Infiltration trench

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Infiltration Overview

Natural or constructed depressions located in permeable soils that capture, store and infiltrate the volume of stormwater runoff associated with a particular design event.

Photo of a Infiltration trench in Lino Lakes

Photo of a Infiltration trench in Stillwater

Infiltration trench articles

• Overview for Infiltration trench
• Types of InfiltratioInfiltration trench
• Design criteria for bInfiltration trench
• Construction specifications for Infiltration trench
• Operation and maintenance of Infiltration trench
• Cost-benefit considerations for Infiltration trench
• External resources for Infiltration trench
• References for Infiltration trench

Design Criteria:

• Contributing drainage area
• Underlying soil types
• Depth to the water table, bedrock or other impeding layer
• Proximity to buildings, drinking water supplies, Karst features, etc.
• Source of stormwater runoff

Benefits

• Reduces volume of stormwater runoff
• Increases ground water recharge
• Improves surface water quality
• Provides thermal benefits (e.g. to cold water fisheries)
• Mimics pre-development hydrology

Limitations

• Unusual construction considerations
• Potential for ground water contamination
• Tendency to lose effectiveness over time due to clogging – if not properly constructed or maintained
• Not recommended for areas with steep slopes
• May require landscaping: consideration should be given to periods on inundation and drought

Description

In general terms, infiltration systems can be described as natural or constructed depressions located in permeable soils that capture, store and infiltrate stormwater runoff within 48 hours. These depressions can be located at the surface of the ground (e.g. infiltration basin) or they can be designed as underground facilities (e.g. structural chamber or excavated pit filled with aggregate such as an infiltration trench). Typically, infiltration systems are designed with one or more pre-treatment facilities or they are designed as off-line facilities.

Warning: Infiltration systems should be located in permeable soils and a minimum 3-foot distance is REQUIRED from the bottom of the seasonally high water table, bedrock or other impeding layer per the Minnesota Pollution Control Agency Construction General Permit (CGP)]

Dry wells and Trenches should be designed to handle the smaller, more frequent rainfall events. Stormwater associated with the larger rainfall events should bypass these practices by a separate pipe or an overflow device. Infiltration basins and underground infiltration systems should be designed to handle both the water quality volume and as the water quantity volume.

Infiltration systems can be designed to address a number of stormwater management issues including: water quality, stormwater runoff reduction, flow attenuation, thermal impacts to cold water fisheries, and groundwater recharge.

Maintenance Requirements

• Establishment of native vegetation may require weeding, watering, pumping, replacement of plants and tree/shrub trimming
• Inspection and removal of sediment accumulation
• Street sweeping of impervious areas adjacent to infiltration practices
• Avoid the application of fertilizer or herbicide in or near infiltration practices

SITE FACTORS

Schematic of Start of Storm Event - Initial runoff & storage

Schematic of Duration of Storm Event - Storage & filtration/infiltration

Schematic of inflitration trench following storm event - Remaining storage drawdown

• Max Drainage Areas: Dry Well/ Trench/ Basin = 1/ 5/ 50 Acres
• Max. Site Slope = 20 percent
• Min. Depth to Bedrock = 3 feet
• Min. Depth to Seasonally High Water Table = 3 feet
• NRCS Soil Type *C & D soils have limited infiltration ability but can be used to match predevelopment conditions = A,B,C*,& D
• Freeze/ Thaw Suitability = Poor - Good
• Potential Hotspot Runoff = NO

MANAGEMENT SUITABILITY

• Water Quality (V_wq) = High
• Med. Channel Protection (V_cp)
• Overbank Flood Protection (Vp₁₀) = Low/Med.
• Extreme Flood Protection (Vp₁₀₀) = Low
• Recharge Volume (V_re) = High

MECHANISMS

• Screening/ Filtration
• Temperature Control
• Transpiration *if vegetated
• Soil Adsorption
• Biological/ Micro. Uptake

POLLUTION REMOVAL*

• Total Suspended Solids (TSS) = 100 percent
• Nutrients - Total Phosphorus/ = 100 percent
• Total Nitrogen = 100 percent
• Metals - Cadmium, Copper, Lead, and Zinc = 100 percent
• Pathogens - Coliform, Streptococci, E. Coli = 100 percent
• Toxins - Hydrocarbon = 100 percent

Pollution removal addresses only the impact on surface water, as there could be some transfer of pollution to the soil layer and groundwater.

