Design of Stormwater Ponds

Design of Stormwater Ponds
and Wetlands Center for Watershed Protection This presentation provides general design guidance and performance criteria that apply to the design and construction of stormwater ponds and wetlands. A series of 20 standard features of ponds and wetlands are discussed in detail. It is assumed that the user has some design background and is familiar with general terminology associated with stormwater treatment practices. Ponds and wetlands are stormwater treatment practices that generally provide both water quality and water quantity storage. Water quality storage volume should be at least equivalent to the entire water quality volume through a combination of a permanent pool, extended detention or shallow marsh. Design variations include: micropool extended detention ponds, wet ponds, wet extended detention ponds, multiple pond systems, and pocket ponds. Dry extended detention ponds that have no permanent pool are not considered to be acceptable for meeting water quality goals; however, dry ponds may have a role in providing flood control and channel protection storage. The following four slides are examples of the various types of designs.

This is an example of a wet pond.
Copyright 2000, CWP

This is an example of a shallow marsh wetland.

This slide shows an example of a multiple cell pond system shortly after construction.
Copyright 2000, CWP

This is a shallow marsh extended detention wetland
This is a shallow marsh extended detention wetland. Note the forebay in the foreground and the safety bench (best seen in the background near the riser structures). Copyright 2000, CWP

Features of the Standard Pond System
Adequate Water Quality Treatment Volume Multiple Treatment Pathways Minimum Pond Geometry Sediment Forebay Non-clogging Low Flow Orifice Riser in Embankment Adjustable Gate Valve Pond Drain Principal Spillway Emergency Spillway The following two slides provide a list of 20 features of standard pond systems that will be discussed in detail during this presentation. The list should be considered a minimum set of design considerations for properly functioning ponds and wetlands.

Features of the Standard Pond System (cont.)
Embankment Specifications Inlet Protection Adequate Outfall Protection Pond Benches Safety Features Pondscaping Plan Wetland Elements Pond Buffers and Setbacks Maintenance Measures Maintenance Access

#1 Adequate Water Quality Treatment Volume
Provide water quality treatment storage for the 90th percentile storm depth per impervious acre For extended detention (ED) ponds, permanent pool should be equal to at least one half the water quality treatment volume Forebay should be sized to provide volume equal to at least 0.1 inch per contributing impervious acre for pretreatment Water quality storage can be provided in multiple cells Provide water quality treatment off-line when topography, head and space permit Stormwater ponds have some of the best water quality performance capabilities of any stormwater treatment practice. This is in large part due to the residence time and settling properties of the permanent pool. Several studies have documented that the majority of the annual pollutant load associated with stormwater runoff is associated with the smaller more frequent events, and that the smaller sized sediment particles carry most of the pollutant load. Consequently, it is recommended that water quality treatment volumes target the smaller storms and that adequate pretreatment is provided. For many of the humid areas of the country, about 90 percent of all rainfall events are in the range of approximately 1 inch. In the absence of a more rigorous rainfall frequency analysis, 1 inch of rainfall per impervious acre is a reasonable criteria to shoot for.

Estimated Sediment Deposition Rate
This slide illustrates estimated sediment deposition rates of stormwater ponds and wetlands. Sedimentation is the primary pollutant removal mechanism in ponds and wetlands, and these data suggest that in-stream ponds and wetlands are less effective at promoting sedimentation than off-stream designs. While generally not as significant, biological uptake by plants and algae is another important pollutant removal mechanism in ponds and wetlands. Copyright 2000, CWP

Pollutant Removal Performance of Stormwater Ponds and Wetlands: By Pond Type
This slide illustrates the pollutant removal capabilities of stormwater ponds and wetlands. The data are taken from the Center for Watershed Protection National Pollutant Removal Performance Database (2000). Copyright 2000, CWP

Irreducible Concentrations in Wastewater Wetlands and Urban Best Management Practices
Water quality parameter (mg/l) Wastewater (Kadlec and Knight, 1996) Wastewater (Reed, 1995) Stormwater BMPs (CWP, 2000) There is a limit to the level of treatment that stormwater ponds and wetlands can provide, sometimes referred to as irreducible concentrations. This concept was originally recognized in process models used to design wastewater treatment wetlands. Essentially, it was determined that wastewater wetlands cannot reduce sediment and nutrient concentrations beyond the rather low levels indicated in the first two columns of data on the slide no matter how much more surface area or treatment volume is provided. The Center’s STP database median effluent concentrations are reported in the fourth column to characterize effluent from STPs.

