Design criteria for filtration

The following terminology is used throughout this design section:

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 filtration practice, but not specifically required by the MPCA CGP.

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

Major design elements

Physical feasibility initial check

Before deciding to use a filtration device for stormwater management, it is helpful to consider several items that bear on the feasibility of using such a device at a given location. The following list of considerations will help in making an initial judgment as to whether or not a filtration device is the appropriate BMP for the site.

Drainage Area: Five acres maximum RECOMMENDED (0.5 to 2 acres is ideal). Surface sand filters can sometimes be sized for drainage areas up to 10 acres, but only with rigorous and committed maintenance schedules, among other design and O&M considerations.

Site Topography and Slopes: It is RECOMMENDED that sloped areas immediately adjacent to the practice be less than 33 percent but greater than 1 percent, to promote positive flow towards the practice.

Soils: No restrictions for media filters with underdrains. Vegetated filters should be sized assuming no losses to infiltration.

Depth to Water Table and Bedrock: A separation distance of at least 3 feet is REQUIRED under the MPCA CGP between the bottom elevation of vegetative filters and the elevation of the seasonally high water table. For purposes of this guidance, it is assumed that the definition of a stormwater pond includes a stormwater filtration system.

Warning: A separation distance of at least 3 feet is REQUIRED under the state CGP between the bottom elevation of vegetative filters and the elevation of the seasonally high water table (does not apply to wet swales)

Site Location/Minimum Setbacks:

Warning: A minimum setback of 50 feet between a stormwater pond and a water supply well is REQUIRED by the Minnesota Department of Health Rule 4725.4350.

Karst: It is HIGHLY RECOMMENDED that underdrains and an impermeable liner be used for sand filters in Karst terrain.

Conveyance

It is Highly Recommended that the designer provides non-erosive flow velocities 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.

Common overflow systems within the structure consist of a yard drain inlet, where the top of the yard drain inlet is placed at the elevation of the shallow ponding area. A stone drop of about 12 inches or small stilling basin could be provided at the inlet of filtration areas where flow enters the practice through curb cuts or other concentrated flow inlets. In cases with significant drop in grade this erosion protection should be extended to the bottom of the facility.

Underdrains

The following are RECOMMENDED for bioretention areas with underdrains.

• The minimum pipe diameter is 4 inches.
• Install 2 or more underdrains for each bioretention 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 system as either bioinfiltration, biofiltration 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.

Pre-treatment

Pre-treatment refers to features of a filtration area that capture and remove coarse sediment particles.

Warning: To prevent clogging of the infiltration or filtration system with trash, gross solids, and particulate matter, use of a pretreatment device such as a vegetated filter strip, vegetated swale, small sedimentation basin (forebay), or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system is REQUIRED.

For applications where runoff enters the filtration area through sheet flow, such as from parking lots, or residential back yards, a grass filter strip with a pea gravel diaphragm is the preferred pre-treatment method. The width of the filter strip depends on the drainage area, imperviousness and the filter strip slope. The minimum RECOMMENDED vegetated filter strip width is 3 feet. The width should increase with increasing slope of the filter strip. Slopes should not exceed 8 percent. Pretreatment filter strips greater than 15 feet in width will provide diminishing marginal utility on the installation cost.

For retrofit projects and sites with tight green space constraints, it may not be possible to include a grass buffer strip. For example, parking lot island retrofits may not have adequate space to provide a grass buffer. For applications where concentrated (or channelized) runoff enters the filtration area, such as through a slotted curb opening, a grassed channel with a pea gravel diaphragm is the preferred pre-treatment method.

The filtration area should be inspected semi-annually to determine if accumulated sediment needs to be removed. Accumulated sediment should be removed from the gravel verge (if applicable) and vegetated filter strip as needed. If the watershed runoff is especially dirty, this frequency may need to be monthly or quarterly. Trash removal should occur in conjunction with removal of debris from the filtration area. During maintenance, check for erosion in the filter strip. If it is visible, it should be repaired with topsoil and re-planted. Vegetation of the filter strip should be designed at least 2 inches below the contributing impervious surface. If, over time, the grade of the vegetated filter strip rises above the adjacent impervious surface draining into it, the grade of the vegetated filter strip needs to be lowered to ensure proper drainage.

