Design criteria for stormwater ponds

This section describes information on design of stormwater ponds.

Physical feasibility initial check

Before deciding to use a pond for stormwater management, it is helpful to consider several items that bear on the feasibility of using a pond at a given location. The following list of considerations will help in making an initial judgment as to whether or not a pond is the appropriate BMP for the site. Note that none of these guidelines are strictly required by the MPCA Construction Stormwater General Permit, and it may be possible to overcome site deficiencies with additional engineering or the use of other BMPs.

• Drainage area – 25 acres minimum Highly Recommended, to ensure hydrologic input sufficient to maintain permanent pool; 10 acres (or less) may be acceptable, particularly if the groundwater table is intercepted and a water balance indicates that a permanent pool can be sustained.
• Space required – Approximately 1 to 3 percent of the tributary drainage area is Recommended for the pond footprint.
• Site topography and slope – It is Highly Recommended that slopes immediately adjacent to ponds be less than 25 percent but greater than 0.5 to 1 percent to promote flow towards the pond.
• Minimum head – The elevation difference Recommended at a site from the inflow to the outflow is 6 to 10 feet, but lower heads will work at small sites.
• Minimum depth to water table – In general, there is no minimum separation distance required with ponds. Intercepting the groundwater table can help sustain a permanent pool. However, some source water protection requirements may dictate a separation distance or an impervious liner if there is a sensitive underlying aquifer and the bottom material of the pond allows for infiltration.
• Soils – Underlying soils of hydrologic group C or D should be adequate to maintain a permanent pool. A liner may be needed for most group A soils and some group B soils, in order to maintain a permanent pool. A site specific geotechnical investigation should be performed. Also, if earthen embankments are to be constructed, it will be necessary to use suitable soils and to follow guidance in NRCS Pond 378 or other guidelines from the Dam Safety Section of the Minnesota Department of Natural Resources.
• Groundwater protection – It is Highly Recommended that ponds treating runoff from potential stormwater hotspots (PSHs) provide excellent pre-treatment capabilities. In some cases (depending on the land use and associated activities), lining the pond may be necessary to protect groundwater, particularly when the seasonally high groundwater elevation is within three feet of the pond bottom.
• Karst – It is Recommended that ponds not be used in karst areas, due to the long term implication of having deep ponded water. If ponds are used in karst areas, impermeable liners and a minimum 3 foot vertical separation from the barotic rock layer are Recommended. Geotechnical investigations are necessary in karst areas.
• Cold water fisheries – Ponds may not be appropriate practices where receiving waters are sensitive cold water fisheries, due to the potential for stream warming from pond outflows. If ponds are used, it is Highly Recommended that the 1-year, 24-hour storm be detained for no longer than 12 hours. If regulatory provisions allow, a smaller permanent pool with more extended detention storage should be considered.
• Shallow soils and bedrock – For situations with shallow bedrock and ground water, pond use is limited due to the available depth, affecting the surface area required as well as the aesthetics of the pond. Consider stormwater wetlands as an alternative.

Warning: Wetlands – It is Required that stormwater ponds not be located in wetlands unless it is mitigated for.

Conveyance

Inflow points

• It is Highly Recommended that pre-treatment be provided to reduce the future pond maintenance burden. If pre-treatment has not been provided in the contributing watershed, then it is Recommended that a forebay be provided at each inlet contributing greater than 10 percent of the total design storm inflow to the pond.

Warning: It is Required that inlet areas be stabilized to ensure that non-erosive conditions exist during events up to the overbank flood event (i.e., Q_p10).

• It is Highly Recommended that rip-rap or other channel liners be extended below the permanent pool elevation.
• It is Highly Recommended that inlet pipe inverts be located at the permanent pool elevation. Submerging the inlet pipe can result in freezing and upstream damage.
• It is Highly Recommended that inlet pipes have a slope of no flatter than 1 percent, to prevent standing water in the pipe and reduce the potential for ice formation.
• It is Highly Recommended that pipes be buried below the frost line, to prevent frost heave and pipe freezing.
• It is Highly Recommended that where open channels are used to convey runoff to the pond, the channels be stabilized to reduce the sediment loads.

Adequate outfall protection

Pond outfalls should be designed to not increase erosion or have undue influence on the downstream geomorphology of the stream.

• It is Highly Recommended that a stilling basin or outlet protection be used to reduce flow velocities from the principal spillway to non-erosive velocities (3.5 to 5.0 feet per second).

Warning: It is Required in the CGP that the V_wq is discharged at no more than 5.66 cubic feet per second per surface area of the pond.