#2 Multiple Treatment Pathways
Provide multiple or redundant treatment Use multiple cells, longer flowpaths, high surface area to volume ratios, and complex microtopography Several design considerations enhance the pollutant removal performance of stormwater ponds and wetlands. Designers should incorporate these features to the maximum extent practicable.

This slide shows an example of a multiple cell wet pond.
Copyright 2000, CWP

#3 Minimum Pond Geometry
Ponds should be wedge-shaped, narrowest at the inlet, and widest at the outlet Minimum length to width ratio should be 1.5:1 (i.e., length equal to 1.5 x width). Greater flowpaths are recommended Maximum depth of the permanent pool should not exceed eight feet (average of 4 to 6 feet) The geometry of ponds and wetlands can also have a significant impact on the pollutant removal performance. The next two slides are representative of the recommendations presented in this slide.

Copyright 2000, CWP Copyright 200, CWP
This slide presents more TSS removal data for ponds and wetlands based on the unit treatment volume provided. Note that pollutant removal is only loosely related to total treatment volume, suggesting that other design factors such as pond geometry are equally important in defining removal rates. Copyright 2000, CWP Copyright 200, CWP

#4 Sediment Forebay - I Each pond shall have a sediment forebay, consisting of a separate cell Size forebay to contain 0.1 inches per impervious acre. Maintain forebay at 3 to 6 feet deep Exit velocities from the forebay shall not be erosive Pretreatment is a critical component of the success and longevity of stormwater pond and wetland performance. Pretreatment is best provided in the form of a sediment forebay, which can also serve to accommodate some portion of the water quality volume. The next slide presents a well-designed forebay.

#4 Sediment Forebay - II Direct maintenance access by heavy equipment should be provided to the forebay Harden forebay bottom to make sediment removal easier Provide a vertical sediment depth marker in the forebay, to measure sediment deposition over time Properly functioning forebays will require maintenance in the form of sediment removal much more frequently than the permanent pool areas. Consequently, it is important to provide adequate access for heavy equipment, as shown in the next slide.

#5 Non-clogging Low Flow Orifice
Provide a submerged, reverse-slope pipe, extending downward from the riser to a release point one foot below the normal pool elevation Low flow orifice shall have a minimum internal diameter of 3 inches Broad crested weirs, protected by a half-round CMP, extending at least 18 inches below the normal pool can also be used One of the most common maintenance and performance problems associated with ponds and wetlands is the clogging of the low flow orifice(s). There are several design specifications that can minimize the likelihood of clogging including reverse slope pipes and “hooded” outlets.

This slide illustrates schematically the reverse slope pipe in a typical ED pond. Other features depicted in the schematic will be discussed in more detail later in the presentation. Copyright 2000, CWP

Here is the headwall of a reverse slope pipe that discharges to the riser in the background. The basin is under construction, and this orifice will be below the permanent pool one construction is complete. Copyright 2000, CWP

This slide shows a weir outlet structure with a half-round CMP hood.
Copyright 2000, CWP

#6 Riser in Embankment Locate riser within the embankment for purpose of maintenance, access, safety and aesthetics Access to the riser by manholes with lockable nuts The riser can be “fenced” with pipe or rebar at inch intervals for safety purposes and to prevent large trash and debris from entering the riser Recommended riser design features include the following points.

Here is an example of a riser located in the embankment of a wet pond.
Copyright 2000, CWP

This slide illustrates the trash rack feature and manhole access to a riser structure.
Copyright 2000, CWP

#7 Adjustable Gate Valve
Both the ED pipe and pond drain pipe should be equipped with an adjustable gate valve Size one pipe schedule higher than the calculated design diameter Locate valves inside of the riser, where they will remain dry and can be operated safely and conveniently Lock the handwheel with a chain to a ringbolt, manhole step, or other fixed object Gate valves are used on the pond drain to drain the pond in emergency situations, or for emergency access. On the ED outlet, gate valves are used to provide the equivalent diameter necessary to provide the required detention time.

This slide shows a gate valve on an ED pipe on the inside of a riser.
Copyright 2000, CWP

This slide illustrates a secured handwheel.
Copyright 2000, CWP

#8 Pond Drain Each pond shall have a ductile iron drain pipe to completely or partially drain the pond Drain pipe should have an inverted elbow within the pond to prevent sediment clogging The diameter of the pipe shall be sufficient to drain the pond within 24 hours Exercise care during pond drawdowns to prevent downstream discharge of sediments or anoxic water It is important to provide a pond or wetland drain to allow for full or at least partial drawdown of the facility for maintenance and safety reasons. Specific design considerations are identified in this slide.