In lieu of grass buffer strips, pre-treatment may be accomplished by other methods such as sediment capture in the curb-line entrance areas. Additionally, the parking lot spaces may be used for a temporary storage and pre-treatment area in lieu of a grass buffer strip. Local requirements may allow a street sweeping program as an acceptable pre-treatment practice. It is HIGHLY RECOMMENDED that pre-treatment incorporate as many of the following as are feasible:

• grass filter strip;
• vegetated swale;
• gravel diaphragm;
• mulch layer;
• forebay;
• flow-through structures; and
• up flow inlet for storm drain inflow.

Treatment

The following guidelines are applicable to the actual treatment area of a filtration facility:

Space Required: Function of available head at site for surface filters. Underground filters generally have little or no surface space requirements except for access.

Slope: The surface slope of media filters should be level to promote even distribution of flow throughout the practice. It is HIGHLY RECOMMENDED that vegetative filters have a maximum slope of 5 percent and a minimum slope of 1 percent.

Depth: The RECOMMENDED elevation difference at a site from the inflow to the outflow is 4 to 6 feet for most sand filters, and 2 to 3 feet for perimeter filters.

Warning: Ground Water Protection: Infiltration of untreated PSH runoff into ground water is PROHIBITED. At confirmed hotspots, it is REQUIRED that the filter is lined and is discharged in a manner that will not mobilize pollutants.

Aesthetics: Vegetative filters can be effectively integrated into the site planning process, and aesthetically designed as attractive green spaces. Media filters are less conducive to site aesthetics, but surface media filters can be designed with turf cover vegetation if desired.

Landscaping

It is HIGHLY RECOMMENDED that impervious area construction be completed and pervious areas established with dense and healthy vegetation (see Minnesota plant lists or [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/stormwater-management/plants-for-stormwater-design.html Plants for Stormwater Design) prior to introduction of stormwater into a filtration practice.

Surface filters can have a grass cover to aid in pollutant adsorption. The grass should be capable of withstanding frequent periods of inundation and drought.

Safety

• No building structures should be constructed on top of underground filters.
• The risk of creating mosquito breeding areas can be minimized by following the recommendations within the mosquito control section.
• It is HIGHLY RECOMMENDED that swale side slopes be set at 1:3 (V:H) or flatter.
• It is HIGHLY RECOMMENDED that perimeter sand filter grates be sufficiently heavy that they cannot be removed easily.
• Fencing may be desirable in surface sand filter designs with significant vertical walls for the sedimentation and/or filter chamber. Fencing can also protect the filter from vandalism and limit animal access.

Warning: It is REQUIRED that underground media filters only be accessed by individuals with appropriate confined space entry training.

Warning: If a dry well or infiltration trench is greater than five feet deep, it is REQUIRED that OSHA health and safety guidelines be followed for safe construction practices.

Additional information on safety for construction sites is available from OSHA.

Design procedure: media filters

The following steps outline a recommended design procedure for media filters. Except where indicated, procedures are consistent with requirements for compliance with the MPCA CGP.

Design steps

Step 1. Preliminary judgment of site conditions and identify the function of the filter.

Make a preliminary judgment as to whether site conditions are appropriate for the use of a surface or perimeter sand filter, and identify the function of the filter in the overall treatment system.

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

• Site drainage area
• Site topography and slopes
• Regional or local depth to groundwater and bedrock
• Site location/minimum setbacks
• Presence of active karst

B. Determine how the media filter will fit into the overall stormwater treatment system.

• Decide whether the filter 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 media filter is most likely to be located.

Step 2. Confirm design criteria and applicability.

A. Determine whether the media filter 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. Select design variant based on physical suitability evaluation

Once the Physical feasibility initial check is complete, apply the better site design principles in sizing and locating the filtration practice(s) on the development site. Given the drainage area, select the appropriate filtration practice for the first iteration of the design process.

Note: Information collected during the Physical feasibility initial check (see Step 2) should be used to explore the potential for multiple filtration practices versus relying on a single facility. The use of smaller filtration practices dispersed around a development is usually more sustainable than a single regional facility that is more likely to have maintenance problems (Source: Wisconsin Department of Natural Resources Conservation Practice Standards, 2004).