• Flared pipe sections that discharge at or near the stream invert or into a step-pool arrangement are Recommended over headwalls at the spillway outlet.
• It is Recommended that tree clearing be minimized along the downstream channel and that a forested riparian zone be reestablished in the shortest possible distance. It is also recommended that excessive use of rip-rap be avoided, to minimize stream warming in channels with dry weather flow.
• Local agencies (Watershed Districts, Watershed Management Organizations (WMOs), municipalities, etc.) may have additional outlet control requirements.

Information: This is an alternative to the preferred active management strategy of drawing down the permanent pool seasonally to provide detention while the permanent pool is frozen.

Pre-treatment

Construction of pre-treatment measures immediately upstream of the main pond is Highly Recommended, to reduce the maintenance requirements and increase the longevity of a stormwater treatment pond. A large portion of the overall sediment load (the heavier sediments) can be captured by relatively small (and therefore relatively easy to clean and maintain) BMPs. The larger pond area can thus be devoted to the settling of finer sediments, allowing it to fill more slowly and therefore requiring less frequent maintenance.

It is therefore Highly Recommended that each pond have a sediment forebay or equivalent upstream pre-treatment (non-pond BMPs may serve as pre-treatment) at each inflow point that contributes greater than 10 percent of the inflow volume. A sediment forebay is a small pool, separated from the permanent pool by barriers such as earthen berms, concrete weirs, or gabion baskets, where initial settling of heavier particulates can occur.

Warning: It is Required that where a forebay is installed, that direct vehicle/equipment access be provided to the forebay for sediment removal and other maintenance activities.

• It is Highly Recommended that flows from forebays enter the permanent pool area with non-erosive outlet conditions.
• It is Recommended that the forebay(s) be sized to contain 10 percent of the water quality volume (V_wq; see Unified sizing criteria) in a pool that is four to six feet deep. The forebay storage volume counts toward the total permanent pool requirement.
• It is Recommended that the forebay(s) be designed with a surface area equivalent to 10 percent of the pond permanent pool surface area or equivalent to 0.1 percent of the drainage area.
• It is Recommended that a fixed vertical sediment depth marker be installed in the forebay to measure sediment deposition over time. The marker should be sturdy and placed deep enough into the bottom of the forebay so that ice movement does not affect its position.
• It is Recommended that the bottom of the forebay be hardened, using concrete, asphalt, or grouted riprap, to make sediment removal easier.

Treatment

Permanent pool and water quality volume (V_wq) sizing for new impervious area

• Under the MPCA Permit, it is Required that stormwater ponds have permanent pool volume (dead storage) equal to at least 1800 cubic feet per acre of drainage to the pond. For example, a 30-acre drainage area requires a permanent pool volume of at least 54,000 cubic feet or 1.24 acre-feet.

Warning: The Required total storage volume (V_ts) equals the sum of the volume in the permanent pool (V_pp below the outlet elevation) plus live storage allocation for water quality volume (V_wq). V_wq equals 1.0 inch of runoff per impervious acre.

Warning: It is Required that permanent pool depths be a minimum of 3 feet and maximum of 10 feet at the deepest points.

• Where phosphorus load reductions are a priority, it is Recommended that a maximum depth of 8 feet be used, to limit the likelihood of stratification and the potential for bottom sediment to release phosphorus.
• If extended detention storage for the Channel Protection Volume (V_cp; see Unified sizing criteria) is provided, it is Recommended that the V_wq be computed and routed through the outlet for V_cp.

Warning: It is Required that the V_wq be released at a rate not to exceed 5.66 cubic feet per second per acre of permanent pool surface area.

• It is Highly Recommended that the V_cp be released over a minimum 24 hour period.
• Where phosphorus load reductions are a priority, permanent pool volumes as large as 3600 cubic feet per acre of drainage are Recommended for enhanced removal.
• To compensate for ice build-up on the permanent pool, it is Highly Recommended that 12 inches (or a volume equal to the average snow melt) of additional storage be provided. This is an alternative to the preferred active management strategy of drawing down the permanent pool seasonally to provide detention while the permanent pool is frozen.
• Using pumps or bubbling systems can reduce ice build-up and prevent the formation of an anaerobic zone in pond bottoms. Caution must be exercised, however, because of the possibility of thin or no ice cover.
• A water balance is Recommended to document sufficient inflows to maintain a constant permanent pool during prolonged dry weather conditions. The basic approach to performing a water balance is as follows:
  • Check maximum drawdown during periods of high evaporation and during an extended period of no appreciable rainfall to ensure that wetland vegetation will survive.
  • The change in storage within a pond = (inflows – outflows).
  • Potential inflows include runoff, baseflow and rainfall (groundwater and surface water).
  • Potential outflows include infiltration, surface overflow and evapotranspiration.
  • Assume no inflow from baseflow, no outflow losses for infiltration and because only the permanent pool volume is being evaluated, no outflow losses for surface overflows. The validity of these assumptions need to be verified for each design.

