is defined as May 1st through October 31st. A one-year guarantee on containerized plants can help to ensure good establishment and decrease weed infestations while maintaining infiltration rates over time through the growth of healthy root systems. Any watering required to keep the plants healthy should be covered under the cost of the warranty period. It is appropriate to require that the contractor provide some form of surety, such as a letter of credit or other security, to the permitting entity for 150 percent of the estimated costs and quantities of all herbaceous plants or seeding for the duration of the 1-year warranty period. Planting and seeding establishment should meet the requirements within MnDOT Section 2571.

Caution: Seeding maintenance requires specialized knowledge and experience in plant and weed identification. Ensure a thorough maintenance plan is established prior to construction and that budget has been allocated for at least three full growing seasons and preferably longer. Native seedings can be more difficult than containerized plantings to establish.

For information on plants recommended or suitable for swales, link here.

### Step 7 – Final stabilization and closeout

As defined in the NPDES/SDS Construction Stormwater permit, final site stabilization is achieved when all soil disturbing activity is completed and the exposed soils have been stabilized with a vegetative cover with a uniform density of at least 70 percent over the entire site or by equivalent means to prevent soil failure. Simply seeding and mulching is not considered acceptable cover for final stabilization. Final stabilization must consist of an established permanent cover, such as a perennial vegetative cover, concrete, riprap, gravel, rooftops, asphalt, etc.

All filtration (filter media) and infiltration areas must be tested for infiltration rates after they are completed in order to submit the NPDES Notice of Termination. It is highly recommended that all infiltration areas are tested prior to project close out, even if an NPDES permit is not required.

MnDOT projects require at least five tests per acre of infiltration area and a minimum of five tests per infiltration area. Infiltration rates shall meet or exceed double the design rate assumed. The test results from a MnDOT project must be submitted to MnDOT.

When a final construction inspection has been completed, log the GPS coordinates for each facility and submit them for entry into the local BMP maintenance tracking database, if applicable.

## Minnesota Department of Transportation example construction protocols

### Preliminary analysis and selection

Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.

Surface area of stormwater control measure (BMP)(ft2) Borings Pits Permeameter tests
< 1000 1 1 5
1000 to 5000 2 2 10
5000 to 10000 3 3 15
>10000 41 41 202

1an additional soil boring or pit should be completed for each additional 2,500 ft2 above 12,500 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2

### Field verification testing prior to pond construction

• Soil hydraulic group represent what is stated in SWPPP (Stormwater Pollution Prevention Plan)
• Seasonally high water table not discovered within 3 feet of the excavated pond base within a test pit
• Commonly will test bottom of proposed pond for soil compaction (subsequent subsoil ripping) prior to media placement
• Commonly will test bottom of proposed pond for insitu infiltration rate by test pit or water filled barrel placed on pond base surface

### Filter media and material testing

• Existing soil (option 1 below) or Washed sand (option 2 below), and compost certification
• Washed course aggregate choker certification
• Other treatment material certification of iron filings, activated charcoal, pH buffers, minerals, etc.
• Geotextile separation fabric certification
• Drain-tile certification (if filtration is specified)
• Seed source certification
• Barrel test verification of infiltration rate using 2.5 feet of imported 3877 Type G media

### Field verification testing/inspection/verification during construction

• Water drains away in 48 hours
• Infiltration drainage rate does not exceed 8.3 inches per hour
• No tracking/equipment in pond bottom
• No sediment deposits from ongoing construction activity, media perimeter controls kept functional
• Forebay is trapping settleable solids, floating materials, and oil/grease
• Area staked off

### Notice of Termination (NOT) verification

• Option 1. Amending existing HSG soils with compost or other treatment material. Test the infiltration rate of each infiltration basin using a double ring infiltrometer prior to completion of the basin. Conduct the test at the finished grade of the basin bottom, prior to blending the compost with the in-situ soils or sand. Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests.
• Option 2. Importing 3877 Type G Filter Topsoil Borrow (may be amended with other treatment material). Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour, or rate specified in the plan. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests. Amend soils with additional washed sand if rates less than specified in the contract, or compost if rates exceed 8.3 inches per hour.

The permanent stormwater management system must meet all requirements in sections 15, 16, and 17 of the CSW permit and must operate as designed. Temporary or permanent sedimentation basins that are to be used as permanent water quality management basins have been cleaned of any accumulated sediment. All sediment has been removed from conveyance systems and ditches are stabilized with permanent cover.

</noinclude>

# Assessing the performance of dry swale (grass swale)

Swales retain solids and associated pollutants by settling and filtering. A typical method for assessing the performance of of BMPs with underdrains is therefore measuring and comparing pollutant concentrations at the influent and effluent. If the swale is designed for infiltration, see Assessing the performance of bioretention.

An online manual for assessing BMP treatment performance was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory. The manual advises on a four-level process to assess the performance of a Best Management Practice.

• Level 1: Visual Inspection. This includes assessments for infiltration practices and for filtration practices. The website includes links to a downloadable checklist.
• Level 2: Capacity Testing. Level 2 testing can be applied to both infiltration and filtration practices.
• Level 3: Synthetic Runoff Testing for infiltration and filtration practices. Synthetic runoff test results can be used to develop an accurate characterization of pollutant retention or removal, but can be limited by the need for an available water volume and discharge.
• Level 4: Monitoring for infiltration or filtration practices

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Additional information on designing a monitoring network and performing field monitoring are found at this link.

# Operation and maintenance of dry swale (grass swale)

Green Infrastructure: Dry swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value. See the section Green Infrastructure for stormwater management.

Photo of a well-maintained dry swale. Courtesy of Limnotech.

The most frequently cited maintenance concern for dry swales is surface soil/media and underdrain clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include:

• standing water after required 48 hour drawdown time;
• clogged soil/media surface;
• clogged inlet, outlet or underdrains; and
• invasive plants that out-compete native vegetation.

## Design phase maintenance

Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include:

## Construction phase maintenance

Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with construction of filtration and infiltration practices the most important action for preventing operation and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line.

Warning: It is required that the contributing drainage area has been fully stabilized prior to bringing the practice on line

Inspections during construction are needed to ensure that the filtration or infiltration practice is built in accordance with the approved design standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.

Dry swale construction inspection checklist.
To access an Excel version of form (for field use), click here.

Construction Sequence Project: Location: Site Status: Date: Time: Inspector: Pre-construction meeting Runoff diverted (Note type of bypass) Facility area cleared Soil tested for permeability Soil tested for phosphorus content (include test method) Verify site was not overdug Project benchmark near site Facility location staked out Temporary erosion and sediment protection properly installed Lateral slopes completely level Soils not compacted during excavation Longitudinal slopes within design range Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence Verify stockpile is not causing compaction and that it is not eroding Was underlying soil ripped or loosened Size, location, and inverts per plans Side slopes stable Groundwater / bedrock verified Stone diaphragm installed per plans Outlets installed pre plans Check dams installed per plans Underdrain installed to grade Pretreatment devices installed per plans Soil bed composition and texture conforms to specifications Inlets installed per plans Underdrain installed per plans For native dry swales, plants and materials ordered 6 months prior to construction For native dry swales, construction planned to allow for adequate planting and establishment of plant community Complies with planting specs Topsoil complies with specs in composition and placement Soil properly stabilized for permanent erosion control Dimensions per plans Pretreatment operational Check dams operational Inlet/outlet/underdrain operational Soil/media/filter bed permeability verified Effective stand of vegetation stabilized Construction generated sediments removed Contributing watershed stabilized before flow is diverted to the practice Comments: Actions to be taken:

## Post-construction operation and maintenance

Proper maintenance is critical to the successful operation of a filtration or infiltration practice. Without regular maintenance, the soil or media of the filtration or infiltration systems can become clogged, losing its ability to conduct and infiltrate water at the designed rate. This can lead to stagnant water, mosquito breeding habitat, and reduction or elimination of pollutant removal capacity.

Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED. Effective long-term operation of filtration and infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value.

## Inspection and maintenance planning

A maintenance plan clarifying maintenance responsibilities is REQUIRED. Effective long-term operation of filtration and infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value. Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards.