Step 4. Compute runoff control volumes

Calculate the Water Quality Volume (V_wq).

Warning: If the media filter is being designed to meet the requirements of the MPCA CGP, the REQUIRED treatment volume is the water quality volume of 1 inch of runoff from the new impervious surfaces created from the project and must be calculated as an instantaneous volume above the media filter.

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 filter.

Step 5. Compute V_wq peak discharge (Q_wq)

A flow regulator (or flow splitter diversion structure) should be supplied to divert the V_wq to the sand filter facility. This is generally accomplished by setting the bypass weir within the diversion to the elevation of the water quality volume within the practice. Please refer to the adjustable diversion detail found in the Computer-aided design and drafting (CAD/CADD) drawings section.

Size low flow orifice, weir, or other device to pass Q_wq.

Step 6. Size filtration basin chamber

Schematic illustrating some of the terms and dimensions used in the Stormwater Manual.

To meet requirements of the Stormwater General Permit (CSW permit), the surface area (A_S, in square feet) of a filtration practice is given by

\(A_S = V_w / (D_O) \)

Where:

V_w is the the water treatment volume of the area contributing runoff to the practice, in cubic feet; and

D_O is the the storage depth of ponded water in the practice, in feet;

Size the depth of the practice the meet the 48 hour draw down time. The max recommended depth of filtration practices is 4 feet

Warning: The water treatment volume must drain with 48 hours (24 hours is RECOMMENDED if discharges from the practice are to a trout stream) per the MPCA CGP)

The water treatment volume is given by

\(V_w = 0.0833 A_c\)

Where

0.0833 = one inch, converted to feet, of runoff captured for filtration, as required by the permit; and

A_c = the impervious surface area contributing to the practice.

The kerplunk method, in which the entire water treatment volume is instantaneously ponded in the filtration practice, is used to size the practice.

For a filtration BMP with sloped sides, the surface area (As) of the practice is the average area of the BMP, given by

\( A_S = (A_O + A_M) / 2 \)

Where

• A_O is the surface area at the overflow; and
• A_M is the surface area at the top of the filtration media

Set preliminary dimensions of filtration basin chamber. The following guidelines are HIGHLY RECOMMENDED.

• The filter media should consist of an 18-inch layer of clean washed medium sand (meeting ASTM C-33 concrete sand) on top of the under-drain system.
• For surface sand filters, if an optional three inches of topsoil are placed over the sand bed, place permeable filter fabric between the topsoil and sand to prevent mixing of the layers. To prevent clogging of the underdrain, use either gravel or a permeable filter fabric.
• Underground sand beds should be protected from trash accumulation by a wide mesh geotextile screen to be placed on the surface of the sand bed. The screen is to be rolled up, removed, cleaned and re-installed during maintenance operations.

Step 7. Pretreatment

Pre-treatment refers to features of a filtration system that capture and remove coarse sediment particles.

Warning: To prevent clogging of the infiltration or filtration system with trash, gross solids, and particulate matter, use of a pretreatment device such as a vegetated filter strip, vegetated swale, small sedimentation basin (forebay), or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system is REQUIRED.

For applications where runoff enters the filtration system through sheet flow, such as from parking lots, or residential back yards, a vegetated filter strip with a pea gravel diaphragm is the preferred pre-treatment method. The width of the filter strip depends on the drainage area, imperviousness and the filter strip slope. The minimum RECOMMENDED vegetated filter strip width is 3 feet. The width should increase with increasing slope of the filter strip. Slopes should not exceed 8 percent. Pretreatment filter strips greater than 15 feet in width will provide diminishing marginal utility on the installation cost.

For retrofit projects and sites with tight green space constraints, it may not be possible to include a grass buffer strip. For example, parking lot island retrofits may not have adequate space to provide a grass buffer. For applications where concentrated (or channelized) runoff enters the filtration system, such as through a slotted curb opening, a vegetated filter strip with a pea gravel diaphragm is the preferred pre-treatment method.