Suitability

General

Stormwater infiltration practices capture and temporarily store stormwater before allowing it to infiltrate into the soil. Design variants include; the infiltration basin, the infiltration trench, the dry well and the underground infiltration system. As the stormwater penetrates the underlying soil, chemical, biological and physical processes remove pollutants and delay peak stormwater flows.

Infiltration practices are applicable to sites with naturally permeable soils and a suitable distance to the seasonally high groundwater table, bedrock or other impermeable layer. They may be used in residential and other urban settings where elevated runoff volumes, pollutant loads, and runoff temperatures are a concern. In applications where the stormwater runoff has a particularly high pollutant load or where the soils have very high Infiltration rates, a significant amount of pre-treatment should be provided to protect the groundwater quality. Sources that include potential stormwater should not be introduced to Infiltration systems. Sources that include potential stormwater hotsposts (PSH) should not be introduced to Infiltration areas.

Function Within Stormwater Treatment Train

Infiltration practices may be located at the end of the treatment train or they can be designed as off-line configurations where the water quality volume is diverted to the 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
• Increase groundwater recharge
• Decrease runoff peak flow rates
• Decrease the volume of stormwater runoff
• Preserve base flow in streams
• Reduce thermal impacts of runoff.

MPCA Permit Applicability

One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA General Stormwater Permit for Construction Activity (MN R100001), commonly called the 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 one acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.

For regulatory purposes, infiltration practices fall under the “Infiltration / Filtration” category described in Part III.C.2 of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given as described in Part III.C.4. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases infiltration 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 stormwater pond design guidance:

REQUIRED: Indicates design standards stipulated by the MPCA Permit (or other consistently applicable regulations).

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

RECOMMENDED: Indicates design guidance that is helpful for infiltration performance but not critical to the design.

Of course, there are situations, particularly retrofit projects, in which an infiltration facility 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 also important to note that additional and potentially more stringent design requirements may apply for a particular infiltration facility, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Information: alert-caution

Of course, there are situations, particularly retrofit projects, in which an infiltration facility 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 also important to note that additional and potentially more stringent design requirements may apply for a particular infiltration facility, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Construction Details

CADD based details for pond systems are contained in the section on Computer-aided design and drafting (CAD/CADD) drawings. The following details, with specifications, have been created for infiltration systems:

• Infiltration Basin
• Infiltration Trench
• Subsurface Infiltration System
• Inlet/outlet Structures

Construction Specifications

Given that the construction of infiltration practices incorporates techniques or steps which may be considered non-standard, it is RECOMMENDED that the construction specifications include the following format and information:

Temporary Erosion Control

• Installation prior to site disturbance
• Catch basin/inlet protection
• Use of BMP as temporary sedimentation basin/

Excavation, Backfill and Grading

• Timing of grading of infiltration practices (relative to total site development)
• Use of low-impact, earth moving equipment
• Controls to ensure site is not over-excavated
• Restoration in the event of sediment accumulation during construction of practice
• Gravel backfill specifications
• Gravel filter specifications
• Filter fabric specifications
• Observation well specifications

Native Plants, Planting and Transplanting

• Site preparation of planting areas
• Timing of native seeding and native planting
• Weed control
• Watering of plant material

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

Construction Sequence Scheduling

• Temporary construction access
• Location of silt fence installation to protect BMPs and downgradient receiving waters
• Removal and storage of excavated material
• Installation of underground utilities
• Rough grading
• Seeding and mulching disturbed areas
• Road construction
• Final grading
• Site stabilization
• Installation of semi-permanent and permanent erosion control measures
• Silt fence removal (often forgotten but an essential step)

Construction Observation

• Adherence to construction documents
• Verification of physical site conditions
• Erosion control measures installed appropriately

Avoid excessive compaction

Warning: It is REQUIRED that in order to prevent soil compaction, the proposed infiltration area be staked off and marked during construction to prevent heavy equipment and traffic from traveling over it.

In addition, it is HIGHLY RECOMMENDED that the side walls of dry wells and infiltration trenches be roughened if they have been smeared by heavy equipment

Stabilize Vegetation Before and After Construction

Excessive sediment loadings can occur without the use of proper erosion and sediment control practices during the construction process.

Warning: It is REQUIRED that upland drainage areas be properly stabilized with a thick layer of vegetation, particularly immediately following construction, to reduce sediment loads.