• A site-specific O&M plan that includes the following considerations should be prepared by the designer prior to putting the stormwater practice into operation:
• Inspection and routine maintenance checklist (see below)
• Operating instructions for any outlet components
• Vegetation maintenance schedule (see item 2 in checklist below and section below)
• A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority to ensure the following:
• Sediment should be cleaned out of any sedimentation chamber when it accumulates to a depth equal to ½ the total depth to the outlet, or when greater than 1.5 feet, whichever is less. The sediment chamber outlet devices should be cleaned/repaired when drawdown times exceed 36 hours. Trash and debris should be removed as necessary; and
• Silt/sediment should be removed from the swale bottom when the accumulation exceeds one inch. When the soil/media’s infiltration capacity diminishes substantially (i.e., when water ponds in flat areas or subtle depressions for more than 48 hours), the top few inches of discolored material (visually different from the unclogged soil below) should be removed, core aeration or cultivation should be conducted as warranted, removed soil should be replaced with fresh soil/media, and appropriate vegetation should be installed (e.g., seed) and secured (e.g., erosion control blanket). Removed sediments should be disposed in an acceptable manner.
• Turf grass swales should be mowed as needed during the growing season to maintain grass heights between 4 and 12 inches.
• Adequate access must be provided for inspection, maintenance and landscaping upkeep, including appropriate equipment and vehicles.
• Maintenance activities should be careful not to cause compaction. No vehicles will be allowed within the footprint of the filtration or infiltration area. Foot traffic and stockpiling should be kept to a minimum.
• Dry swales generally should not be used as dedicated snow storage areas, but can be with the following considerations.
• Snow storage should not occur in areas designated as potential stormwater hotspots for road salt.
• Areas designed for infiltration should be protected from excessive snow storage where sand and salt is applied.
• Specific snow storage areas should be assigned that will provide some filtration before the stormwater reaches the BMP areas. NOTE: Chloride will not be attenuated in filtration or infiltration BMPs such as dry swales.
• When used for snow storage, or if used to treat parking lot runoff, the BMP area should be planted with salt tolerant and non-woody plant species.
• BMPs should always be inspected for sand build-up on the surface following the spring melt event.
• General maintenance activities and schedule are provided below.

Dry swale operation and maintenance checklist.
To access an Excel version of form (for field use), click here.

Maintenance Item Project: Location: Site Status: Date: Time: Inspector: Contributing areas clean of litter and vegetative debris Filtration or infiltration facility clean Inlets and outlets clear Vegetation maintenance complies with O&M plan Vegetation meets performance standards (including control of specified invasive species) Plant composition according to O&M plan Minimum mowing depth not exceeded No evidence of erosion Dewaters between storms within 48 hours Area clean of sediment Contributing drainage area stabilized and free of erosion Winter accumulation of sand removed each spring Good condition, no need for repair No evidence of erosion No evidence of any blockages No evidence of structural deterioration Encroachment on easement area (if applicable) Complaints from residents (if applicable) Any public hazards (specify) Comments: Actions to be taken:

### Summary of typical maintenance regime

The list below highlights the assumed maintenance regime for a dry swale.

• First year after planting
• Adequate water is crucial to plant survival and temporary irrigation may be needed unless rainfall is adequate until plants mature
• Inspect after significant rain events (e.g. >0.5 inch)
• As needed
• Prune and weed to maintain appearance
• Remove trash and debris
• Mow filter strip/grass channel (if present)
• Replace vegetation whenever the percent cover of acceptable vegetation falls below 90 percent or project specific performance requirements are not met. If vegetation suffers for no apparent reason, consult with horticulturist and/or test soil as needed
• Repair any structural damage to check dams or tie-in to downstream channel
• Semi-annually
• Inspect inflow and pretreatment systems for clogging (off-line systems) and remove any sediment
• Inspect filter strip/grass channel for erosion or gullying. Sod as necessary
• Herbaceous vegetation, trees and shrubs should be inspected to evaluate their health and replanted as appropriate to meet project goals
• Remove any dead or severely diseased vegetation
• Annually in fall
• Inspect and remove any sediment and debris build-up in pretreatment areas
• Inspect inflow points and infiltration surface for buildup of road sand associated with spring melt period, remove as necessary to maintain infiltration rates and volume capacity, and replant areas that have been impacted by sand/salt build up
• Check structural stability of check dams
• Annually in spring
• Cut back and remove previous year’s plant material and remove accumulated leaves if needed (or conduct controlled burn where appropriate)

## Estimated hours to perform maintenance activities

All estimated hours listed below would be to perform maintenance on a dry swale system approximately 1,000 square feet in size that has adequate pretreatment and where seed and/or live plants have been installed appropriately. The times do not include travel times.

• Plant Establishment Period (First two years)
• Monthly weeding – 12 visits (6 per year) at 1 hour per visit
• Vegetation replacement – 1 overseeding or replanting effort, 2 hours (assuming 10 percent warrants replacement)
• Spring cleanup (cut back of previous years vegetation) – 2 cleanups (1 per year) at 2 hours each
• Erosion, sediment, and pretreatment cleanout – 2 cleanouts (1 per year) at 1 hour each (assuming vacuum truck clean-out of any sump catch basins)
• Regular Maintenance (After first two years)
• Bi-monthly (every other month) weeding – 3 visits per year at 1 hour per visit
• Vegetation replacement – 1 overseeding or replanting effort per year on average, 1 hour (assuming 5 percent warrants replacement)
• Spring cleanup (cut back of previous years vegetation) – 1 per year at 2 hours
• Erosion, sediment, and pretreatment cleanout – 2 hours per year on average (assuming vacuum truck clean-out of any sump catch basins once per year, and at least one bi-yearly (every other year) sediment removal from the bottom of the swale)

## Erosion protection and sediment monitoring, removal, and disposal

Regular inspection of not only the BMP but also the immediate surrounding catchment area is necessary to ensure a long lifespan of the water quality improvement feature. Erosion should be identified as soon as possible to avoid the contribution of significant sediment to the BMP.

Pretreatment devices need to be maintained for long-term functionality of the entire BMP. Accumulated sediment in filter strips, rock diaphragms, water quality sump catch basins, or any pretreatment features will need to be inspected yearly.

Timing of cleaning of these features is dependent on their design and sediment storage capabilities. In watersheds with erosion or high sediment loadings, the frequency of clean out will likely be increased. A vacuum truck is typically used for sediment removal. It is possible that any sediment removed from pretreatment devices or from the bottom of a dry swale may contain high levels of pollutants. All sediments, similar to those retrieved from a stormwater pond during dredging, may be subjected to the MPCA’s guidance for reuse and disposal.

Sediment loading can potentially lead to a drop in infiltration or filtration rates. It is recommended that infiltration performance evaluations follow the four level assessment systems in Stormwater Treatment: Assessment and Maintenance (Gulliver et al., 2010). See Assessing the performance of dry swale (grass swale) for a summary of assessment methods.

## Seeding, planting, and landscaping maintenance

Plant selection during the design process is essential to limit the amount of maintenance required. It is also critical to identify who will be maintaining the BMP in perpetuity and to design the plantings or seedings accordingly. The decision to install containerized plants or to seed will dictate the appearance of the BMP for years to come. If the BMP is designed to be seeded with an appropriate native plant based seed mix, it is essential the owner have trained staff or the ability to hire specialized management professionals. Seedings can provide plant diversity and dense coverage that helps maintain drawdown rates, but landscape management professionals that have not been trained to identify and appropriately manage weeds within the seeding may inadvertently allow the BMP to become infested and the designed plant diversity be lost. The following are minimum requirements for seed establishment and plant coverage.

• At least 50 percent of specified vegetation cover at end of the first growing season, not including REQUIRED cover crop
• At least 90 percent of specified vegetation cover at end of the third growing season, not including REQUIRED cover crop
• Supplement seeding/plantings to meet project specifications if cover requirements are not met
• Tailor percent coverage requirements to project goals and vegetation. For example, percent cover required for turf after one growing season would likely be 100 percent, whereas it would be lower for other vegetation types.

For proper nutrient control, swales must not be fertilized unless a soil test from a certified lab indicates nutrient deficiency. If this is the case, apply the minimum rate of appropriate nutrients to provide a suitable environment for vegetation establishment while also minimizing the mobilization (and loss) of nutrients to downstream receiving waters. Irrigation may be needed during establishment, depending on soils, precipitation, and if stormwater flows are kept off-line during establishment.

Weeding is especially important during the plant establishment period, when vegetation cover is not 100 percent yet. Some weeding will always be needed. It is also important to budget for some plant replacement (at least 5 to 10 percent of the original plantings or seedings) during the first few years in case some of the plants or seed that were originally installed don’t become vigorous. It is HIGHLY RECOMMENDED that the install contractor be responsible for a plant warranty period. Typically, plant warranty periods can be 60 days or up to one year from preliminary acceptance through final inspections. If budget allows, installing larger plants (#1 container vs. 4” pot) during construction can decrease replacement rates if properly cared for during the establishment period.