The filtration system should be inspected semi-annually to determine if accumulated sediment needs to be removed. Accumulated sediment should be removed from the gravel verge (if applicable) and vegetated filter strip as needed. If the watershed runoff is especially dirty, this frequency may need to be monthly or quarterly. Trash removal should occur in conjunction with removal of debris from the filtration system. During maintenance, check for erosion in the filter strip. If it is visible, it should be repaired with topsoil and re-planted. Vegetation of the filter strip should be designed at least 2 inches below the contributing impervious surface. If, over time, the grade of the vegetated filter strip rises above the adjacent impervious surface draining into it, the grade of the vegetated filter strip needs to be lowered to ensure proper drainage.

The type of vegetation in the bioretention cell determines the appropriate flow velocity for which the pre-treatment device should be designed. For tree-shrub-mulch bioretention cells, velocity through the pre-treatment device should not exceed 1 foot per second, which is the velocity that causes incipient motion of mulch. For grassed bioretention cells, flow velocity through the pre-treatment device should not exceed 3 feet per second. In all cases, appropriate maintenance access should be provided to pre-treatment devices.

In lieu of grass buffer strips, pre-treatment may be accomplished by other methods such as sediment capture in the curb-line entrance areas. Additionally, the parking lot spaces may be used for a temporary storage and pre-treatment area in lieu of a grass buffer strip. If bioretention is used to treat runoff from a parking lot or roadway that is frequently sanded during snow events, there is a high potential for clogging from sand in runoff. Local requirements may allow a street sweeping program as an acceptable pre-treatment practice. It is HIGHLY RECOMMENDED that pre-treatment incorporate as many of the following as are feasible:

• vegetated filter strip;
• vegetated swale;
• gravel diaphragm;
• mulch layer;
• forebay;
• flow-through structures; and
• up flow inlet for storm drain inflow.

Step 8. Design inlets, pre-treatment facilities, under-drain system, and outlet structures

• Pre-treatment of runoff in a sand filter system must be provided. Inlets to surface sand filters must be provided with energy dissipaters. Exit velocities from the pretreatment device must be non-erosive.
• An emergency or bypass spillway must be included in the surface sand filter to safely pass flows that exceed the design storm flows. The spillway prevents filter water levels from overtopping the embankment and causing structural damage. The emergency spillway should be located so that downstream buildings and structures will not be impacted by spillway discharges.
• The allowable materials for sand filter construction are detailed in the table below.

This table shows sand material specifications.
Link to this table

Parameter	specification	Size	Notes
Sand	clean AASHTO M-6 or ASTM C-33 concrete sand	0.02” to 0.04”	Sand substitutions such as Diabase and Graystone #10 are not acceptable. No calcium carbonated or dolomitic sand substitutions are acceptable. Rock dust cannot be substituted for sand.
Underdrain Gravel	AASHTO M-43	1.5” to 3.5”
Geotextile Fabric (if required)	ASTM D-4833 (puncture strength - 125 lb.) ASTM D-1117 (Mullen Burst Strength - 400 psi) ASTM D-4632 (Tensile Strength - 300 lb.)	0.08” thick equivalent opening size of #80 sieve	Must maintain 125 gpm per sq. ft. flow rate. Note: a 4” pea gravel layer may be substituted for geotextiles meant to separate sand filter layers.
Impermeable Liner (if required)	ASTM D-4833 (thickness) ASTM D-412 (tensile strength 1,100 lb., elongation 200%) ASTM D-624 (Tear resistance - 150 lb./in) ASTM D-471 (water adsorption: +8 to -2% mass)	30 mil thickness	Liner to be ultraviolet resistant. A geotextile fabric should be used to protect the liner from puncture.
Under-drain Piping	ASTM D-1785 or AASHTO M-278	minimum 4” rigid schedule 40 PVC	3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” washed #57 stone over pipes; not necessary underneath pipes

Step 9. Compute overflow weir sizes

Surface sand filter:

• If appropriate, plan outlet protection for overflow from sedimentation chamber.
• Size bypass weir at elevation in filtration chamber above stand pipe to handle surcharge of flow through filter system within 48 hours.

Perimeter sand filter:

• Size overflow weir at end of pretreatment to handle excess inflow, set at V_wq elevation

Follow the design procedures identified in the section on Unified sizing criteria to determine the volume control and peak discharge requirements for water quality, recharge (not required), channel protection, overbank flood and extreme storm. Adapt these values to local regulations, if any exist.

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 filtration practice defined above: pond elevation and area (defines the pond volume), filtration 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 unintended consequences upstream (i.e. flooding).