Warning: If infiltration practices are in-place during construction activities, it is REQUIRED that sediment and runoff be kept away the infiltration area, such as with diversion berms and soil-stabilizing vegetation around the perimeter of the practice.

Correctly Install Filter Fabrics

Large tree roots should be trimmed flush with the sides of dry wells and infiltration trenches to prevent puncturing or tearing of the filter fabric during subsequent installation procedures. When laying out the geotextile, the width should include sufficient material to compensate for perimeter irregularities in the dry well or trench and for a 6-inch minimum top overlap. The filter fabric itself should be tucked under the sand layer on the bottom of the dry well of infiltration trench, and stones or other anchoring objects should be placed on the fabric at the trench sides to keep the excavation open during windy periods. Voids may occur between the fabric and the excavated sides of the practice. Natural soils should be placed in any voids to ensure fabric conformity to the excavation sides.

Carefully Finish Final Grading

Initial infiltration basin excavation should be carried to within 2 feet of the final elevation of the basin floor.

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

The final phase excavation should remove all accumulated sediment and be done by light tracked equipment to avoid compaction of the basin floor and provide a well-aerated, highly porous surface texture.

Keep infiltration practices “Off-line” until construction Is complete

Warning: It is REQUIRED that Ssediment and runoff be kept completely away from the infiltration area during construction. Thus, infiltration practices should never serve as Ssediment control devices during site construction.

It is HIGHLY RECOMMENDED that construction of Ssediment practices be suspended during snowmelt or rainfall, in order to prevent soil smearing, clumping, or compaction.

Establish Permanent Vegetation

• Establishing dense vegetation on the basin side slopes is HIGHLY RECOMMENDED, to reduce erosion and sloughing and
• Provide a natural means of maintaining relatively high Iinfiltration rates. Vegetative cover at inflow points to the {Glossary#B|basin]] is also HIGHLY RECOMMENDED to provide erosion protection and reduce sediment accumulation. The use of native grasses is RECOMMENDED for seeding primarily due to their adaptability to local climates and soil conditions.
• Inspections during construction are needed to ensure that the Iinfiltration practice is built in accordance with the approved design and 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 designer.

Post-Construction Operation and Maintenance

Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED.

Effective long-term operation of infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Some important post-construction maintenance considerations are provided below.

• A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority.
• Adequate access must be provided for all infiltration practices for inspection, maintenance, and landscaping upkeep, including appropriate equipment and vehicles.
• General iinfiltration trench maintenance activities and schedule are provided in the table below

Typical maintenance problems for infiltration trenches and basins.
Link to this table

Problem	Practices Applied To	Comments
Clogging, sediment deposition	Both	Key issue for infiltration practice. Requires vigilant inspection and maintenance.
Surface Vegetation	Both	Often important to maintain vigorous growth at the base of infiltration practices (basins). Important to restrict woody vegetation from the surface of infiltration trenches.
Erosion of contributing land or in channels leading to practice	Both	In these practices, it is important to monitor not only the practice itself, but also upland infiltration to minimize the sediment load.
Damage to filter fabric	Trench	Infrequent but important maintenance concern.
Scouring at Inlet	Both	Similar issues to Ponds. Need to promote non-erosive flows that are evenly distributed
Access Issues	Both	Similar issues to Ponds. Need access for inspection and maintenance.
Concrete Failure	Basins, if they include a riser structure	Similar issues to ponds and wetlands.
Problems with the Embankment	Basins	Similar issues to dry ponds.

Typical maintenance activities for infiltration trenches and infiltration basins.
Link to this table

Activity	Schedule
Replace pea gravel/topsoil and top surface filter fabric (when clogged).	As needed
Ensure that contributing area, practice and inlets are clear of debris. Ensure that the contributing area is stabilized. Remove sediment and oil/grease from pre-treatment devices, as well as overflow structures. Mow grass filter strips should be mowed as necessary. Remove grass clippings. Repair undercut and eroded areas at inflow and outflow structures	Monthly
Inspect pre-treatment devices and diversion structures for sediment build-up and structural damage. Remove trees that start to grow in the vicinity of the trench.	Semi-annual Inspection
Disc or otherwise aerate basin bottom. De-thatch basin bottom.	Annually
Scrape basin bottom and remove sediment. Restore original crosssection and infiltration rate. Seed or sod to restore ground cover.	Every 5 years
Perform total rehabilitation of the trench to maintain design storage capacity. Excavate trench walls to expose clean soil	Upon Failure

Construction and Maintenance Costs

The Integrated stormwater management section outlines a cost estimation method which site planners could use to compare the relative construction and maintenance costs for structural best management practices. These curves 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 the cost estimation worksheet, will allow designers to avoid over or under estimation of fixed costs.