Weeding in years after initial establishment should be targeted and thorough. Total eradication of aggressive weeds at each maintenance visit will ultimately reduce the overall effort required to keep the BMP weed free. Mulch is generally not recommended for use in swales since flowing water typically washes it downstream; however, mulch may be appropriate in planting beds or around individual trees on upper sideslopes and adjacent areas.

Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape. Trash removal is important for prevention of mosquitoes and for the overall appearance of the BMP.

## Sustainable service life

The service life of swales depends upon the pollutant of concern.

### Infiltration rate service life before clogging

It is known that plant roots are essential in macropore formation, which helps maintain infiltration into soil. If proper pretreatment is present, service life for infiltration should be unlimited. However, if construction site runoff (or another source of fines) is not prevented from entering the swale, clogging will occur, limiting or eliminating the infiltration function of the system, thus requiring restorative maintenance or repair (Brown and Hunt, 2010).

### Nitrogen reduction

Nitrogen removal is not a primary function of dry swales.

### Phosphorus reduction

Phosphorus (P) removal in swales is achieved primarily through infiltration and sorption of phosphorus to trapped sediments. Sediment bound phosphorus is removed through sedimentation, while removal of soluble phosphorus depends on the type of soil/media used. If the soil/media is already saturated with P (i.e., its P binding sites are full), it will not be able to retain additional dissolved P and the P in stormwater will tend to leach from the soil/media as it passes through the biofilter (Hunt et al., 2006). It is highly recommended that the P-index of the media at installation be below 30, which equates to less than 36 milligrams per kilogram P, to ensure P removal capacity. Laboratory research has suggested an oxalate extractable P concentration of 20 to 40 milligrams per liter will provide consistent removal of P (O’Neill and Davis, 2012). Leaching of phosphorus from soil or media is a concern for filtration swales (those having an underdrain). For information on phosphorus leaching from bioretention media, link here.

### Heavy metals retention

Metals are typically retained in infiltration systems (including dry swales) through sedimentation and adsorption processes. Since there are a finite amount of sorption sites for metals in a particular soil/media, there will be a finite service life for the removal of dissolved metals. Morgan et al. (2011) investigated cadmium, copper, and zinc removal and retention with batch and column experiments. Using synthetic stormwater at typical stormwater concentrations, they found that 6 inches of filter media composed of 30 percent compost and 70 percent sand will last 95 years until breakthrough (i.e., when the effluent concentration is 10 percent of the influent concentration). They also found that increasing compost from 0 percent to 10 percent more than doubles the expected lifespan for 10 percent breakthrough in 6 inches of filter media for retainage of cadmium and zinc. Using accelerated dosing laboratory experiments, Hatt et al. (2011) found that breakthrough of Zn was observed after 2000 pore volumes, but did not observe breakthrough for Cd, Cu, and Pb after 15 years of synthetic stormwater passed through the media. However, concentrations of Cd, Cu, and Pb on soil/media particles exceeded human and/or ecological health levels, which could have an impact on disposal if the soil/media needed replacement. Since the majority of metals retainage occurs in the upper 2 to 4 inches of the soil/media (Li and Davis, 2008), long-term metals capture may only require rejuvenation of the upper portion of the media.

### Polycyclic aromatic hydrocarbons (PAHs) reduction

Accumulation of polycyclic aromatic hydrocarbons (PAHs) in sediments has been found to be so high in some stormwater retention ponds that disposal costs for the dredging spoils were prohibitively high. Research has shown that rain gardens, on the other hand, are “a viable solution for sustainable petroleum hydrocarbon removal from stormwater, and that vegetation can enhance overall performance and stimulate biodegradation.” (Lefevre et al., 2012). Dry swales provide some of the same functions as rain gardens, and therefore would be expected to provide some PAH management. However, swale performance in PAH management has not been the focus of any identified studies.

## Typical maintenance problems and activities

The following table summarizes common maintenance concerns, suggested actions, and recommended maintenance schedule.

Typical maintenance problems and activities for dry swales

Inspection Focus Common Maintenance Problems Maintenance Activity Recommended Maintenance Schedule
Drainage Area and Drawdown Time Clogging, sediment deposition Ensure that contributing catchment areas to practice, and inlets are clear of debris Monthly
Erosion of catchment area contributing significant amount of sediment In case of severely reduced drawdown time, scrape bottom of basin and remove sediment. Disc or otherwise aerate/scarify basin bottom. De-thatch if basin bottom is turf grass. Restore original design cross section or revise section to increase infiltration rate and restore with vegetation as necessary. Upon identification of drawdown times longer than 48 hours or upon complete failure
Site Erosion Scouring at inlets Correct earthwork to promote non‐erosive flows that are evenly distributed As necessary
Unexpected flow paths into practice Correct earthwork to eliminate unexpected drainage or created additional stable inlets as necessary As necessary
Vegetation Reduced drawdown time damaging plants Correct drainage issues as described above Replace with appropriate plants after correction of drainage issues
Severe weed establishment Limit the ability for noxious weed establishment by properly mowing, mulching or timely herbicide or hand weeding. Refer to the MDA Noxious Weed List Bi‐monthly April through October
Vegetative cover Add seed/plants to maintain ≥95% vegetative cover. Bi‐monthly April through October

## Maintenance agreements

A Maintenance Agreement is a legally binding agreement between two parties, and is defined as ”a nonpossessory right to use and/or enter onto the real property of another without possessing it.“ Maintenance Agreements are often required for the issuance of a permit for construction of a stormwater management feature and are written and approved by legal counsel. Maintenance Agreements are often similar to Construction Easements. A Maintenance Agreement is required for one party to define and enforce maintenance by another party. The Agreement also defines site access and maintenance of any features or infrastructure if the property owner fails to perform the required maintenance.

Maintenance Agreements are commonly established for a defined period such as five years for a residential site or 10 to 20 years for a commercial/governmental site after construction of the infiltration or filtration practice. Maintenance agreements often define the types of inspection and maintenance that would be required for that infiltration or filtration practice and what the timing and duration of the inspections and maintenance may be. Essential inspection and maintenance activities include but are not limited to drawdown time, sediment removal, erosion monitoring and correction, and vegetative maintenance and weeding. If maintenance is required to be performed due to failure of the site owner to properly maintain the infiltration or filtration practices, payment or reimbursement terms of the maintenance work are defined in the Agreement. Below is an example list of maintenance standards from an actual Maintenance Agreement.

1. Live plantings and seeding areas shall be watered as necessary to achieve performance standards.
2. Weeding and vegetation management (e.g., mowing, spot spraying) shall be conducted as necessary to achieve performance standards.
3. Dead plant material, garbage, and other debris shall be removed from the swale at least annually.
4. Silt/sediment should be removed from the swale bottom when the accumulation exceeds one inch.
5. Side slopes must be inspected for erosion and the formation of rills or gullies at least annually, and erosion problems must be corrected immediately.
6. If properly planned, designed, constructed, and maintained (including protected from sediment and compaction and incorporating sufficient pretreatment), a dry swale is likely to retain its effectiveness for well over 20 years. After that time, inspection will reveal whether sedimentation warrants scraping out the swale bottom and replanting it.

In some project areas, a drainage easement may be required. Having an easement provides a mechanism for enforcement of maintenance agreements to help ensure swales are maintained and functioning. Drainage easements also require that the land use not be altered in the future. Drainage easements exist in perpetuity and are required property deed amendment to be passed down to all future property owners.

As defined by the Maintenance Agreement, the landowner should agree to provide notification immediately upon any change of the legal status or ownership of the property. Copies of all duly executed property transfer documents should be submitted as soon as a property transfer is made final.

## Maintenance inspection reports

To link to the maintenance inspection report, click here. The contents of the inspection form are provided below. For another source of information on visual indicators, see Chesapeake Stormwater visual indicators form.

Maintenance Inspection Report for Dry Swale with Check Dams and Stormwater Step Pool. Can be used for wet swales with exceptions, as noted in footnotes.