Warning: The following item is specifically REQUIRED by the MPCA Permit:

Volume: The Permittee(s) must design infiltration or filtration systems that provide a water quality volume (calculated as an instantaneous volume) of one (1) inch of runoff (or one (1) inch minus the volume of stormwater treated by another system on the site) from the new impervious surfaces created by the project.

Period of Inundation: filtration 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 invert of the outlet structure or drain tile or bottom of the facility. It is assumed that this range is less than 1/5 the bounce in the filtration practice.

Step 10. Prepare vegetation and landscaping plan

See Major design elements section for guidance on preparing vegetation and landscaping management plan.

Step 11. Prepare operations and maintenance (O&M) plan

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

Step 12. Prepare cost estimate

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

Design procedure: vegetative filters

The following steps outline a recommended design procedure for vegetative filters 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

Make a preliminary judgment as to whether site conditions are appropriate for the use of a vegetative filter, and identify the function of the filter in the overall treatment system

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

• Site drainage area
• Site topography and slopes
• Regional or local depth to ground water and bedrock
  • Dry Swale: Bottom of facility to be at least three feet above the seasonably high water table.
  • Wet Swale: The seasonally high water table may inundate the swale; but not above the design bottom of the channel.
• Site location/minimum setbacks.
• Presence of active karst.

B. Determine how the vegetative filter will fit into the overall stormwater treatment system

• Decide whether the filter 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 vegetative filter is most likely to be located.

Step 2. Confirm design criteria and applicability

A. Determine whether the vegetative filter 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. 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 filtration 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 filtration practices versus relying on a single facility. The use of smaller filtration practices dispersed around a development is usually more sustainable that a single regional facility that is more likely to have maintenance problems (Source: Wisconsin Department of Natural Resources Conservation Practice Standards, 2004)

Step 4. Compute runoff control volumes and other key design parameters

Calculate the Water Quality Volume (V_wq), Channel Protection Volume (V_cp), Overbank Flood Protection Volume (V_p10), and the Extreme Flood Volume (V_p100).

If the vegetative filter is being designed to meet the requirements of the MPCA Permit, the REQUIRED treatment volume is the water quality volume of 1 inch of runoff from the new impervious surfaces created from the project. 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 filter.

For filter strips, compute the following design parameters:

a. Calculate the maximum discharge loading per foot of filter strip width

\( q = (0.00236/n)Y^{1.67}S^{0.5} \)

Where:

q = discharge per foot of width of filter strip, from Manning’s equation (cfs/ft);

Y = allowable depth of flow (inches) (3 to 4 inch maximum);

S = slope of filter strip (percent) (2 to 6 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 Q_wq

c. Minimum Filter Width (in feet) = Q_wq / q

Where:

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

Step 5. Determine pre-treatment method

Warning: Pre-treatment for vegetative filters is REQUIRED

One alternative is a level spreader that allows coarse sediment to settle and evenly distributes flow across the full width of the filter. Pre-treatment could be provided with plunge pools where concentrated flows enter and with level spreaders where lateral flows enter. Additional pre-treatment measures include filter strips and street/parking lot sweeping. Street/parking lot sweeping may be considered pre-treatment in the case of a parking lot island or other area where spatial limitations make structural pre-treatment measures unfeasible.

Storage volume created for pre-treatment counts toward the total Vwq requirement, and should be subtracted from the V_wq for subsequent calculations.

Step 6. Preliminary design

Wet and dry swales:

• Size bottom width, depth, length, and slope necessary to store V_wq with less than 18 inches of ponding at the downstream end.
• Slope should not exceed 5 percent (1 to 2 percent recommended)
• Bottom width should range from 2 to 8 feet
• Ensure that side slopes are no greater than 3:1 (4:1 recommended)

If the system is on-line, channels should be sized to convey runoff from the overbank flood event (V_p10) safely with a minimum of 6 inches of freeboard and without damage to adjacent property. The peak velocity for the 2-year storm must be nonerosive for the soil and vegetative cover provided.

The channel and under-drain excavation should be limited to the width and depth specified in the design. The bottom of the excavated trench shall not be loaded in a way that causes soil compaction, and scarified prior to placement of gravel and permeable soil. The sides of the channel shall be trimmed of all large roots. The sidewalls shall be uniform with no voids and scarified prior to backfilling.