The design infiltration rates

The table below lists the specific site components that are specific to infiltration practices. Not included in this table are those cost items that are common to all construction projects, such as mobilization, traffic control, erosion and sediment control, permitting, etc.

Design Procedure

The following steps outline a recommended design procedure for infiltration practices in compliance with the MPCA Permit for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.

Design Steps

Step 1. Make a preliminary judgment as to whether site conditions are appropriate for the use of an infiltration practice, and identify the function of the practice in the overall treatment system.

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

• Site drainage area (See the Summary of infiltration practices for given drainage areas table below)
• Site topography and slopes
• Soil infiltration capacity
• Regional or local depth to groundwater and bedrock
• Site location/ minimum setbacks
• Presence of active Karst

B. Determine how the infiltration practice will fit into the overall stormwater treatment system:

• Decide whether the infiltration practice 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 infiltration practice is most likely to be located.

{{:Summary of infiltration practices for given drainage areas}}

Step 2. Confirm design criteria and applicability.

A. Determine whether the infiltration practice must comply with the MPCA Permit.

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.

If the initial evaluation indicates that an infiltration practice would be a good BMP for the site, it is RECOMMENDED that a minimum of three soil borings or pits be dug (in the same location as the proposed infiltration practice) to verify soil types and infiltration capacity characteristics and to determine the depth to groundwater and bedrock.

It is RECOMMENDED that the minimum depth of the soil borings or pits be five feet below the bottom elevation of the proposed infiltration practice.

It is HIGHLY RECOMMENDED that soil profile descriptions be recorded and include the following information for each soil horizon or layer (Source: Site Evaluation for Stormwater Infiltration, Wisconsin Department of Natural Resources Conservation Practice Standards, 2004):

• Thickness, in inches or decimal feet
• Munsell soil color notation
• Soil mottle or redoximorphic feature color, abundance, size and contrast
• USDA soil textural class with rock fragment modifiers
• Soil structure, grade size and shape
• Soil consistency, root abundance and size
• Soil boundary
• Occurrence of saturated soil, impermeable layers/lenses, groundwater, bedrock or disturbed soil
• It is HIGHLY RECOMMENDED that the field verification be conducted by a qualified geotechnical professional.

Step 4. Compute runoff control volumes.

Calculate the Water Quality Volume (V_wq), Channel Protection Volume (Vcp), Overbank Flood Protection Volume (Vp₁₀), and the Extreme Flood Volume (Vp₁₀₀) (see Unified sizing criteria).

Warning: If the infiltration practice is being designed to meet the requirements of the MPCA Permit, the REQUIRED treatment volume is the water quality volume of ½ inch of runoff from the new impervious surfaces created from the project (or 1 inch for certain protected waterbodies)

If part of the overall V_wq is to be treated by other BMPs, subtract that portion from the V_wq to determine the part of the V_wq to be treated by the infiltration practice.

The design techniques in this section are meant to maximize the volume of stormwater being infiltrated. If the site layout and underlying soil conditions permit, a portion of the Channel Protection Volume (V_cp), Overbank Flood Protection Volume (Vp₁₀), and the Extreme Flood Volume (Vp₁₀₀) may also be managed in the infiltration practice (see Step 7).

Details on the Unified sizing criteria are found in the Unified sizing criteria section.

Step 5. Select design variant based on Physical Suitability Evaluation.

Once the Physical Suitability Evaluation is complete, it is HIGHLY RECOMMENDED that the designer apply the better site design principles in sizing and locating the infiltration practice(s) on the development site. Given the water quality volume and the drainage area, select the appropriate infiltration practice for the first iteration of the design process.

Note: Information collected during the site suitability evaluation (see Steps 1 and 3) should be used to explore the potential for multiple infiltration practices versus relying on a single infiltration facility. The use of smaller infiltration practices dispersed around a development is usually more sustainable than a single regional facility that is more likely to have maintenance and ground-water mounding problems (Source: Site Evaluation for Stormwater Infiltration, Wisconsin Department of Natural Resources Conservation Practice Standards, 2004).