Date: ____________________________________________________________________

Drainage Area Stabilization (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)

• Erosion control/planting/seeding necessary: __________________________________________________
• Mowing, pruning and debris removal necessary: _______________________________________________
• Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Inlets & Pretreatment Structures (Inspect in Spring and Fall)

• Repair needed: _________________________________________________________________________
• Debris & sediment removal required: _______________________________________________________
• Erosion evident: _________________________________________________________________________
• Water by-passing inlet: ___________________________________________________________________
• Vegetation control necessary: _____________________________________________________________
• Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Swale (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)

• Condition of infiltration area1: ______________________________________________________________
• Condition of check dams: _________________________________________________________________
• Surface erosion evident: __________________________________________________________________
• Debris/sediment removal required: _________________________________________________________
• Weeding and pruning necessary: ___________________________________________________________
• Mulch replacement necessary3: _____________________________________________________________
• Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Outlet/Emergency Overflow (Inspect in Spring and Fall)

• Overflow type: _________________________________________________________________________
• Debris/sediment removal required: _________________________________________________________
• Repair needed: _________________________________________________________________________
• Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

1For wet swale, check condition of inundated area
2For wet swale with check dam, drawdown applies to the water elevation at the botton of weir
3Not applicable for wet swale

# Calculating credits for dry swale (grass swale)

 Recommended pollutant removal efficiencies, in percent, for dry swale BMPs. Sources. NOTE: removal efficiencies are 100 percent for water that is infiltrated. TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen TSS TP PP DP TN Metals Bacteria Hydrocarbons 68 link to table link to table link to table 35 80 0 80
Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

This page provides a discussion of how dry swales can achieve stormwater credits. Swales with and without underdrains are both discussed, with separate sections for each type of system as appropriate.

## Overview

Schematic showing characteristics of a dry swale.

Dry swales, sometimes called grass swales, are similar to bioretention cells but are configured as shallow, linear channels. Dry swales function primarily as a conveyance BMP, but provide treatment of stormwater runoff, particularly when used in tandem with check dams that temporarily retain water in a series of cells. Dry swales with an underdrain and engineered soil media are considered a filtration practice. Dry swales with in-situ soils capable of infiltration, (A or B soils) are considered infiltration practices. Dry swales are designed to prevent standing water. Dry swales typically have vegetative cover such as turf or native perennial grasses.

### Pollutant Removal Mechanisms

Dry swales without check dams or with underdrains primarily remove pollutants through filtration during conveyance of stormwater runoff. Dry swales may also provide some volume reduction benefits through infiltration and evapotranspiration during conveyance or below an underdrain. Water quality treatment is also recognized through biological and microbiological uptake, and soil adsorption. Check dams may be incorporated into dry swale design to enhance infiltration.

### Location in the Treatment Train

Dry swales may be located throughout the treatment train, including the main form of conveyance between or out of BMPs, at the end of the treatment train, or designed as off-line configurations where the water quality volume is diverted to the filtration or infiltration practice. In any case, the practice may be applied as part of a stormwater management system to achieve one or more of the following objectives:

• reduce stormwater pollutants (filtration or infiltration practices)
• increase groundwater recharge (infiltration practices)
• decrease runoff peak flow rates (filtration or infiltration practices)
• decrease the volume of stormwater runoff (infiltration practices)
• preserve base flow in streams (infiltration practices)
• reduce thermal impacts of runoff (filtration or infiltration practices)

## Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article.

Dry swale practices generate credits for volume, TSS,and TP. Dry swale practices with an underdrain do not substantially reduce the volume of runoff but may qualify for a partial volume credit as a result of evapotranspiration, infiltration occurring through the sidewalls above the underdrain, and infiltration below the underdrain piping. Dry swale practices are effective at reducing concentrations of other pollutants including metals and hydrocarbons. They are generally not effective at removing bacteria. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

### Assumptions and Approach

In developing the credit calculations, it is assumed the swale is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the Manual.

Warning: Pretreatment is required for all filtration and infiltration practices

Unlike other BMPs such as bioretention and permeable pavement, credits for swales are calculated in two ways. First, if check dams are incorporated into the design, the water quality volume (VWQ) is assumed to be delivered instantaneously to the BMP and stored as water ponded behind the check dam, above the soil or filter media, and below the overflow point of the check dam. VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch times new impervious surface area. For MIDS, the VWQ is 1.1 inches times impervious surface area.

Second, if check dams are not incorporated into the swale, water will infiltrate into the underlying soil or filter media as it is conveyed along the swale. The amount of water captured in this manner is a function of the underlying soil permeability and the length of time flowing water is in contact with the soil, which in turn is affected by the slope of the swale.

### Volume credit calculations - check dams and no underdrain

Schematic illustrating terms and dimensions used for volume and pollutant calculations, no underdrain.

Volume credits are typically calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff from the existing stormwater collection system. When check dams are incorporated into the design, these credits are assumed to be instantaneous values entirely based on the capacity of the BMP for any storm event. Instantaneous volume reduction, or event based volume reduction of a BMP can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.

Credits for dry swales with check dams are dependent on multiple design factors of the swale channel and side slopes, as well as infiltration rates for underlying soils. The water quality volume (Vwq) achieved behind each check dam (instantaneous volume) is given by

$V_{wq} = [h^2 * (h * H + B_w)]/(2S)$

where

h = check dam height (inches);
H = horizontal component of the swale side slope (1 vertical : H horizontal)(unitless);
S = slope (unitless); and
Bw = channel bottom width (inches).

Convert the volume to cubic feet by dividing by 1728.

Add the Vwq for each check dam together to obtain the cumulative water quality volume for the swale.

For an example calculation, link here.

Some of the VWQ will be lost to evapotranspiration rather than all being lost to infiltration. In terms of a water quantity credit, this differentiation is unimportant, but it may be important if attempting to calculate actual infiltration into the underlying soil.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and water quality volume. See footnote1 for how these were determined.

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.

### Volume credit calculations - check dams with an underdrain

Schematic illustrating terms and dimensions used for volume and pollutant calculations, with underdrain.
Schematic illustrating the different water loss terms for a swale (biofiltration) BMP with a raised underdrain.

Volume credit for a swale with check dams and an underdrain is the same as for a biofiltration BMP, although some of the BMP configurations differ somewhat. Volume credits are available only if the BMP permanently removes a portion of the stormwater runoff via infiltration through sidewalls or beneath the underdrain piping, or through evapotranspiration. These credits are assumed to be instantaneous values based on the design capacity of the BMP for a specific storm event. Instantaneous volume reduction, also termed event based volume reduction, can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.

Volume credits for a dry swale with check dams and underdrains are calculated by a combination of infiltration through the unlined sides and bottom of the basin (the area below the underdrain), the volume loss through evapotranspiration (ET), and the retention volume below a raised underdrain, if applicable (this is based on the assumption that this stored water will infiltrate into the underlying soil). The main design variables impacting the volume credits include whether the underdrain is elevated above the native soils and if an impermeable liner on the sides or bottom of the basin is used. Other design variables include surface area at the check dam overflow, media top surface area, underdrain location, and basin bottom locations, total depth of media, soil water holding capacity and media porosity, and infiltration rate of underlying soils.

Information: For the following equations, units most commonly used in practice are given and unit correction factors are based on those units

The following calculations are for a single check dam. To get the total volume credit add the volumes for each check dam.

The volume credit (V) for a dry swale with a check dam and underdrain, in cubic feet, is given by

$V = V_{inf_B} + V_{inf_s} + V_{ET} + V_U$

where:

Vinfb = volume of infiltration through the bottom of the basin (cubic feet);
Vinfs = volume of infiltration through the sides of the basin (cubic feet);
VET = volume reduction due to evapotranspiration (cubic feet); and
VU = volume of water stored beneath the underdrain that will infiltrate into the underlying soil (cubic feet).

Volume credits for infiltration through the bottom of the basin (Vinfb) are accounted for only if the bottom of the basin is not lined. As long as water continues to draw down, some infiltration will occur through the bottom of the BMP. However, it is assumed that when an underdrain is included in the installation, the majority of water will be filtered through the media and exit through the underdrain. Because of this, the drawdown time is likely to be short. Volume credit for infiltration through the bottom of the basin is given by

$V_{inf_B} = A_B\ DDT\ I_R/12 / 2$

where

AB = surface area at the bottom of the basin (square feet); and
DDT = drawdown time for ponded water (hours).

Because of the slope in a swale and the resulting unequal pool depth behind a check dam, a correction factor of 2 is included in the above equation.

Information: The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to IR. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.

The drawdown time is typically a maximum of 48 hours, which is designed to be protective of plants grown in the media. The Construction Stormwater permit requires drawdown within 48 hours and recommends 24 hours when discharges are to a trout stream. With a properly functioning underdrain, the drawdown time is likely to be considerably less than 48 hours.