Information: All pond designs should incorporate an access bench

Pond liners

It is Highly Recommended that pond liners be considered in circumstances where a permanent pool is needed but difficult to maintain due to site conditions, or where seepage from the pond into the groundwater would otherwise occur but must be avoided. This includes:

• Areas with Hydrologic Group A soils, gravel, or fractured bedrock
• Potential stormwater hotspots (PSHs)
• Karst terrain

If geotechnical tests confirm the need for a liner, acceptable options include: (a) 6 to 12 inches of clay soil, including bentonite, (minimum 15 passing the #200 sieve and a maximum permeability of 1 x 10^-5 centimeters per second, (b) a 30 milimeter poly-liner, or (c) engineering design as approved on a case-by-case basis by MPCA or appropriate review agency.

Grading and site layout

The site layout and pond grading affect the pollutant removal capability of the pond as well as the ease of maintenance. Performance is enhanced when multiple treatment pathways are provided by using multiple cells, longer flowpaths, high surface area to volume ratios, complex microtopography, and/or redundant treatment methods (combinations of pool, extended detention, and marsh). It is Recommended that a berm or simple weir be used instead of pipes to separate multiple ponds, because of the higher freezing potential of pipes. Specific guidelines are provided below:

• It is Highly Recommended that pond side slopes within the permanent pool (below the aquatic bench) not exceed 1:2 (V:H).
• It is Highly Recommended that side slopes to the pond should be 1:3 (V:H) or flatter, and that they terminate on an access bench.
• It is Recommended that approximately 15 percent of the permanent pool surface area be allocated to a shallow (i.e., less than or equal to 18 inches in depth) zone along the perimeter to promote a shallow marsh littoral zone.
• It is Recommended that the minimum length to width ratio for ponds be 1.5:1.
• It is Recommended that the maximum drainage area to surface area ratio be 100:1.
• It is Recommended that to the greatest extent possible, ponds should be irregularly shaped and long flow paths should be maintained.

Pond benches

All pond designs should incorporate an access bench (a shallow slope area adjacent to the pond, providing equipment access and preventing people from slipping into the water) and a submerged aquatic bench (a shallow slope area just inside the pond perimeter, facilitating the growth of aquatic plants). This is a Highly Recommended design practice that may be required by local authorities. Mosquito breeding concerns exist along bench areas. Therefore, it is Highly Recommended that designers follow recommendations from the Metropolitan Mosquito Control District.

• Access Bench: It is Highly Recommended that an access bench extending 10 feet outward from the permanent pool edge to the toe of the pond side slope be provided. Narrower benches may be used on sites with extreme site limitations. The maximum cross-slope of the access bench should be 0.06:1 (V:H), or 6 percent. Access benches are not needed when the pond side slopes are 1:4 (V:H) or flatter.
• Aquatic Bench: It is Highly Recommended that an irregularly configured aquatic bench, extending up to 10 feet inward from the normal shoreline and graded no more than 18 inches below the permanent pool water surface elevation, be incorporated into the pond.

Maintenance access

Warning: It is Required that adequate maintenance access, typically with a minimum width of 8 feet, be provided.

If feasible, it is Recommended that the access be 10 feet wide, have a maximum slope of 0.15:1 (V:H) or 15 percent, and be appropriately stabilized for use by maintenance equipment and vehicles. Steeper grades may be allowable if designed using appropriate materials for the grade.

Warning: It is Required that the maintenance access extends to the forebay, access bench, riser, and outlet, and allows vehicles to turn around.

Riser in embankment

Warning: It is Required that the riser be located so that short-circuiting between inflow points and the riser does not occur.

It is Recommended that the riser be located within the embankment for maintenance access, prevention of ice damage, and aesthetics.

Spillway design

The principle spillway (riser) should be designed for the desired release rates while keeping the future maintenance needs in mind. Lessening the potential for clogging and freezing, creating safe access paths for inspection and maintenance, barring access to children and vandals, and allowing safe draw down of the permanent pool, when necessary, are goals of riser design that consider long-term maintenance needs.

