Performance Criteria: Stormwater Ponds
Stormwater ponds are practices that have a combination of a permanent pool, extended detention or shallow marsh equivalent to the entire WQv. Design variants include:
This section presents criteria for:
A summary of the design criteria is provided in Table 1. The table separates required or minimum design elements from design criteria that would act primarily as guidance.
Notes: Dry extended detention ponds that have no permanent pool are not considered an acceptable option for meeting WQv due to poor pollutant removal and chronic maintenance problems.
The term "pocket" refers to a pond or wetland that has such a small contributing drainage area that little or no baseflow is available to sustain water elevations during dry weather. Instead, water elevations are heavily influenced and, in some cases, maintained by a locally high water table.
Stormwater ponds can also be used to provide Channel Protection volume as well as overbank and extreme flood attenuation.
Table 1. Design Criteria: Stormwater Ponds
Please note that judgement is needed to separate minimum design elements from guidance. When crafting a local or state design manual, the jurisdiction will need to go through a process involving stakeholders to select which design elements are necessary in all cases.
Pond Feasibility Criteria
Stormwater ponds should have a minimum contributing drainage area of ten acres or more (25 or more are preferred), unless groundwater is confirmed as the primary water source (i.e., pocket pond).
This requirement ensures that the permanent pond cab be maintained by the runoff from the contributing drainage. A water balance analysis may replace this requirement. In addition, the specific drainage area requirements will vary based on regional rainfall and temperature.
Stormwater ponds should not be located within jurisdictional waters, including wetlands.
Dam safety review may be required for stormwater ponds. Stormwater ponds are typically required to be classified as a "Low Hazard" for dam impact.
Consult dam safety requirements to specify the review process for your locality. Also refer to the construction specifications section for an example.
Pond Conveyance Criteria
Conveyance should be provided which does not cause erosion.
When reinforced concrete pipe is used for the principal spillway to increase its longevity, "O-ring" gaskets (ASTM C361) should be used to create watertight joints.
A forebay should be provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the pond.
Inlet areas should be protected to reduce erosion.
Inlet pipes to the pond can be partially submerged.
Note: In cold climates, criteria may be changed to disallow submerged inlets.
Adequate Outfall Protection
Outfalls should be constructed such that they do not increase erosion or have undue influence on the downstream geomorphology of the stream.
Flared pipe sections that discharge at or near the stream invert or into a step-pool arrangement should be used at the spillway outlet.
The channel immediately below the pond outfall shall be modified to prevent erosion and conform to natural dimensions in the shortest possible distance, typically by use of large rip-rap placed over filter cloth.
A stilling basin or outlet protection shall be used to reduce flow velocities from the principal spillway to non-erosive velocities.
Consult Soil and Water Conservation Engineering (Schwab et al.) for data on erosive velocities.
If a pond daylights to a channel with dry weather flow, care should be taken to minimize tree clearing along the downstream channel, and to reestablish a forested riparian zone in the shortest possible distance. Excessive use of rip-rap should be avoided to reduce stream warming.
When a pond is located in karst topography, gravelly sands or fractured bedrock, a liner may be needed to sustain a permanent pool of water. If geotechnical tests confirm the need for a liner, acceptable options include: (a) 6 to 12 inches of clay soil (minimum 15% passing the #200 sieve and a minimum permeability of 1 x 10-5 cm/sec), (b) a 30 ml poly-liner (c) bentonite, or (d) use of chemical additives (see NRCS Agricultural Handbook No. 387, dated 1971, or Engineering Field Manual).
In some regions, a pond liner may be required for all ponds, particularly for protection of sole source aquifers.
Pond Pretreatment Criteria
A sediment forebay is important for maintenance and longevity of a stormwater treatment pond.
Each pond should have a sediment forebay or equivalent upstream pretreatment. The forebay should consist of a separate cell, formed by an acceptable barrier.
The forebay shall be sized to contain 0.1 inches per impervious acre of contributing drainage, and should be 4 to 6 feet deep. The forebay storage volume can count toward the total WQv requirement. Exit velocities from the forebay should be non-erosive.
Direct maintenance access for appropriate equipment should be provided to the forebay.
A fixed vertical sediment depth marker should be installed in the forebay to measure sediment deposition over time.
The bottom of the forebay may be hardened to make sediment removal easier.
Pond Treatment Criteria
Provide water quality treatment storage to capture the computed WQv from the contributing drainage area through any combination of permanent pool, extended detention (WQv-ED) or marsh.
If ED is provided in a pond, storage for Cpv-ED and Wqv-ED shall be computed and routed separately (i.e., the WQv cannot be met simply by providing Cpv storage for the one year storm).
In the Wet Extended Detention Pond Design, at least 50% of the (WQv) should be stored in the permanent pool.
It is generally desirable to provide water quality treatment off-line when topography, head and space permit (i.e., apart from stormwater quantity storage).
Water quality storage can be provided in multiple cells. 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, ED, and marsh).
Minimum Pond Geometry
The minimum length to width ratio for the pond is 1.5:1 (i.e., length relative to width). Long flow paths and irregular shapes are recommended.
Pond Landscaping Criteria
The perimeter of all deep pool areas (four feet or greater in depth) should be surrounded by two benches:
A safety bench that extends 15 feet outward from the normal water edge to the toe of the pond side slope. The maximum slope of the safety bench shall be 6%.