Volume credit for infiltration through the sides of the basin is accounted for only if the sides of the basin are not lined with an impermeable liner. Volume credit for infiltration through the sides of the basin is given by

$V_{inf_s} = (A_O - A_U)\ DDT\ I_R/12$

where

AO = the surface area at the overflow (square feet); and
AU = the surface area at the underdrain (square feet).
Information: The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to IR. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.

This equation assumes water will infiltrate through the entire sideslope area during the period when water is being drawn down. This is not the case, however, since the water level will decline in the BMP. The MIDS calculator assumes a linear drop in water level and thus divides the right hand term in the above equation by 2.

Volume credit for media storage capacity below the underdrain (VU) is accounted for only if the underdrain is elevated above the native soils. Volume credit for media storage capacity below the underdrain is given by

$V_U = (n-FC)\ D_U\ (A_U + A_B)/2$

where

AB = surface area at the bottom of the media (square feet);
n = media porosity (cubic feet per cubic foot);
FC is the field capacity of the soil, in cubic feet per cubic foot; and
DU = the depth of media below the underdrain (feet).

This is an instantaneous volume. This will somewhat overestimate actual storage when the majority of water is being captured by the underdrains. This equation assumes water between the soil porosity and field capacity will infiltrate into the underlying soil.

The volume of water lost through ET is assumed to be the smaller of two calculated values: potential ET and measured ET. Potential ET (ETpot) is equal to the amount of water stored in the basin between field capacity and the wilting point. Measured ET (ETmea) is the amount of water lost to ET as measured using available data and is assumed to be 0.2 inches/day. ETmea is converted to ET by multiplying by a factor of 0.5. ET is considered to occur over a period equal to the drawdown time of the basin. Volume credit for evapotranspiration is given by the lesser of

$ET_{mea} = (0.2/12)\ A\ 0.5\ t$ $ET_{pot} = D\ A\ C_S$

where

t = time over which ET is occurring (days);
D = depth being considered (feet);
A = area being considered (square feet); and
CS = soil water available for ET, generally assumed to be the water between field capacity and wilting point.

ET is likely to be greater if one or more trees is planted in the swale. The MIDS calculator increases the above ET credit by a factor of 3 when a tree is planted in the swale, but this credit is not available for swales. See Plants for swales for information about trees that might acceptable in swales.

Provided soil water content is greater than the wilting point, ET will continually occur during the non-frozen period. However, because the above volume calculations are event based, t will be equal to the time between rain events. In the MIDS calculator, a value of 3 days is used because this is the average number of days between precipitation events. ET will occur over the entire media depth. D may therefore be set equal to the media depth (DM). In this case, the value for A would be the average area through the entire depth of the media. The MIDS calculator limits ET to the area above the underdrain. If infiltration is being computed through the bottom and sidewalls of the basin, then CS would be field capacity minus the wilting point of soils (cubic feet per cubic foot) since water above the field capacity would infiltrate (or go to an underdrain).

The volume of water passing through underdrains can be determined by subtracting the volume loss (V) from the volume of water instantaneously captured by the BMP. No volume reduction credit is given for filtered stormwater that exits through the underdrain, but the volume of filtered water can be used in the calculation of pollutant removal credits through filtration.

The volume reduction credit (V) can be converted to an annual volume if desired. This conversion can be generated using the MIDS calculator or other appropriate modeling techniques. The MIDS calculator obtains the percentage annual volume reduction through performance curves developed from multiple modeling scenarios using the volume reduction capacity for swales, the infiltration rate of the underlying soils, and the contributing watershed size and imperviousness.

### Volume credit calculations - no check dam

When a check dam is not incorporated into the design, water will infiltrate into the soil or media as it is conveyed along the swale. Volume credits for swales without check dams can be calculated using an appropriate model, such as the MIDS calculator or soil infiltration models (e.g. Green and Ampt).

### Water quality credit calculations

Water quality credits applied to dry swales can be calculated by rainfall event or annual rainfall. This value is obtained from the infiltration and filtration volume capacity of the BMP (calculated above).

#### Total suspended solids

TSS reduction credits correspond with volume reduction through infiltration and filtration of water captured by the swale and are given by

$M_{TSS} = M_{TSS_i} + M_{TSS_f}$

where

MTSS = TSS removal (pounds);
MTSS_i = TSS removal from infiltrated water (pounds); and
MTSS_f = TSS removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The event-based mass of pollutant removed through infiltration, in pounds, is given by

• filtration (underdrain) - $M_{TSS_i} = 0.0000624\ (V_{inf_b} + V_{inf_s} + V_U)\ EMC_{TSS}$
• infiltration - $M_{TSS_i} = 0.0000624\ V_{WQ}\ EMC_{TSS}$

where

EMCTSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMCTSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on EMC values for TSS, link here or here.

Removal for the filtered portion is less than 100 percent. The event-based mass of pollutant removed through filtration, in pounds, is given by

$M_{TSS_f} = 0.0000624\ (V_{total} - (V_{inf_b} + V_{inf_s} + V_U))\ EMC_{TSS}\ R_{TSS}$

where

Vtotal is the total volume of water captured by the BMP (cubic feet); and
RTSS is the TSS pollutant removal percentage for filtered runoff.

The Stormwater Manual provides a recommended value for RTSS of 0.68 (68 percent) removal for filtered water. Alternate justified percentages for TSS removal can be used if proven to be applicable to the BMP design.

The above calculations may be applied on an event or annual basis and are given by

$M_{TSS_f} = 2.72\ F\ V_{F_{annual}}\ EMC_{TSS}\ R_{TSS}$

where

F is the fraction of annual volume filtered through the BMP; and
Vannual is the annual volume treated by the BMP, in acre-feet.

#### Total phosphorus

Total phosphorus (TP) reduction credits correspond with volume reduction through infiltration and filtration of water captured by the swale and are given by

$M_{TP} = M_{TP_i} + M_{TP_f}$

where

• MTP = TP removal (pounds);
• MTP_i = TP removal from infiltrated water (pounds); and
• MTP_f = TP removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by

• filtration (underdrain) - $M_{TP_i} = 0.0000624\ (V_{inf_b} + V_{inf_s} + V_U)\ EMC_{TP}$
• infiltration - $M_{TP_i} = 0.0000624\ V_{WQ} \ EMC_{TP}$

where

• EMCTP is the event mean TP concentration in runoff water entering the BMP (milligrams per liter).

The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.

The filtration credit for TP in a swale with underdrains assumes removal rates based on the soil media mix used and the presence or absence of amendments. Soil mixes with more than 30 mg/kg phosphorus (P) content are likely to leach phosphorus and do not qualify for a water quality credit. If the soil phosphorus concentration is less than 30 mg/kg, the mass of phosphorus removed through filtration, in pounds, is given by

$M_{TP_f} = 0.0000624\ (V_{total} - (V_{inf_b} + V_{inf_s} + V_U))\ EMC_{TP}\ R_{TP}$

Information: Soil mixes C and D are assumed to contain less than 30 mg/kg of phosphorus and therefore do not require testing

Again, assuming the phosphorus content in the media is less than 30 milligrams per kilogram, the removal efficiency (RTP) provided in the Stormwater Manual is a function of the fraction of phosphorus that is in particulate or dissolved form, the depth of the media, and the presence or absence of soil amendments. For the purpose of calculating credits it can be assumed that TP in storm water runoff consists of 55 percent particulate phosphorus (PP) and 45 percent dissolved phosphorus (DP). The removal efficiency for particulate phosphorus is 80 percent. The removal efficiency for dissolved phosphorus is 20 percent if the media depth is 2 feet or greater. The efficiency decreases by 1 percent for each 0.1 foot decrease in media thickness below 2 feet. If a soil amendment is added to the BMP design, an additional 40 percent credit is applied to dissolved phosphorus. Thus, the overall removal efficiency, (RTP), expressed as a percent removal of total phosphorus, is given by

$R_{TP} = (0.8 * 0.55) + (0.45 * ((0.2 * (D_{MU_{max=2}})/2) + 0.40_{if amendment is used})) * 100$

where

• the first term on the right side of the equation represents the removal of particulate phosphorus;
• the second term on the right side of the equation represents the removal of dissolved phosphorus; and
• DMUmax=2 = the media depth above the underdrain, up to a maximum of 2 feet.