Non-clogging low flow orifice

It is Highly Recommended that the low flow orifice be adequately protected from clogging by either an acceptable external trash rack (recommended minimum orifice of 3 inches) or by internal orifice protection that may allow for smaller diameters (recommended minimum orifice of 1 inch). The Recommended method is a submerged reverse-slope pipe that extends downward from the riser to an inflow point at least one foot below the normal pool elevation (see CADD designs). This should also draw from at least 6 inches below the typical ice layer. To avoid release of deposited sediment, the pipe should not be installed on the pond floor.

Alternative methods are to employ a broad crested rectangular, V-notch, or proportional weir, protected by a half-round CMP that extends at least 12 inches below the normal pool. It is Highly Recommended that the minimum weir slot width be 3 inches, especially when the slot is tall. It is Recommended that hoods over orifices be oversized to account for ice formation.

Trash racks

Warning: It is Required that basin outlets be designed to prevent discharge of floating debris.

It is Highly Recommended that the principal spillway openings be equipped with removable trash racks to prevent clogging by large debris and to restrict access to the interior for safety purposes. US EPA guidance on control of floatables suggests that openings in the range of 1.5 inches are both cost-efficient and effective in removing floatables and large solids.

It is Recommended that trash racks be installed at a shallow (~15°) angle to prevent ice formation.

Baffle weirs (essentially fences in the pond) can prevent ice reformation during the spring melt near the outlet by preventing surface ice from blocking the outlet structure.

Pond drain

It is Highly Recommended that each pond be equipped with a drain that can dewater the pond to the maximum extent possible within 24 hours. The drain pipe should have an elbow or protected intake extending at least 1 inches above the bottom of the permanent pool to prevent deposited sediment from clogging the pipe or being re-released while the pond is being drained.

Adjustable gate valve

It is Recommended that the pond drain and possibly the low flow orifice be equipped with an adjustable gate valve (typically a handwheel activated knife gate valve). These valves should be located inside the riser, where they (a) will not normally be inundated and (b) can be operated in a safe manner. To prevent vandalism that alters the pond level, the handwheel should be chained to a ringbolt, manhole step or other fixed object.

It is Recommended that both the low flow orifice pipe and the pond drain be sized one pipe size greater than the calculated design diameter and the gate valve be installed and adjusted to an equivalent orifice diameter.

Riser access

It is Recommended that lockable manhole covers and manhole steps within easy reach of valves and other controls be installed, to allow for maintenance access and prevent vandalism.

Emergency spillway

Warning: It is Required that an emergency spillway should be provided to pass storms in excess of the pond hydraulic design.

Warning: It is also Required that the spillway be stabilized to prevent erosion and designed in accordance with applicable dam safety requirements (NRCS Pond Standard 378 and Mn/DNR dam safety guidelines). The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges. If the spillway crosses the maintenance access, materials meeting the appropriate load requirements must be selected.

Landscaping

Landscaping plan

It is Highly Recommended that a landscaping plan for the stormwater pond and the surrounding area be prepared to indicate how aquatic and terrestrial areas will be stabilized, and established with vegetation (see vegetation for guidance on vegetation). Landscaping plans should also include maintenance schedules. It is Highly Recommended that the plan be prepared by a qualified professional. The following guidance suggests how landscaping can be incorporated into pond design.

Woody vegetation should not be planted or allowed to grow within 15 feet of the toe of the embankment or 25 feet from the inlet and outlet structures.

Wherever possible, wetland plants should be encouraged in a pond design, either along the aquatic bench (fringe wetlands), the access bench and side slopes (ED wetlands) or within shallow areas of the pool itself.

The best elevations for establishing wetland plants, either through transplantation or volunteer colonization, are within six inches (plus or minus) of the normal pool.

The soils of a pond buffer are often severely compacted during the construction process to ensure stability. The density of these compacted soils can be so great that it effectively prevents root penetration, and therefore, may lead to premature mortality or loss of vigor. Consequently, it is advisable to excavate large and deep holes around the proposed planting sites, and backfill these with uncompacted topsoil or other organic material.

As a rule of thumb, planting holes should be three times deeper and wider than the diameter of the rootball (of balled and burlap stock), and five times deeper and wider for container grown stock. This practice should enable the stock to develop unconfined root systems.

Species that require full shade, are susceptible to winterkill, or are prone to wind damage should be avoided. Extra mulching around the base of the tree or shrub is strongly recommended as a means of conserving moisture and suppressing weeds.

Pond buffers and setbacks

Warning: It is Required (Minnesota Department of Health Rule 4725.4350) that a minimum horizontal distance between a water-supply well and the ordinary high water level of a storm water retention pond is 35 feet.