An aquatic bench that extends up to 15 feet inward from the normal shoreline and has a maximum depth of eighteen inches below the normal pool water surface elevation.
Pond Benches can be important for safety reasons and establishment of emergent vegetation.
A landscaping plan for a stormwater pond and its buffer should be prepared to indicate how aquatic and terrestrial areas will be vegetatively stabilized and established.
Manuals should also provide landscaping guidance for stormwater ponds, including lists of appropriate plant species.
Wherever possible, wetland plants should be encouraged in a pond design, either along the aquatic bench (fringe wetlands), the safety 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 is 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.
As a rule of thumb, planting holes should be 3 times deeper and wider than the diameter of the rootball (of balled and burlap stock), and 5 times deeper and wider for container grown stock. This practice should enable the stock to develop unconfined root systems. Avoid species that require full shade, are susceptible to winterkill, or are prone to wind damage. 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
Pond buffers can be important in providing ample space for access and safety. The buffer can be planted or left in trees to discourage resident goose populations.
A pond buffer should be provided that extends 25 feet outward from the maximum water surface elevation of the pond. The pond buffer should be contiguous with other buffer areas, that are required by existing regulations (e.g., stream buffers). An additional setback may be provided to permanent structures.
Woody vegetation may not be planted on allowed to grow within 15 feet of the toe of the embankment and 25 feet from the principal spillway structure.
The purpose of this regulation is to prevent failure of the embankment or principal spillway.
Existing trees should be preserved in the buffer area during construction. It is desirable to locate forest conservation areas adjacent to ponds. To discourage resident geese populations, the buffer can be planted with trees, shrubs and native ground covers.
Annual mowing of the pond buffer is only required along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow (mowing every other year) or forest.
Pond Maintenance Criteria
Establishment of a maintenance plan is an important aspect of Stormwater Treatment Practice (STP) guidance. Maintenance is essential to the proper functioning of STPs. Having a financial mechanism or authority responsible for the maintenance of the facility from the outset is imperative.
Maintenance responsibility for a pond and its buffer should be vested with a responsible authority by means of a legally binding and enforceable maintenance agreement that is executed as a condition of plan approval.
The principal spillway should be equipped with a removable trash rack.
Sediment removal in the forebay should occur every 5 to 7 years or after 50% of total forebay capacity has been lost.
Sediments excavated from stormwater ponds that do not receive runoff from designated hotspots are not considered toxic or hazardous material, and can be safely disposed by either land application or land filling. Sediment testing may be required prior to sediment disposal when a hotspot land use is present.
Sediment removed from stormwater ponds should be disposed of according to an approved erosion and sediment control plan.
A maintenance right of way or easement should extend to a pond from a public or private road.
Maintenance access shall be at least 12 feet wide; have a maximum slope of no more than 15%; and should be appropriately stabilized to withstand maintenance equipment and vehicles.
The maintenance access should extend to the forebay, safety bench, riser, and outlet and be designed to allow vehicles to turn around.
Non-clogging Low Flow Orifice
A non-clogging low flow orifice must be provided.
The low flow orifice shall have a minimum diameter of 3 inches, and shall be adequately protected from clogging by an acceptable external trash rack.
This minimum may be increased in cold climates to deal with issues related to freezing pipes. On the other hand, smaller diameters may be allowed if internal orifice protection is provided.
The preferred method is a submerged reverse-slope pipe that extends downward from the riser to an inflow point one foot below the normal pool elevation.
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.
The use of horizontal perforated pipe protected by geotextile and gravel is not recommended. Vertical pipes may be used as an alternative if a permanent pool is present.
Riser in Embankment
The riser shall be located within the embankment for maintenance access, safety and aesthetics.
Access to the riser is to be provided by lockable manhole covers, and manhole steps within easy reach of valves and other controls (the principal spillway opening can be "fenced" with pipe or rebar at 8 inch intervals for safety purposes).
Each pond shall have a drain pipe that can completely or partially drain the pond. The drain pipe shall have an elbow within the pond to prevent sediment deposition, and a diameter capable of draining the pond within 24 hours.
Care shall be exercised during pond drawdowns to prevent downstream discharge of sediments or anoxic water and rapid drawdown. The approving jurisdiction shall be notified before draining a pond.
Adjustable Gate Valve
Both the WQv-ED pipe and the pond drain shall be equipped with an adjustable gate valve (typically a handwheel activated knife gate valve).
Valves shall be located inside of the riser at a point where they (a) will not normally be inundated and (b) can be operated in a safe manner.
Both the WQv-ED pipe and the pond drain shall be sized one pipe size greater than the calculated design diameter.
To prevent vandalism, the handwheel shall be chained to a ringbolt, manhole step or other fixed object.
The adjustable gate valve allows for rapid use of the pond drains and also allows for adjustment of the WQv-ED pipe to meet appropriate detention times.
Fencing of ponds is not generally desirable, but may be required by the local review authority. A preferred method is to manage the contours of the pond to eliminate dropoffs and other safety hazards.
Side slopes to the pond shall not exceed 3:1 (h:v), and shall terminate on a safety bench. Both the safety bench and the aquatic bench may be landscaped to prevent access to the pool. The bench requirement may be waived if slopes are 4:1 or gentler.
The principal spillway opening shall not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.
Warning signs prohibiting swimming and skating may be posted.