## Methods for calculating credits

This section provides specific information on generating and calculating credits from swale BMPS for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

### Credits based on models

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for dry swales. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

1. Model name and version
2. Date of analysis
3. Person or organization conducting analysis
4. Detailed summary of input data
5. Calibration and verification information
6. Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for constructed basin BMPs. In using this table to identify models appropriate for constructed ponds and wetlands, use the sort arrow on the table and sort by Constructed Basin BMPs. Models identified with an X may be appropriate for using with constructed basins.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse Manu-
factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X X No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X X X No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X X X No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X Yes Yes Yes
i-Tree-Hydro X No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets No No Yes Computes volume reduction for trees, only.
LSPC X X X Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator X Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet X No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X X X X Yes Yes Yes
PCSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X [ No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA X No No Yes
SELECT X X X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
SHSAM X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X X X Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X Yes Yes Yes
XPSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.

### MIDS Calculator

Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach.

### Credits Based on Reported Literature Values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or event mean concentration (EMC) of the dry swale. A more detailed explanation of the differences between mass load reductions and EMC reductions can be found here.

Designers may use the pollutant reduction values reported here or may research values from other databases and published literature.

Designers who opt for this approach should:

• Select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database.
• Select a pollutant removal reduction from literature that studied a dry swale device with site characteristics and climate similar to the device being considered for credits.
• When using data from an individual study, review the article to determine that the design principles of the studied dry swale are close to the design recommendations for Minnesota, as described here, and/or by a local permitting agency.
• Preference should be given to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed dry swale, considering such conditions as watershed characteristics, swale sizing, and climate factors.

• Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon
• Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations
• Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics
• Applicable to Filters, Wet Ponds, Porous Pavements, Soakage Trenches, Flow through Stormwater Planters, Infiltration Stormwater Planters, Vegetated Infiltration Basins, Swales, and Treatment Wetlands
• The Illinois Green Infrastructure Study
• Figure ES-1 summarizes BMP effectiveness
• Provides values for TN, TSS, peak flows / runoff volumes
• Applicable to Permeable Pavements, Constructed Wetlands, Infiltration, Detention, Filtration, and Green Roofs
• New Hampshire Stormwater Manual
• Volume 2, Appendix B summarizes BMP effectiveness
• Provides values for TSS, TN, and TP removal
• Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
• BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
• Appendix B provides pollutant removal performance curves
• Provides values for TP, TSS, and Zn
• Pollutant removal broken down according to land use
• Applicable to Infiltration Trench, Infiltration Basin, Bioretention, Grass Swale, Wet Pond, and Porous Pavement
• Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices: Final Report
• Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
• Applicable to Wet Basins, Stormwater Wetlands, Bioretention Filter, Sand Filter, Infiltration Trench, and Filter Strips/Grass Swales

### Credits Based on Field Monitoring

Field monitoring may be used to calculate stormwater credits in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

1. Establish the objectives and goals of the monitoring.
1. Which pollutants will be measured?
2. Will the monitoring study the performance of a single BMP or multiple BMPs?
3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
4. Will the results be compared to other BMP performance studies?
5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
2. Plan the field activities. Field considerations include:
1. Equipment selection and placement
2. Sampling protocols including selection, storage, delivery to the laboratory
3. Laboratory services
4. Health and Safety plans for field personnel
5. Record keeping protocols and forms
6. Quality control and quality assurance protocols
3. Execute the field monitoring
4. Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

• Chapter 2: Designing the Program
• Chapters 3 & 4: Methods and Equipment
• Chapters 5 & 6: Implementation, Data Management, Evaluation and Reporting
• Chapter 7: BMP Performance Analysis
• Chapters 8, 9, & 10: LID Monitoring
Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes the following chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP:

• Chapter 4: Stormwater Characterization
• 4.2: General Characteristics and Pollutant Sources
• 4.3: Sources of Stormwater Quality data
• Chapter 8: Performance Evaluation
• 8.1: Methodology Options
• 8.5: Evaluation of Quality Performance for Individual BMPs
• 8.6: Overall Hydrologic and Water Quality Performance Evaluation
• Chapter 10: Hydrologic Evaluation
• 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices.

In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The information contained in this White Paper would be of use to those considering the monitoring of a manufactured BMP. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).

Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include:

• Chapter 4: Monitoring Methods and Equipment
• Chapter 5: Analytical Methods and Laboratory Selection
• Chapter 6: Monitoring Site Selection
• Chapter 8: Equipment Installation and Maintenance
• Chapter 10: Pre-Storm Preparation
• Chapter 11: Sample Collection and Handling
• Chapter 12: Quality Assurance / Quality Control
• Chapter 13: Laboratory Reports and Data Review
• Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice, involving:

• Level 1: Visual Inspection
• Level 2: Capacity Testing
• Level 3: Synthetic Runoff Testing
• Level 4: Monitoring
• Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

## Other pollutants

According to the International BMP Database, studies have shown dry swales are effective at reducing concentration of other pollutants as well including solids, bacteria, metals, and nutrients. This database provides an overview of BMP performance in relation to various pollutant categories and constituents that were monitored in BMP studies within the database. The report notes that effectiveness and range of unit treatment processes can vary greatly depending on BMP design and location. Table 3-4 shows a list of the constituents and associated pollutant category for the monitored “media filters” data. The constituents shown all had data representing decreases in effluent pollutant loads for the median of the data points and the 95% confidence interval about the median. If dry swale design utilizes a bioretention base, additional pollutant removals may be applicable as well (For more information see the bioretention credit article ). Pollutant removal percentages for dry swale BMPs can also be found on the WIKI page.

Pollutant Category Constituent Treatment Capabilities

(Low = < 30%; Medium = 30-65%;

High = 65 -100%)
Metals1 Cd, Cr, Cu, Zn Medium
As2,Fe, Ni, Pb Medium/High
Nutrients Total Nitrogen, TKN Low
Bacteria Fecal Coliform, E. coli Low
Organics Medium

1Results are for total metals only
2Information on As was found only in the International Stormwater Database where removal was found to be low

• Ahearn, Dylan, and Richard Tveten. "Legacy LID: Stormwater Treatment in Unimproved Embankments along Highway Shoulders in Western Washington." In Proceedings of the 2008 International Low Impact Development (LID) Conference, pp. 16-19. 2008.
• Barrett, Michael E., Michael Vincent Keblin, Patrick M. Walsh, Joseph F. Malina Jr, and Randall J. Charbeneau. Evaluation of the performance of permanent runoff controls: summary and conclusions. No. TX-99/2954-3F,. 1998.
• Barrett, Michael E., Patrick M. Walsh, Joseph F. Malina Jr, and Randall J. Charbeneau. "Performance of vegetative controls for treating highway runoff." Journal of environmental engineering 124, no. 11 (1998): 1121-1128.
• Barrett, Michael, Anna Lantin, and Steve Austrheim-Smith. "Storm water pollutant removal in roadside vegetated buffer strips." Transportation Research Record: Journal of the Transportation Research Board 1890, no. 1 (2004): 129-140.
• Bureau of Environmental Services. 2006. Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon. Bureau of Environmental Services, Portland, Oregon.
• California Stormwater Quality Association. "California Stormwater BMP Handbook-New Development and Redevelopment." California Stormwater Quality Association, Menlo Park, CA (2003).
• Caltrans. 2004. BMP Retrofit Pilot Program Final Report, Report No., CTSW-RT-01-050. Division of Environmental Analysis, California Dept. of Transportation, Sacramento, CA
• CDM Smith. 2012. Omaha Regional Stormwater Design Manual Chapter 8 Stormwater Best Management Practices. Kansas City, MO.
• Dorman, M. E., H. Hartigan, F. Johnson, and B. Maestri. Retention, detention, and overland flow for pollutant removal from highway stormwater runoff: interim guidelines for management measures. Final report, September 1985-June 1987. No. PB-89-133292/XAB.
• Consultants, Geosyntec, and Wright Water Engineers. "Urban stormwater BMP performance monitoring." (2002).
• Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
• Gulliver, J. S., A. J. Erickson, and PTe Weiss. "Stormwater treatment: Assessment and maintenance." University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. http://stormwaterbook. safl. umn. edu (2010).
• Guo, James CY, Gerald E. Blackler, T. Andrew Earles, and Ken MacKenzie. "Incentive index developed to evaluate storm-water low-impact designs." Journal of Environmental Engineering 136, no. 12 (2010): 1341-1346.
• Harper, Harvey H. "Effects of stormwater management systems on groundwater quality." FDEP Project# WM190. Florida Department of Environmental Regulation, Tallahassee, FL (1988).
• Jaffe, et. al. 2010. The Illinois Green Infrastructure Study. Prepared by the University of Illinois at Chicago, Chicago Metropolitan Agency for Planning, Center for Neighborhood Technology, Illinois-Indiana Sea Grant.
• Jurries, Dennis. "Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal." Quality, S. o. OD o. E.(Ed.).
• Kearfott, Pamela J., Michael E. Barrett, and Joseph F. Malina. Stormwater quality documentation of roadside shoulders borrow ditches. Center for Research in Water Resources, University of Texas at Austin, 2005.
• Kim, Yun Ki, and Seung Rae Lee. "Field infiltration characteristics of natural rainfall in compacted roadside slopes." Journal of geotechnical and geoenvironmental engineering 136, no. 1 (2009): 248-252.
• Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
• New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B. Concord, NH.
• Transportation Officials, Oregon State University. Dept. of Civil, Environmental Engineering, University of Florida. Dept. of Environmental Engineering Sciences, GeoSyntec Consultants, and Low Impact Development Center, Inc. Evaluation of Best Management Practices for Highway Runoff Control. No. 565. Transportation Research Board, 2006.
• State of California, Department of Transportation. 2013. Caltrans Stormwater Monitoring Guidance Manual. Sacramento, CA.
• TetraTech. 2008. BMP Performance Analysis. Prepared for US EPA Region 1, Boston, MA.
• Water Environment Federation. 2014. Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices. A White Paper by the National Stormwater Testing and Evaluation of Products and Practices (STEPP) Workgroup Steering Committee.
• WEF, ASCE/EWRI. 2012. Design of Urban Stormwater Controls, WEF Manual of Practice No. 23, ASCE/EWRI Manuals and Reports on Engineering Practice No. 87. Prepared by the Design of Urban Stormwater Controls Task Forces of the Water Environment Federation and the American Society of Civil Engineers/Environmental & Water Resources Institute.
• Weiss, Peter T., John S. Gulliver, and Andrew J. Erickson. "The Cost and Effectiveness of Stormwater Management Practices Final Report." (2005).