This slide shows a drain with a gate valve inside a riser.
Copyright 2000, CWP

#9 Principal Spillway - I
Principal spillways shall be designed in accordance with local embankment specifications. NRCS Pond Specifications Code are often available locally and serve as good guidance Provide capacity to accommodate the design storm with adequate height to the crest of the emergency spillway Crest elevation of the principal spillway shall be no less than one foot below the emergency spillway crest The next two slides provide design guidance for principle spillways. It is important to be familiar with the local requirements which may include peak discharge control and minimum freeboard requirements. These requirements may vary depending on size of facility and type of land use, among other things.

#9 Principal Spillway - II
Design riser to go from weir flow control to barrel flow control without going into orifice control Reinforced concrete pipe and cast-in-place reinforced concrete box culverts are recommended to increase longevity Equip principal spillway with a removable trash rack

This slide shows a principal spillway with many of the key design features incorporated.
Copyright 2000, CWP

#10 Emergency Spillway Provide emergency spillway for large flood flows. Design in accordance with NRCS Pond Code, National Engineering Handbook, or local equivalent Earthen spillways shall be trapezoidal and located in undisturbed earth (i.e., not in fill) Side-slopes should be no greater than 2:1 (h:v) Provide an inlet channel, level section, and an exit channel (8 feet minimum width with non-erosive velocities through the control section and exit channel) An emergency spillway should be provided in almost all instances. It is important to be familiar with the local requirements for providing safe passage of larger flood events. Local regulations may include peak discharge control, minimum width and minimum freeboard requirements. These requirements may vary depending on size of facility and type of land use, among other things.

The earthen emergency spillway in this slide is located to the right of the principal spillway.
Copyright 2000, CWP

The emergency spillway in this slide is constructed of rip-rap.
Copyright 2000, CWP

#11 Embankment Specifications - I
Design and construct in a manner to prevent dam breach or seepage (exact criteria depend on local requirements or NEH dam safety criteria) Minimum top width of 10 feet (or 16’ to 26’ if it is a roadway embankment) Increase dam height to account for settlement (often 5 to 10 %) Provide cutoff trench and impervious core located along centerline of dam Similar to emergency spillways, it is important to be familiar with the local regulations and requirements pertaining to dam safety. Requirements often vary depending on dam classification. Key considerations to be aware of are provided on the next two slides.

#11 Embankment Specifications - II
Combined upstream and downstream side slopes of embankment shall not be steeper than 5:1 (h:v) with neither slope exceeding 2:1 Provide freeboard, depending on dam classification (usually 1 to 2 feet) Provide anti-seep collars or seepage diaphragms for all conduit pipes through the embankment greater than six inches in diameter Most of these criteria are designed to ensure safety and prevent dam breach. The “combined upstream and downstream side slopes” refers to the addition of the two h:v ratios. For example, a 4:1 upstream slope and a 3:1 downstream slope would result in a 7:1 combined slope.

This slide is a dry pond but nevertheless illustrates proper design features of an embankment including slopes, stabilizing vegetation, and proper compaction. Note the absence of woody vegetation, which is generally prohibited on embankments for structural stability reasons. Copyright 2000, CWP

#12 Inlet Protection Inlets to ponds should be adequately protected with rip-rap to avoid erosion and scouring. Flared end section pipes help reduce the erosion potential Inlet pipes to the pond can be partially submerged in warmer climates Provide a forebay at each inlet (unless it provides less than 10% of the total design storm inflow rate to the pond) The following inlet protection criteria should be considered with pond and wetland design.

This slide shows rip-rap lined inlets to avoid scour and erosion.
Copyright 2000, CWP

Here is another example of inlet protection.
Copyright 2000, CWP

#13 Adequate Outfall Protection - I
Provide flared end pipe sections that discharge at or near the stream invert Minimize tree clearing along the downstream channel, and reestablish a forested riparian zone in the shortest possible distance Outfall protection is a critical design consideration, particularly with respect to the downstream channel stability.

Here is an example of a flared end section outfall where tree clearing has been minimized. This outfall is lacking additional protection in the form of rip-rap which may result in a scour hole and subsequent head cutting. Copyright 2000, CWP

#13 Adequate Outfall Protection - II
Provide a pilot channel underdrain pipe (located 2 to 3 feet below the rip-rap) to prevent excessive warming of dry weather flows. Protect the channel with shade trees Modify the channel immediately below the pond outfall to conform to natural dimensions, stabilize with large rip-rap place over filter cloth Provide a stilling basin to reduce flow velocities from the principal spillway, when necessary Additional design considerations for outfall protection are provided in this slide.