Step 6. Size infiltration practice (Note: Steps 6, 7, 8 and 9 are iterative).

After following the steps outlined above, the designer will presumably know the location of naturally occurring permeable soils, the depth to the water table, bedrock or other impermeable layer, and the contributing drainage area. While the first step in sizing an infiltration practice is selecting the type of infiltration practice for the site, the basic design procedures are very similar.

Infiltration Location: Given the steps performed in the Physical Suitability Evaluation, identify the most suitable location for the infiltration practice.

Warning: Pre-treatment prior to infiltration is REQUIRED to remove total suspended solids and other pollutants associated with stormwater (see Step 9)

Infiltration Rates: If the infiltration rate is not measured, the Design infiltration rates table above provides infiltration rates for the design of infiltration practices. These infiltration rates represent the long-term infiltration capacity of a practice and are not meant to exhibit the capacity of the soils in the natural state. Select the design infiltration rate from the Design infiltration rates table based on the least permeable soil horizon within the first five feet below the bottom elevation of the proposed infiltration practice.

The infiltration capacity and existing hydrologic regime of natural basins are inheritably different than constructed practices and may not meet the General Permit requirements for constructed practices. In the event that a natural depression is being proposed to be used as an infiltration system, the design engineer must demonstrate the following information: infiltration capacity of the system under existing conditions (inches/hour), existing drawdown time for the high water level (HWL) and a natural overflow elevation. The design engineer should also demonstrate that operation of the natural depression under post-development conditions mimics the hydrology of the system under pre-development conditions.

If the infiltration rates are measured the tests shall be conducted at the proposed bottom elevation of the infiltration practice. If the infiltration rate is measured with a double-ring infiltrometer the requirements of ASTM D3385 shall be used for the field test.

The measured infiltration rate shall be divided by a correction factor selected from the table below. The correction factor adjusts the measured infiltration rates for the occurrence of less permeable soil horizons below the surface and the potential variability in the subsurface soil horizons throughout the infiltration site. This correction factor also accounts for the long-term infiltration capacity of the stormwater management facility.

Total correction factors Ddvided into measured infiltration rates

To select the correction factor from the Total correction factors Ddvided into measured infiltration rates table above, determine the ratio of the design infiltration rates for each location an infiltration measurement was performed. To determine this ratio, the design infiltration rate (the Design infiltration rates ) for the surface textural classification is divided by the design infiltration rate for the least permeable soil horizon. For example, a device with a loamy sand (0.8 inch/hour.) at the surface and least permeable layer of loam (0.3 inches/hour.) will have a design infiltration rate ratio of about 2.7 and thus a correction factor of 3.5. The depth of the least permeable soil horizon should be within 5 feet of the proposed bottom of the device or to the depth of a limiting layer. In this exercise, if an infiltration rate of 2.5 inches/hr is measured, the adjustment rate would be 0.71 inch/hour.

Depth: The depth of an infiltration practice is a function of the maximum drawdown time and the design infiltration rate. Given the assumed infiltration rate for the practice, determine the maximum depth as follows:

\(D = i x t\)

Where:

D = maximum depth of practice (inches)

i = infiltration rate (inches/hour)

t = maximum drawdown time (48 hours)

Effective Infiltration Area: Given the water quality volume (Vwq) and the maximum depth of the practice (D) calculate the effective infiltration area where the effective infiltration area is defined as the area of the facility that is used to infiltrate runoff and does not include the area used for site access, berms and/or pre-treatment. For above ground practices that are rectangular in nature (infiltration basins with 1V:3H side slopes or steeper)

\(A_i = V_w/D\)

Where:

A_i = effective infiltration area at the bottom of practice (ft²)

V_w = design volume (e.g. V_wq) (feet³)

D = maximum depth of practice (feet) Note: bottom of the infiltration practice must be at least three feet from the seasonally high ground-water table.

For above ground practices that have angular sides slopes (infiltration basins with sides slopes shallower that 1V:3H)

\(Ai = V_w/D\)

Where:

A_i = effective infiltration area at half the volume of the practice* (see figure below) (ft²)

V_w = design volume (e.g. Vwq) (feet³)

D = maximum depth of practice (feet) Note: bottom of the infiltration practice should be at least 3 feet from the seasonally high groundwater table.