It is Highly Recommended that a pond buffer extending 25 feet outward from the maximum water surface elevation of the pond be provided. Permanent structures (e.g., buildings) should not be constructed within the buffer. This distance may be greater under local regulations.

The pond buffer should be contiguous with other buffer areas that are required by existing regulations (e.g., stream buffers).

It is Highly Recommended that existing trees should be preserved in the buffer area during construction. It is desirable to locate forest conservation areas adjacent to ponds. To help discourage resident geese populations, the buffer can be planted with trees, shrubs and native ground covers.

Warning: If the pond will be used for temporary sediment control during construction, the associated permanent pool volume REQUIRED is either the 2 year, 24 hour storm runoff volume draining to the pond (with minimum 1800 cubic feet for each acre draining to the basin), or in the absence of such a calculation, 3600 cubic feet for each acre draining to the basin. It is also REQUIRED that sediment deposited during construction be removed before normal operation begins (refer to MPCA Permit for additional design requirements).

Safety

Warning: It is Required that public safety be considered in every aspect of pond design.

• The principal spillway opening should not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter should be fenced to prevent a hazard.
• The access and aquatic benches should be landscaped to prevent access to the pond.
• Warning signs prohibiting swimming, skating, and fishing should be posted.
• Pond fencing is generally not encouraged because the fence limits access to emergency personnel. A preferred method is to grade the pond to eliminate steep drop-offs or other safety hazards. Designers should check local requirements since fencing is required by some municipalities.
• Dam safety regulations should be strictly followed with pond design to ensure that downstream property and structures are adequately protected.

Design procedure

The following steps outline a recommended design procedure for a wet extended detention pond (wet sedimentation basin) in compliance with the MPCA CGP for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.

Step 1: Make a preliminary judgment as to whether site conditions are appropriate

Warning: For sites covered under the MPCA Construction Stormwater General Permit or under the MPCA Municipal Separate Storm Sewer System General Permit, the permittee must first consider implementing on-site volume control practices.

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

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

• Site drainage area
• Depth to water table
• Depth to bedrock
• Presence of wetlands
• Soil characteristics
• Receiving water(s)

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

• Decide whether the pond is the only BMP to be employed, or if are there other BMPs (including other ponds) addressing some of the treatment requirements.
• Determine whether the pond needs to treat water quality (V_wq), quantity (V_cp, Q_p, Q_f), or both (see Unified sizing criteria).
• Determine whether the pond is being designed as a wet sedimentation basin under the MPCA General Stormwater Permit for Construction Activities (CGP).
• Determine whether the pond will be used as a temporary sediment basin during construction.
• Decide where on the site the pond is most likely to be located.

Warning: If the pond will be used for temporary sediment control during construction, the associated permanent pool volume REQUIRED is either the 2 year, 24 hour storm runoff volume draining to the pond (with minimum 1800 cubic feet for each acre draining to the basin), or in the absence of such a calculation, 3600 cubic feet for each acre draining to the basin. It is also REQUIRED that sediment deposited during construction be removed before normal operation begins (refer to MPCA Permit for additional design requirements).

Step 2. Confirm design criteria and applicability.

A. Determine whether the pond must comply with the MPCA CGP.

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. Confirm site suitability.

A. Perform field verification of site suitability.

• If the initial evaluation indicates that a pond would be a good BMP for the site, it is Recommended that one boring per acre with a minimum of three soil borings or pits be dug in the same location as the proposed pond to verify soil types and to determine the depth to ground water 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 pond.
• It is Highly Recommended that the field verification be conducted by a qualified geotechnical professional.

B. Perform water balance calculations, if needed.

Schematic plan drawing illustrating pond storage allocations. Allocations include the permanent pool (V_pp), water quality volume (V_wq), channel protection volume (V_cp), overbank flood volume (V_p10), and extreme flood volume(V_p100). A typical pond plan and profile CADD file exists for this layout.

Step 4. Compute runoff control volumes and permanent pool volume.

Calculate the Permanent Pool Volume (V_pp), Water Quality Volume (V_wq), Channel Protection Volume (V_cp), Overbank Flood Protection Volume (V_p10), and the Extreme Flood Volume (V_p100)(see Unified sizing criteria).

\(V_{pp} = 1800 * A\)

or

\(V_{pp} = 0.0417 A \)

Warning: If the pond is being designed as a wet detention pond for new construction under the MPCA CGP Permit, then a permanent pool volume equal to 1800 cubic feet for each acre draining to the pond is Required.

