This section describes information on design of stormwater ponds. CADD drawings can be found at this link
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.
Pond outfalls should be designed to not increase erosion or have undue influence on the downstream geomorphology of the stream.
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% 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.
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 ground water would otherwise occur but must be avoided. This includes:
If geotechnical tests confirm the need for a liner, acceptable options include: (a) six to 12 inches of clay soil, including bentonite, (minimum 15% passing the #200 sieve and a maximum permeability of 1 x 10-5 cm/sec), (b) a 30 ml poly-liner, or (c) engineering design as approved on a case-by-case basis by MPCA or appropriate review agency.
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:
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.
It is Required that a maintenance access, with a minimum width of 8’, 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%, and be appropriately stabilized for use by maintenance equipment and vehicles. Steeper grades may be allowable if designed using appropriate materials for the grade.
It is Required that the maintenance access extends to the forebay, access bench, riser, and outlet, and allows vehicles to turn around.
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.
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.
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") or by internal orifice protection that may allow for smaller diameters (recommended minimum orifice of 1"). 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” 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”, especially when the slot is tall. It is Recommended that hoods over orifices be oversized to account for ice formation.
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.
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 12” 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.
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.
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.
It is Required that an emergency spillway should be provided to pass storms in excess of the pond hydraulic design. 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
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 (see bioretention specifications).
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.
It is Required (Minnesota Department of Health Rule 4725.4350) that a 50’ setback between high water levels of stormwater ponds and public water supply wells be provided.
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.
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.
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:
B. Determine how the pond will fit into the overall stormwater treatment system.
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.
A. Perform field verification of site suitability.
B. Perform water balance calculations, if needed.
Calculate the Permanent Pool Volume (Vpp), Water Quality Volume (Vwq), Channel Protection Volume (Vcp), Overbank Flood Protection Volume (Vp10), and the Extreme Flood Volume (Vp100)(see Uniform sizing criteria).
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 Vwq 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.
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).
The water quality volume, Vwq, can be calculated in different ways, depending upon what it discharges to a water:
where IC = new impervious area (in)
It is Recommended that the Channel Protection Volume, (Vcp), be based on the 1-yr, 24-hr rainfall event, though local ordinances may be more restrictive. It should be noted that the Vcp is inclusive of the Vwq. In other words, the Vwq is contained within the Vcp.
If part of the overall Vwq is to be treated by other BMPs, subtract that portion from the Vwq to determine the part of the Vwq 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 Vcp, Overbank Flood Protection Volume (Vp10), and Extreme Flood Control Criteria (Vp100) to determine the volume to be treated by the pond.(see figure to right)
The preliminary grading plan can be developed with the following procedure:
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.
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 Table 12.8.6 (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).
Table 12.8.6: Example spreadsheet - cumulative volume above normal surface elevation.
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).
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% or more of the total design inflow, with a Recommended volume equal to 10% 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.
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 (see Figure 12.POND.1).
The average snowmelt volume can be computed from the following equation:
A series of maps have been prepared in Chapter 2 (Figures 2.5 - 2.7) 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.
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. For this discussion, an emergency drain is 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), while a water quality (low flow) outlet is 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 Vwq is computed as:
From the stage-storage table, find the elevation associated with Vwq. Calculate the approximate average head on the water quality outlet as:
The required orifice cross sectional area can then be indirectly computed using the orifice equation
The rate of discharge from the orifice for any head value hwq on the orifice can then be computed as:
Channel protection outlet: an outlet designed to release Vcp over a period of 24 hours (minimum Vcp detention time is recommended to be 12 hours). The Vcp pool elevation can be read from the pond stage-storage relationship.
Assuming an orifice is also used to release Vcp, the invert of the Vcp orifice may be placed at the Vwq pool elevation (Ewq).
The average release rate for Vcp is computed as
From the stage-storage table, find the elevation associated with Vcp. The average head on the channel protection outlet can then be calculated as
Again, the required orifice cross sectional area can then be indirectly computed using the orifice equation:
The diameter of the orifice is then
The rate of discharge from the channel protection orifice for any head value hcp on the channel protection outlet can then be computed as
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.
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.
A skimmer or similar device is Required to prevent the discharge of floating debris.
The following items are some of the key guidelines to adhere to in the design of spillways and embankments.
The NRCS has compiled additional design guidance and requirements for spillways and embankments (NRCS Pond 378 Conservation Practice Standard for MinnesotaP.
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.
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.
Maintenance access to the pond, forebay, and inlet and outlet structures is Required. The access routes should be designed with a minimum 10’ width and maximum 15% 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.
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.
Check that Vwq is detained for an average of 12 hours.
Check that the Vwq release rate does not exceed 5.66 cfs/acre of pond area.
Determine applicable requirements for Vcp volume and release rate, and verify that the pond performs adequately for the appropriate design event.
Determine applicable requirements for Qp10 and Qp100 release rates (e.g., pre-development rates), and check pond release rates (and freeboard) for the appropriate design events.
A landscaping and planting plan by a qualified professional for the pond and surrounding area should be prepared, utilizing native vegetation wherever possible.
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.
Refer to the Cost Considerations section for information on preparing a cost estimate for stormwater ponds. Construction
See CADD designs for design details for pond systems. The following details, with specifications, have been created for stormwater ponds: