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.
NOTE: The methodology described on this page can also be used to calculated credits for high-gradient stormwater step-pool.
| TSS | TP | PP | DP | TN | Metals | Bacteria | Hydrocarbons |
|---|---|---|---|---|---|---|---|
| 68 | link to table | link to table | link to table | 35 | 80 | 0 | 80 |
| TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen | |||||||
| NOTE: removal efficiencies are 100 percent for water that is infiltrated. | |||||||
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. For more information on using models to calculate credits see Methods and resources for calculating credits.
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
- providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
- complying with permit requirements, including antidegradation (see Construction permit; Municipal (MS4) permit);
- meeting the MIDS performance goal; or
- meeting or complying with water quality objectives, including total maximum daily load (TMDL) wasteload allocations (WLAs).
Contents
Overview
Dry swales, sometimes called grass swales, are similar to bioretention practices 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 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 a treatment train, including the main form of conveyance between or out of BMPs, at the end of a 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 baseflow 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). For specific tools and methods that can be used to calculate credits see Methods and resources for calculating credits.
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
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.
Volume credit calculations - check dams with an 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, soil porosity (f), and design 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:
IR = design infiltration rate of underlying soil (inches per hour);
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 be acceptable in swales.
Provided soil water (moisture) 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 68 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
$^R_{TP} = (0.68 * 0.55) + (0.45 * ((0.2 * (D_{MU_{max=2}})/2) + 0.40_{if amendment is used})) * 100 ^$
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.
The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.
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 |
||
Related articles
- Methods and resources for calculating credits
- Dry swales
- Terminology for swales
- Overview for dry swale (grass swale)
- Types of infiltration
- Types of filtration
- Design criteria for dry swale (grass swale)
- Construction specifications for dry swale (grass swale)
- Operation and maintenance of dry swale (grass swale)
- Assessing the performance of dry swale (grass swale)
- Cost considerations for dry swale (grass swale)
- Case studies for dry swale (grass swale)
- Plants for swales
- Check dams for stormwater swales
- Requirements, recommendations and information for using dry swale (grass swale) without an underdrain in the MIDS calculator
- Requirements, recommendations and information for using dry swale (grass swale) with an underdrain in the MIDS calculator
- Requirements, recommendations and information for using swale side slope as a BMP in the MIDS calculator