In the case where the entire V_wq is to be treated with other BMPs and the pond is being constructed only for rate control, a permanent pool may not be required, although it still may be desirable.

Warning: If the pond will be used for temporary sediment control during construction, the associated permanent pool volume Required is either the 2 year, 24 hour storm runoff volume draining to the pond (with minimum 1800 cubic feet for each acre draining to the basin), or in the absence of such a calculation, 3600 cubic feet for each acre draining to the basin.

Warning: It is also Required that sediment deposited during construction be removed before normal operation begins (refer to MPCA Permit for additional design requirements).

The water quality volume, V_wq, can be calculated in different ways, depending upon what it discharges to a water:

For normal waters

\(V_{wq} = 0.0833 IC\)

For special waters

\(V_{wq} = 0.833 * IC\)

where

IC = new impervious area (acres)

It is Recommended that the Channel Protection Volume, (V_cp), be based on the 1-yr, 24-hr rainfall event, though local ordinances may be more restrictive. It should be noted that the V_cp is inclusive of the V_wq. In other words, the V_wq is contained within the V_cp.

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 pond. It is assumed that the pond will be the only BMP used for rate control for larger storms. If this is the case, the pond should be designed to treat the entirety of these runoff control volumes. If some portion of these control volumes is treated by other BMPs, it can be subtracted from the overall V_cp, Overbank Flood Protection Volume (V_p10), and Extreme Flood Control Criteria (V_p100) to determine the volume to be treated by the pond.(see figure to right)

Step 5. Determine location and preliminary geometry.

The preliminary grading plan can be developed with the following procedure:

1. Locate the pond in the lowest elevation area of the site (not in a jurisdictional wetland) and provide space around the pond for maintenance access.

1. 10 foot minimum width is Highly Recommended.
2. Establish a primary outlet elevation (normal water level) and/or a pond bottom elevation.
3. Provide storage for the permanent pool below the primary outlet elevation in the main pond area.
4. Include an aquatic bench extending into the permanent pool and a access bench extending out from the permanent pool.
5. Considering the desired pond footprint during the V_wq, V_cp, V_p10, and V_p100 design storms, allocate storage volume above the primary outlet elevation for V_wq, V_cp, V_p10, and V_p100, respectively. While developing the grading plan, consider the desired (or required) length to width ratio and side slopes detailed earlier in this section (or in applicable regulations).
6. Once the preliminary grading plan has been developed, determine the associated stage-storage relationship for water surface elevations through the maximum expected levels.

Warning: Adequate maintenance access (typically 8 feet) is Required.

The approximate storage corresponding to a given stage (elevation) can be determined using the average end area method. The area within each of the closed contour lines on the grading plan representing the pond is measured, and the average area of each set of adjacent contours is computed. The approximate volume between the two contours is then calculated as the average area multiplied by the elevation difference.

\(V_{1-2} = ((A_1 + A_2)/2) (E_2 - E_1)\)

where:

V_1-2 = the volume between contour 1 and contour 2;

A₁ and A₂ = the areas within closed contours 1 and 2, respectively; and

E₂ and E₁ = the elevations of contours 1 and 2, respectively.

Cumulative volume above the bottom of the pond, or above the normal water surface elevation, can be calculated by adding subsequent incremental volumes. This is readily accomplished with the use of a spreadsheet prepared as follows in the table below (the first row of the table below contains the spreadsheet column header, the second row is column description, and the third, fourth, and fifth rows provide an example, with a permanent pool elevation of 902).

The table below is an example spreadsheet - cumulative volume above normal surface elevation.

This table shows an example spreadsheet - Cumulative volume above normal surface elevation.
Link to this table

Spreadsheet Column Header	Elevation	Area	Average Area	Depth	Volume	Cumulative Volume	Volume Above Permanent Pool
Spreadsheet Column Description	Elevation of Pond Contour Line	Area enclosed by Contour Line	Average area of current and previous rows	Elevation difference between current and previous rows	Volume between current and previous contour	Volume between current and lowest contour	Volume between current and permanent pool contour
Example value	900	1000	N/A	N/A	N/A	0	N/A
Example value	902	1200	1100	2	2200	2200	0
Example value	904	1600	1400	2	2800	5000	2800

The stage-storage relationship will be used to develop a stage-storage-discharge table as outlet structures are designed. This is an iterative process that may include revising the preliminary grading plan and subsequently redetermining the stage-storage relationship (or using an acceptable model to check).