# Cost considerations for dry swale (grass swale)

Information: The following table is generally applicable to filtration BMPs and not specifically to swales. Some items may not be applicable.

Cost estimates that site planners can use to compare the relative construction and maintenance costs for structural best management practices are excellent for purposes of comparison. However, it is recommended that construction and maintenance budgets should be based on site specific information. Utilizing the table below and cost estimation worksheets will allow designers to more accurately estimate the cost of a filtration BMP.

Cost components for filtration practices.

Implementation Stage Primary Cost Components Basic Cost Estimate Other Considerations
Site Preparation Tree & plant protection Protection Cost ($/acre) x Affected Area (acre) Removal of existing structures, topsoil removal and stockpiling Topsoil salvage Salvage cost ($/acre) x Affected Area (acre)
Clearing & grubbing Clearing Cost ($/acre) x Affected Area (acre) Site Formation Excavation / grading X-ft Depth Excavation Cost ($/acre) x Area (acre) Soil & rock fill Hauling material material, tunneling
Hauling material offsite Excavation Cost x (% of Material to be hauled away)
Structural Components Under-drains Under-drain cost ($/lineal foot) x length of device Pipes, catchbasins, manholes, valves, vaults Vault structure (for media filters) ($/structure)
Media (for media filters) Media cost ($/cubic yard) X filter volume (cubic yard) Inlet structure (for vegetative filters ($/structure)
Outlet structure (for vegetative filters) ($/structure) Site Restoration Filter strip Sod cost ($/square foot) x filter strip area Tree protection, soil amendments, seed bed preparation, trails
Soil preparation Topsoil or amendment cost ($/acre) x Area (acre) Seeding Seeding Cost ($/acre) x Seeded Area (acre)
Planting / transplanting Planting Cost ($/acre) x Planted Area (acre) Annual Operation, Maintenance, and Inspection Debris removal Removal Cost ($/acre) x Area (acre) x Frequency (2x / 1yr) Vegetation maintenance, cleaning of structures
Sediment removal Removal Cost ($/acre) x Area (acre) x Frequency (1x / 5yr) Gate / valve operation Operation Cost ($) x Operation Frequency (2x / 1 yr)
Inspection Inspection Cost ($) x Inspection Frequency (6x / 1 yr) Mowing (for some vegetative filters) Mowing Cost ($) x Mowing Frequency (4x / 1 yr)

# Case studies for dry swale (grass swale)

This page provides one detailed case study and several short summaries of case studies for dry swales.

## International Plaza Drive Improvements

Project Summary – International Plaza Drive Improvements
Location: City of St. Ann, Missouri
Owners: City of St. Ann, Missouri
Designer: HR Green
Year of Completion: 2012
Design Features: Dry swale, rocked inlets and check dams
Total Drainage Area: 6-acres
Total Construction Cost: $770,500 Views of International Plaza Drive and pavement failure and repairs. HR Green worked with the City of St. Ann, Missouri to design a point of entry into the City. The pavement at this location was in disrepair. Evident ponding of stormwater occurred at the location and was one of the reasons for the pavement failure. This project is generally located south of Interstate 70, between Cypress Road and Ashby Road within the city limits of St. Ann, Missouri. International Plaza Drive is a designated collector roadway , running in the east-west direction, serving as a means to move traffic from local streets to nearby arterial roads. Improvements for this project include the complete reconstruction of the roadway with new (PCC) pavement and integral curb, striped bike lanes, and a meandering concrete sidewalk/pathway. To address the requirements of the regulatory agencies with oversight, namely the Metropolitan St. Louis Sewer District (MSD), the City utilized engineered structures to address the water quality needs of the project. This approach minimized maintenance costs and maintenance activities associated with the use of a . The project was designed in 2011 and constructed in 2012. ### Design consideration summary Typical dry swale cross section. Click on image for better resolution. This project road design inverted the proposed roadway section and directed all of the roadway’s stormwater runoff into the center median. The existing curb inlets and appurtenances were no longer needed and were removed. The new drainage pattern results in stormwater runoff to four that receive flow from a total of 6.01 acres, with approximately forty to fifty percent of that total area being impervious surface. The project extent is 2.20-acres Design general performance criteria for stormwater management for this project was based on Metropolitan St. Louis Sewer District (MSD) Section 4.080. Stormwater management was required because the redevelopment area exceeded 1 acre of disturbed area. The dry swale drainage areas outside of the pavement section include an area inlet to serve as an overflow structure to accept flow from the typical 15-year, 20-min design storm. or were installed within the dry swale areas such that the maximum water elevation in these areas will be three (3) inches below the adjacent top of curb; any flow in excess of this volume will flow over the ditch checks and into the downstream area inlet. Depths of flow (15-year, 20-min) above the earthen check dams were calculated in order to verify that stormwater would not overtop the adjacent top of curb and onto the roadway. Berm placement is approximately every 200 to 400 feet along the dry swale and is constructed with a 1:1 slope and a two-foot width at the top. Soils at the site were Hydrologic Soil Group C and D. View of an installed dry swale during a rain event and during dry conditions. The design intent was to follow all applicable rules and regulations, but circumstances within the design process required deviations from the guidelines, as noted below: • Dry swale Area Side Slopes, 2:1 • Though 3:1 side slopes are preferred within dry swale areas, deviation from this preference was required in order to accommodate the total (WQV) for the entire project while staying within existing right-of-way limits, which is a requirement of the funding mechanism. Multiple options were investigated with respect to the total WQV, all which eventually resulted in adjusting the dry swale side slopes within the project area to 2:1. On a similar typical section for dry swales, the Maryland Stormwater Design Manual (2000) notes that 2:1 or flatter side slopes are to be used; the proposed side slopes do not exceed this slope requirement. • To provide a means of protecting the side slopes from erosion due to concentrated stormwater runoff, a curb-cut detail with rock lining has been provided (see cross-section image on right) in the construction plans and this report for review. The intent of the rock lining is to slow the concentrated flow from the gutter to a velocity that will not erode the side slopes. While these dry swales areas were sized to hold flows generated from a 1.14-in rainfall event, inevitably more intense storms will fall within the region. In these situations, the proposed area inlets installed at the downstream ends of the dry swale areas will capture excess stormwater and will be routed downstream to the existing system, consisting of intakes and pipes. Some design complications were realized on this project as compliance with hydraulic grade line (HGL) requirements was difficult to attain in all areas of the storm sewer design. As a minimum, MSD notes that “…the [HGL] at any inlet or storm manhole shall not be higher than two (2) feet below the inlet sill or top of manhole.” However, due to the assumed HGL starting point within the Hydraflow model (pipe enters two reaches downstream of the connection point to the existing system), there are instances in the design where this requirement was unable to be attained. Through trial and error, the pipe sizes and flowlines of the proposed storm sewer system were tweaked in an attempt to correct this problem to no avail. Since the differential flow for the project is essentially non-existent, it is assumed that the existing system currently operates under this condition as well. ### Applicable Costs The cost to complete the roadway, bike and pedestrian sidewalks and associated items, signals, lighting, signing and striping, and landscaping and streetscaping was$770,428.