Step 7. Compute number of check dams (swales) or berms (filter strip)

Wet and Dry Swales: Checkdams

• Design to contain entire V_wq.
• Channel slopes between 1 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 1 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 V_wq at the downstream end should not exceed 18 inches.

Filter Strips: Berms

• Size outlet pipes to ensure that the bermed area drains within 24 hours.
• Specify grasses resistant to frequent inundation within the shallow ponding limit.
• Berm material should be of sand, gravel and sandy loam to encourage grass cover(Sand: ASTM C-33 fine aggregate concrete sand 0.02 to 0.04 inch, Gravel: AASHTO M-43 ½ to 1 inch)
• Size filter strip to contain the V_wq within the wedge of water backed up behind the berm.
• Maximum berm height should be 12 inches.
• Pervious berms to be a sand/gravel mix (35 to 60 percent sand, 30 to 55 percent silt, and 10 to 25 percent gravel). Berms are to have overflow weirs with 6-inch minimum head.

Step 8: Calculate draw-down time

Dry swale: The bed of the dry swale consists of a permeable soil layer of at least 30 inches in depth, above an 8-inch diameter perforated PVC pipe (AASHTO M 252) longitudinal under-drain in a 12-inch gravel layer. The soil media should have an infiltration rate of at least 0.5 feet per day (fpd) with a maximum of 1.5 fpd and contain a high level of organic material to facilitate pollutant removal. 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 V_wq and allow for full filtering through the permeable soil layer.

Step 9. 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 6 inches of freeboard.

Step 10. Design low flow control at downstream headwalls and checkdams

Design control to pass V_wq in 48 hours.

Step 11. Design inlets, sediment forebay(s), and under-drain system (dry swale)

Inlets to swales must be provided with energy dissipaters such as riprap or geotextile reinforcement. Pre-treatment of runoff in both a dry and wet swale system 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 pre-treatment for lateral sheet flows. The under-drain system should discharge to the storm drainage infrastructure or a stable outfall. For a wet swale, do not use an under-drain system.

Step 12. Check volume, peak discharge rates and period of inundation 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 filtration practice defined above: pond elevation and area (defines the pond volume), filtration 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 Filtration systems shall be sufficient to filter 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 filtration practice or treat excess water quality volume (V_wq) in an upstream or downstream BMP (see Step 5).

B. Period of Inundation

Filtration 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 invert of the outlet structure or drain tile or bottom of the facility. It is assumed that this range is less than 1/5 the bounce in the filtration practice.

Step 13: Prepare Vegetation and Landscaping Plan

A landscaping plan for a dry or wet swale 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.

This table shows sand material specifications.
Link to this table

Parameter	specification	Size	Notes
Sand	clean AASHTO M-6 or ASTM C-33 concrete sand	0.02” to 0.04”	Sand substitutions such as Diabase and Graystone #10 are not acceptable. No calcium carbonated or dolomitic sand substitutions are acceptable. Rock dust cannot be substituted for sand.
Underdrain Gravel	AASHTO M-43	1.5” to 3.5”
Geotextile Fabric (if required)	ASTM D-4833 (puncture strength - 125 lb.) ASTM D-1117 (Mullen Burst Strength - 400 psi) ASTM D-4632 (Tensile Strength - 300 lb.)	0.08” thick equivalent opening size of #80 sieve	Must maintain 125 gpm per sq. ft. flow rate. Note: a 4” pea gravel layer may be substituted for geotextiles meant to separate sand filter layers.
Impermeable Liner (if required)	ASTM D-4833 (thickness) ASTM D-412 (tensile strength 1,100 lb., elongation 200%) ASTM D-624 (Tear resistance - 150 lb./in) ASTM D-471 (water adsorption: +8 to -2% mass)	30 mil thickness	Liner to be ultraviolet resistant. A geotextile fabric should be used to protect the liner from puncture.
Under-drain Piping	ASTM D-1785 or AASHTO M-278	minimum 4” rigid schedule 40 PVC	3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” washed #57 stone over pipes; not necessary underneath pipes

Step 14. Prepare Operation and Maintenance (O&M) 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.