Since there is potentially a significant amount of infiltration that could occur though the sides of the practice, the design engineer should take this surface area into consideration thereby potentially reducing the overall footprint of the stormwater infiltration practice.

For underground practices (e.g. infiltration trenches, dry wells, subsurface infiltration practices):

\(A_i = V_w/nD\)

Where:

A_i = effective infiltration area is the sum of the bottom area and the sides of the practice* (feet²)

V_w = design volume (e.g. V_wq) (feet³)

n = porosity of filter media (range of porosity values for sands and gravels: 0.25 to 0.5)

D = maximum depth of practice (feet) Note: maximum of 12 feet, and separated by at least 3 feet from seasonally high ground-water table

Since underground facilities have potentially more surface area in contact with permeable soils, these practices should take these areas into consideration. Only that portion of the sides that is in contact with naturally permeable material should be used in calculating the effective infiltration area of the practice.

For subsurface infiltration practices, use the procedure described above or technique recommended by manufacturer and approved by the local or state authority.

Volume: The preliminary volume of the infiltration practice is determined by multiplying the average basin area by the depth of the practice.

The total storage volume for infiltration basin and underground infiltration systems is:

\(V = A x D\)

Where:

V = Design volume for infiltration basin and underground infiltration system (ft³)

A = average basin area (square feet)

D = depth of practice (feet)

For those practices that do not involve a media filter (e.g. infiltration basin and underground infiltration systems) this volume represents the total storage volume (design volume) of the practice. For those practices which do involve a media filter (e.g. infiltration trenches and dry wells) this volume represents the void space and the total storage volume will be greater. The following formula can be used to determine the total storage volume (design volume):

The total storage volume for infiltration trenches and dry wells

\(V_t = A x n x D_i\)

Where:

V_t = Design volume for infiltration trenches and dry wells (feet³)

A = average basin area (square feet)

n = porosity of filter media (range of porosity values for sands and gravels: 0.25 to 0.5)

D = depth of practice (feet)

Step 7. Size outlet structure and/or flow diversion structure, if needed (Note: Steps 6, 7, 8 and 9 are iterative).

It is HIGHLY RECOMMENDED that the outlet for the infiltration practice shall safely convey stormwater using all of the following mechanisms (Infiltration Basin, Wisconsin Department of Natural Resources Conservation Practice Standard, 10/04).

Drawdown valve: infiltration systems may be designed with a drawdown valve for the removal of standing water for maintenance and winter diversion.

Emergency spillway:

Warning: A means to release discharge in excess of the infiltration volume safely into the downstream stormwater conveyance system is REQUIRED.

Freeboard: It is HIGHLY RECOMMENDED that two feet of freeboard be provided from the 100-year flood elevation of the infiltration practice to the lowest basement floor elevation of residential, commercial, industrial and institutional buildings located adjacent to the BMP, unless local requirements recommend otherwise.

Drop Structure: Infiltration trenches or subsurface infiltration systems may be designed with a drop structure sized to handle the overflow. This additional volume of stormwater may be directed into the existing stormwater system or it may be diverted to a downstream BMP

Step 8. Perform ground-water mounding analysis (Note: Steps 6, 7, 8 and 9 are iterative).

Ground water 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 infiltraand seasonally saturated soils (or from bedrock) is required (5 feet RECOMMENDED) to maintain the hydraulic capacity of the practice and provide adequate water quality treatment. A groundwater mounding analysis is RECOMMENDED to verify this separation for infiltration practices.

The most widely known and accepted analytical methods to solve for groundwater mounding are based on the work by Hantush (1967) and Glover (1960). The maximum groundwater mounding potential should be determined through the use of available analytical and numerical methods. Detailed groundwater mounding analysis should be conducted by a trained hydrogeologist or equivalent as part of the site design procedure.

Step 9. Determine pre-treatment volume and design pre-treatment measures (Note: Steps 6, 7, 8 and 9 are iterative).

See the section on pre-treatment earlier in this section for specific pre-treatment design guidance

Step 10. Check volume, peak discharge rates and period of inundation against State, local and watershed organization requirements (Note: Steps 6, 7, 8 and 9 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, channel protection, overbank flood and extreme storm.

Perform hand calculations or model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site (see also the section on stormwater modeling). This includes defining the parameters of the infiltration practice defined above: elevation and area (defines the storage volume), infiltration rate and method of application (effective infiltration 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 (back to Step 5).