Summary of costs for dry swale case study - International Plaza Drive Improvements.

Description Unit Quantity Unit cost (dollars) Extension (dollars)
Furnish/Place Type 1 ditch liner Cubic yard 3.2 80 257.78
Ketucky Bluegrass sodding Cubic yard 1226.3 6 7.357.87
Bermuda or Zoysia sodding Square yard 1963.6 7 13,744.87
Mulch Cubic yard 25.7 30 771.40
3/8 inch gravel Cubicyard 268.5 40 10,738.73
3/4 inch gravel Cubic yard 268.5 40 10,783.73
Sand Cubic yard 268.5 25 6,711.71
Bioretention soil media Cubic yard 1073.9 50 53,693.67
MSD Type 4 geotextile Square yard 1000.7 11 11007.88
Plugs Each 1898 5 9490.00
Shrubs (#5 container or 1-gallon container) Each 30 90 2.700
Trees (2.5-inch caliper or 1-gallon container) Each 3 325 975.00

### Maintenance

The ongoing maintenance of the dry swale was taken on by the City of St. Ann (link here for information on operation and maintenance of dry swales). Maintenance of the dry swale included the following.

• The dry swale area is intended to temporarily store stormwater and let it slowly soak into the ground. If standing water remains for longer than 3 days, the top of the planting bed has likely clogged. Clogging can be repaired by raking the surface, soil aeration (poking holes in the top layer of soil), or replacing the top 2-3 inches of planting soil and mulch.
• The plant species in the dry swale area were specifically selected and for this project included, but were not limited to, Fox Sedge, Soft Rush, Little Bluestem, Marsh Milkweed, Horsetail, Copper Iris, and New England Aster. These plants typically won’t require watering or fertilizing beyond their first year. Watering during drought periods will help plant appearance. As needed or desired, the City will replant the dry swale areas with compatible MSD approved plant species. Link here for information on plants for swales in Minnesota.
• The City will trim plants as needed to give them the “shape” they like. Between November and March, plants will be cut near the ground surface. The City will leave seed heads through winter for bird and wildlife watching.
• Weeds will be pulled and spot use of herbicides will be implemented, but blanket herbicide application is not recommended. The insects and birds living around a healthy dry swale area keep mosquito populations in check.
• Rainwater flowing into the basin may carry trash and debris. Inlet pipes and “pop-ups” will be checked and cleaned as needed. Trash, debris, leaf litter, and sediment will be removed regularly to ensure that the dry swale area is not clogged and does not fill-in. Trash, debris, and sediment collected in the dry swale area will be disposed with other household waste. Material suspected to be polluted by oil, old paint chips (lead), or other chemicals that could potentially be hazardous should be properly tested and disposed in accordance with state and federal hazardous waste regulations.
• When cleaning a BMP, standing “clear, unpolluted water” can be decanted and discharged to the storm system. Water that has become turbid during cleaning should be either, (1) pumped and hauled to an acceptable wastewater disposal facility or (2) treated by filtration, such as pumped through a bag filter, and discharged to the sanitary sewer system. A special discharge permit from MSD is not required for discharging to the sanitary system if the total volume is less than 10,000 gallons. The flow rate pumped into the sanitary system shall not exceed 50 gallons per minute (gpm).

An Annual BMP Maintenance Report is submitted to MSD by the City of St. Ann before March 31st. A MSD inspector periodically inspects the BMP.

## Links to and summaries of other case studies

Below are summaries of additional case studies for dry swales..

### Gene Green Beltway 8 Park - Houston, Texas

A project incorporating bioswales into a local park located east of downtown Houston, Texas. The document describing the project provides a summary of the project, including design, expected pollutant removal, LEED features. Includes detail drawings; soil mixes used; information on underdrains, curbs, and inlets; plant materials; landscaping; maintenance; cost information; photos and schematics. Source: Houston Land Water Sustainability Forum.

### North Carolina

This PowerPoint presentation, Low Impact Development in the Transportation Environment: Lessons Learned in North Carolina (Winston, 2014), summarizes several case studies utilizing swales. The focus is on different design options and features and their effect on swale performance. Includes some design information, performance results, and numerous photos, schematics, and other images.

### Springfield, Oregon

Three bioswales were constructed at the Lane Transit District (LTD) Springfield Station, Springfield, Oregon. Two swales function as major elements and one as a minor element. This case study provides a minimum of information on design but provides a summary of monitoring conducted at the site. Link.

### Oregon Department of Environmental Quality

Biofilters For Stormwater Discharge Pollution Removal - Excerpt from this document: "This document is an attempt to compile the best available information on the design and use of biofilters (bioswales, vegetated filter strips, and constructed wetlands) so that those sites that may have an application of one or the other of these vegetated filtering systems will have information to make the best decision on the design, construction, implementation, and maintenance of these Best Management Practices. It is not a design manual but a practical, based on experience and knowledge of sites that implemented these BMPs, useful information on what works and does not work when designing, constructing, and operating them."

### Cornell University

"A bioswale was installed as part of the Nevin Welcome Center building project in 2010. The project, which includes a green roof, and several other sustainable features, received LEED Gold from the U.S. Building Council. The bioswale was designed to slow and clean storm water runoff from the parking lot while providing an attractive garden landscape, which is more ecologically-minded than a traditional storm drain system. The garden is used as a teaching landscape to showcase the benefits and functions of a bioswale garden."

The study report for this project focuses on sustainable features of this project.

# External resources for dry swale (grass swale)

## Dry swales

### Case studies for dry swales

See section on Stormwater manuals

# References for dry swale (grass swale)

## References

• Brown, R.A. and Hunt, W.F. 2010. Impacts of construction activity on bioretention performance. Journal of Hydrologic Engineering. 15(6):386-394.
• Erickson, A.J., P.T. Weiss, and J.S. Gulliver. 2013. Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance. Springer Publishing, New York, NY, USA.
• Gulliver, J.S., A.J. Erickson, and P.T. Weiss (editors). 2010. Stormwater Treatment: Assessment and Maintenance. University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN.
• Hatt, B.E., Steinel, A., Deletic, A., and Fletcher, T.D. 2011. Retention of heavy metals by stormwater filtration systems: Breakthrough analysis. Water, Science, and Technology. 64(9):1913-1919.
• Hunt, W.F., Jarrett, A.R., Smith, J.T., and Sharkey, L.J. 2006. Evaluating bioretention hydrology and nutrient removal at three field sites in North Carolina. Journal of Irrigation and Drainage Engineering. 132(6):600-608.
• Lefevre, G.H., P.J. Novak, R.M. Hozalski. 2012. Fate of naphthalene in laboratory-scale bioretention cells: implications for sustainable stormwater management. Environmental Science and Technology 46(2):995-1002.
• Li, H. and Davis, A.P. 2008. Heavy metal capture and accumulation in bioretention media. Environmental Science & Technology. 42:5247-5253.
• Morgan, J.G., K.A. Paus, R.M. Hozalski and J.S. Gulliver. 2011. Sorption and Release of Dissolved Pollutants Via Bioretention Media. SAFL Project Report No. 559. September 2011.
• North Carolina Department of Environment and Natural Resources. 2009. NCDENR Stormwater BMP Manual – Section 14, Grassed Swale.
• O’Neill, S.W. and Davis, A.P. 2012. Water treatment residual as a bioretention amendment for phosphorus. I: Evaluation studies. Journal of Environmental Engineering. 138(3), 318-327.
• Virginia Department of Conservation and Recreation (VA DCR). 2011. Virginia DCR Stormwater Design Specification No. 3 – Grass Channels. Version 1.8, March 1, 2011. Division of Soil and Water Conservation. Richmond, VA.
• Virginia Department of Conservation and Recreation (VA DCR). 2011. Virginia DCR Stormwater Design Specification No. 10 – Dry Swales. Version 1.9, March 1, 2011. Division of Soil and Water Conservation. Richmond, VA.