Step 6. Determine pre-treatment (sediment forebay) volume

In the absence of adequate upstream treatment by other BMPs, it is Highly Recommended that a sediment forebay or similarly effective pre-treatment system be provided at each inlet providing 10 percent or more of the total design inflow, with a Recommended volume equal to 10 percent of the permanent pool volume in a pool 4 to 6 feet deep (at shallower depths, the risk of sediment resuspension in the pre-treatment area increases). The forebay storage volume counts toward the total permanent pool requirement. The storage volumes from other BMPs used upstream in the treatment train count toward the water quality volume (Vwq) requirement and thus may be subtracted from it.

Step 7. Consider water quality treatment volume variations for frozen conditions

When the pond and sediment forebay are frozen, much of the storage is rendered ineffective because stormwater runoff can flow over the ice and bypass the intended treatment. To alleviate this problem, additional extended detention storage (which is available even under frozen conditions) can be designed into the pond by increasing the extended detention storage volume designated for water quality control, or by adding a weir structure to the sediment forebay overflow area.

This schematic illustrates seasonal operation for snowmelt runoff management.

The average snowmelt volume can be computed from the following equation

\(A_{sv} = (A_{sd} S_{nw}) - I_{vol}\)

where:

A_sv = Average snowmelt volume (depth/unit area)

A_sd = Average snowpack depth at the initiation of the snowmelt period

S_nw = Typical snowpack water at time of melt

I_vol = Estimated infiltration volume likely to occur during a 10-day melt period.

A series of maps that will allow the designer to determine the average depth of snowpack existing at the start of spring snowmelt, the water content of the snowpack during the month of March, and the expected infiltration.

Step 8: Size and design outlet structures

The following outlet stages should be included in the pond design. It is possible to design one device to meet all stages. Equations included in this step are based on certain assumptions about which types of outlet structures will be used to control the various stages. If the designer chooses to use different structure types, the specific equations used to determine stage-discharge relationships will change, but the general approach will remain the same.

Emergency drain: a drawdown pipe sized to drain the pond within 24 hours to allow access for riser repairs and sediment removal, or to lower the permanent pool in late fall (to provide additional storage during frozen conditions)

Water quality (low flow) outlet: an outlet (typically an orifice) designed to release Vwq with an average detention time of 12 hours. After designing the orifice, a check should be made to verify that the release rate is no greater than 5.66 cfs/acre of pond surface area. (Calculation steps adapted from Vermont Stormwater Management Manual.)

The average release rate for V_wq is computed as

\(Q_{wq_avg} = V_{wq} / t_{wq}\)

where:

t_wq = the intened V_wq detention time.

From the stage-storage table, find the elevation associated with V_wq. Calculate the approximate average head on the water quality outlet as

\(h_{wq_avg} = (E_{wq} - E_{PermPool}) / 2\)

where:

E_wq = the V_wq pool elevation; and

E_PermPool = the elevation of the permanent pool (the invert of the water quality orifice)

The required orifice cross sectional area can then be indirectly computed using the orifice equation

\(Q_{wq_avg} = CA_{wq} \sqrt{2gh _{wq avg}}\)

where:

C = the orifice coefficient (0.6 is typically used, but may not apply in all cases);

A_wq = the orifice area; and

g = gravitational acceleration.

The diameter of the orifice is then d_wq = 2(feet)

\(d_{wq} = 2 \sqrt{Awq / \pi}\)

The rate of discharge from the orifice for any head value hwq on the orifice can then be computed as

\(Q_{wq} = CA_{wq} \sqrt{2ghwq}\)

Channel protection outlet: an outlet designed to release V_cp over a period of 24 hours (minimum Vcp detention time is recommended to be 12 hours). The V_cp pool elevation can be read from the pond stage-storage relationship.

Assuming an orifice is also used to release V_cp, the invert of the V_cp orifice may be placed at the V_wq pool elevation (E_wq).

The average release rate for V_cp is computed as

\(Q_{cp_avg} = (V_{cp} - V_{wq} / t_{cp}) - Q_{wq}\)

where:

t_cp = the intended channel protection volume detention time

Q_wq= computed (using the above equation for Q_wq), with the head value calculated as

\(h_{wq} = (E_{wq} + E_{cp} / 2) - E_{PermPool}\)

From the stage-storage table, find the elevation associated with V_cp. The average head on the channel protection outlet can then be calculated as

\(h_{cp_avg} = (e_{cp} - E_{wq}) / 2\)

Again, the required orifice cross sectional area can then be indirectly computed using the orifice equation

\(Q_{cp_avg} = CA_{cp} \sqrt{2ghcp avg}\)

The diameter of the orifice is then

\(D_{cp} = 2\sqrt{Acp / \pi}\)

The rate of discharge from the channel protection orifice for any head value hcp on the channel protection outlet can then be computed as

\(Q_{cp} = CA_{cp} \sqrt{2ghcp}\)

The combined flow out of the water quality orifice and channel protection orifice at a given water surface elevation can be computed by adding together the discharges from the two structures, for the head values corresponding to the specified water surface elevation.