The following items are specifically REQUIRED by the MPCA Permit:

Warning: Volume: Infiltration or filtration systems shall be sufficient to infiltrate or filter a water quality volume of ½ inch of runoff from the new impervious surfaces created by the project (or 1 inch for certain protected waterbodies). If this criterion is not met, increase the storage volume of the infiltration practice or treat excess water quality volume (Vwq) in an upstream or downstream BMP (see Step 5).

Warning: Peak Discharge Rates: Since most infiltration systems are not designed for quantity control they generally do not have peak discharge limits. However outflow must be limited such that erosion does not occur down gradient.

Warning: Period of Inundation: Infiltration practices 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. 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. The period of inundation is defined as the time from the high water level in the practice to 3 to 6 inches above the bottom of the facility. It is assumed that this range is less than 1/5 the bounce in the infiltration practice.

Other design requirements may apply to a particular site. The applicant should confirm local design criteria and applicability (see Step 3).

Step 11. Prepare Vegetation and Landscaping Plan.

A landscaping plan for an infiltration basin or trench should be prepared to indicate how the enhanced swale system will be stabilized and established with vegetation. Landscape design should specify proper grass species and wetland plants based on specific site, soils and hydric conditions present along the channel. Further information on plant selection and use occurs in the Minnesota plant lists section.

Step 12. Prepare Operation and Maintenance (O&M) Plan.

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

Step 13. Prepare Cost Estimate.

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

Links to Other Manuals

Georgia Stormwater Management Manual 2001. Atlanta Regional Commission.

Vermont Stormwater Management Manual. 2002.

Wisconsin DNR Site Evaluation for Stormwater Infiltration Conservation Practice Standard 1002 2004.

Wisconsin DNR Infiltration Basin Conservation Practice Standard 1003 2004.

References

ASTM standard D2487-00 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)

Bouwer, H. and R. C. Rice. 1989. Effect of Water Depth in Ground-water Recharge Basins on Infiltration. Journal of Irrigation and Drainage Engineering, Vol. 115, No. 4, pp. 556-567.

Georgia Stormwater Management Manual. 2001.

Glover, R., 1960. Mathematical Derivations as Pertain to Ground-water Recharge. Report CER60REG70. Agricultural Research Service, USDA, Fort Collins, Colorado. 81 pp.

Hantush, M., 1967. Use of Soil Texture, Bulk Density and Slope of water Retention Curve to Predict Saturated Hydraulic Conductivity. Water Resources Research, 3(1): 227-234.

Rawls, W., D. Brakensiek, and K. Saxton, 1982. Estimation of Soil Water Properties, Transactions of the American Society of Agricultural Engineers. Vol. 25, No. 5, pp. 1316 – 1320 and 1328.

Rawls, W., D. Giminez, and R. Grossman, 1998. Use of Soil Texture, Bulk Density and Slope of water Retention Curve to Predict Saturated Hydraulic Conductivity, ASAE. Vol. 41(4), pp. 983 – 988.

Metropolitan Council Urban Small Sites Best Management Practice Manual. 2001.

Minnesota Pollution Control Agency Stormwater Best Management Practices Manual. 2000.

Natural Resources Conservation Service, 1986. Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55).

South Washington Watershed District (SWWD), 2005. 2004 Infiltration Monitoring Program Final Report. Emmons and Olivier Resources, Inc.

>United States Department of Agriculture Natural Resources Conservation Service. TR-55, Urban Hydrology for Small Watersheds. Washington D.C., 1975.

U.S. Department of Agriculture, Natural Resources Conservation Service, 2005. National Soil Survey Handbook, title 430-VI. (Online) Available: http://soils.usda.gov/technical/handbook/.

U.S. Environmental Protection Agency, When are Storm Water Discharges Regulated as Class V Wells, June 2003

University of Wisconsin – Extension Wisconsin Storm Water Manual: Technical Design Guidelines for Storm Water Management Practices. 2000.

Winer, R. 2000. National Pollutant Removal Performance Database.

Wisconsin Department of Natural Resources Site Evaluation for Stormwater Infiltration Conservation Practice Standard 1002. 2004.

Wisconsin Department of Natural Resources Infiltration Basin Conservation Practice Standard 1003. 2004.

Wisconsin Department of Natural Resources. Personal conversation with Roger Bannerman. July 2005.