• Overbank flood control outlet: an outlet (typically a weir) with its invert at the V_cp pool elevation, designed to release Q_p10 at pre-development rates (recommended).
• Extreme storm control outlet: an outlet with its invert at or slightly above the V_p10 pool elevation, designed to release Q_p100 at pre-development rates (recommended), or at minimum to safely pass the Q_p100 with 1 to 2 feet of freeboard below the top of the embankment. Check with local officials to determine whether a principal spillway can be used to manage extreme storm flows, or if an emergency spillway (broad-crested weir or earthen embankment, not susceptible to obstruction) is necessary.

Using the determined size information, incorporate the outlet structures into the pond design. Be aware of concerns associated with frozen conditions, particularly the risk of clogging or blockage of outlet structures with ice and the importance of burying pipes below the frost line.

Warning: A skimmer or similar device is REQUIRED to prevent the discharge of floating debris.

Step 9: Design spillway and embankments

The NRCS has compiled additional design guidance and requirements for spillways and embankments (NRCS Pond 378 Conservation Practice Standard for Minnesota. The following items are some of the key guidelines to adhere to in the design of spillways and embankments.

• It is Required that the emergency overflow be stabilized.
• It is Required that embankments be overfilled by at least 5 percent to account for settling.
• The Required minimum embankment width is 6 feet (wider for embankment heights greater than 10 feet or if maintenance access will be required).
• It is Required that embankments be adequately stabilized with vegetation or other measures.
• It is Highly Recommended that side slopes be no steeper than 1:3 (V:H).

Step 10: Design inlets

To prevent freezing and blockage of inflow, it is Highly Recommended that inlet pipes not be fully submerged and that they be buried below the frost line. The Minnesota Department of Transportation has developed frost and thaw depths for several Minnesota sites.

It is also Highly Recommended to design the inlet to reduce or prevent scour, by including riprap or flow diffusion devices such as plunge pools or berms.

Step 11: Design sediment forebay

The size of the sediment forebay was determined in Step 6. It is Recommended that a sediment marker be included in the forebay to indicate the need for sediment removal in the future. A hard bottom surface in the forebay is also Recommended in order to make sediment removal easier.

As discussed in Step 6, a weir structure added to the forebay will ensure that some pre-treatment storage is available, even when the normal forebay is frozen.

Step 12: Design maintenance access and safety features

Warning: Maintenance access to the pond, forebay, and inlet and outlet structures is Required.

The access routes should be designed with a minimum 8 feet width and maximum 15 percent slope.

Safety features such as obstructive planting that make access difficult, signs warning against fishing and swimming, fencing, and grates over outlet structures should be included as appropriate. Aesthetic enhancements such as trails or benches can also be included.

Warning: If an outlet structure is greater than five feet deep, it is Required that OSHA health and safety guidelines be followed for safe construction and access practices.

Additional information on safety for construction sites is available from OSHA. OSHA has prepared a flow chart which will help site owners and operators determine if the site safety plan must address confined space procedures.

Step 13: Check expected pond performance against regulatory requirements.

Check that V_wq is detained for an average of 12 hours.

Check that the V_wq release rate does not exceed 5.66 cubic feet per second per acre (cfs/acre) of pond area.

Determine applicable requirements for V_cp volume and release rate, and verify that the pond performs adequately for the appropriate design event.

Determine applicable requirements for Q_p10 and Q_p100 release rates (e.g., pre-development rates), and check pond release rates (and freeboard) for the appropriate design events.

Step 14: Prepare vegetation and landscaping plan.

A landscaping and planting plan by a qualified professional for the pond and surrounding area should be prepared, utilizing native vegetation wherever possible.

Step 15: Prepare operation and maintenance plan.

Preparation of a plan for operation and maintenance of the pond and associated structures and landscaping is Required. See the Operation and Maintenance section for further details.

Step 16: Prepare cost estimate.

Refer to the Cost Considerations section for information on preparing a cost estimate for stormwater ponds.

See CADD designs for design details for pond systems. The following details, with specifications, have been created for stormwater ponds:

• Stormwater pond plan
• Stormwater pond profile
• Pond inlet
• Riser pipe outlet structure