#14 Pond Benches Provide two benches for all deep pool areas (four feet or greater in depth) Safety Bench- extends feet outward from the shoreline to the top of the pond sideslope (5% maximum cross-slope) Aquatic Bench- extends 15 feet, on average, inward from normal shoreline, maximum depth of 18 inches Pond benches provide both safety and habitat benefits to these facilities. Both benches should be incorporated into pond designs.

This slide shows the diverse vegetation that can be incorporated into an aquatic bench.
Copyright 2000, CWP

Both a safety bench and aquatic bench can be seen in this slide.
Copyright 2000, CWP

#15 Safety Features - I Fencing of ponds is not generally desirable, but may be required by local regulations Safety can be provided by managing contours of ponds to eliminate drop-offs and other hazards Sideslopes to the pond shall not exceed 3:1 (h:v), and shall terminate on a safety bench Safety should be a primary consideration in the design of pond and wetland facilities, particularly for facilities that will be easily accessed by the public.

This is a more aesthetically pleasing fence, but perhaps not as much of a deterrent.
Copyright 2000, CWP

This slide shows a good example of mild sideslopes.
Copyright 2000, CWP

#15 Safety Features - II Landscape both the safety bench and the aquatic bench to prevent access Limit the principle spillway opening to prevent access by small children Provide warning signs prohibiting swimming and skating Other important safety features to be incorporated in the design of ponds and wetlands include the above.

The aquatic bench vegetation of this wetland prevents easy access to the water.
Copyright 2000, CWP

This riser (principle spillway) has adequate protection against access by small children.
Copyright 2000, CWP

#16 Pondscaping Plan Prepare a pondscaping plan that indicates how aquatic and terrestrial areas will be stabilized with vegetation Consult local guidance documents or NRCS extension office for appropriate plant lists Depending on the pond or wetland design, there may be as many as 6 pondscaping zones Pondscaping is a technique that utilizes native trees, shrubs, herbaceous plants, and wetland species to meet specific functional design objectives within a stormwater pond or wetland system.

This slide schematically illustrates possible pondscaping zones to include in a design. Each zone has a separate functional goal. In addition, different types of vegetation are best suited to survive in each zone. While zone 6 favors upland species such as oaks, zone 2 would incorporate plants such as pickerel. Local soil and water conservation districts or extension agencies may be the best source of local plant information.. Copyright 2000, CWP

This slide illustrates many of the 6 zones
This slide illustrates many of the 6 zones. Note that this facility functions as an aesthetic feature, and grass is used near the pond to allow for public viewing. Copyright 2000, CWP

The following two slides are representative of the level of detail that is desirable for the preparation of a pondscaping plan. This particular example is for an ED wetland. Above is the plan view of the facility. Copyright 2000, CWP

The second sheet of the plan provides the planting details, including zones, species, quantity and planting sequence. When pondscaping is done well, it can provide significant opportunities for habitat enhancement, such as nesting areas and refuge. Copyright 2000, CWP

Water fowl often find refuge on larger stormwater ponds
Water fowl often find refuge on larger stormwater ponds. The Canada Geese illustrated in this slide can be a major pollutant source. Where desired, pondscaping can be used to minimize their access. Substituting grass at the edge of the pond with shrubs acts as a major deterrent. Copyright 2000, CWP

Dense wetland vegetation provides refuge for birds and other wildlife.
Copyright 2000, CWP

#17 Wetland Elements Encourage wetland plants whenever possible in the design (along the aquatic bench, the safety bench and sideslopes, or within shallow areas of the pool itself) Establish wetland plants, either through transplantation or volunteer colonization, within six inches (plus or minus) of the normal pool Feature 17 provides guidance on some of the unique and defining features associated with stormwater wetlands. Where feasible, wetland vegetation should also be provided along the aquatic bench and other wet areas of wet ponds.

For wetland design, special attention should be paid to the establishment and diversity of vegetation. The next two slides show big-leaved arrowhead (Sagittaria) with spike bush (Eleocharis), two common wetland species in the mid-Atlantic region. Copyright 2000, CWP

Pollutant Removal Pathways within Stormwater Wetlands
Sedimentation Adsorption to sediments/vegetation/detritus Physical filtration of runoff Microbial uptake/transformation Uptake by wetland plants Uptake by algae Extra detention and/or retention Wetlands have several pollutant removal pathways that make them excellent stormwater treatment practices. Many of these removal mechanisms are also functioning in stormwater ponds (e.g., sedimentation and uptake by algae). Incorporating specific design features (described in the following slides) will enhance these pollutant removal pathways.

Creating Effective and Diverse Stormwater Wetlands
Design to Maintain a Constant Pool Elevation and Baseflow—Maintain Water Balance Need Ability to Regulate Water Levels for Planting and Maintenance Forebay to Trap Sediments, Dissipate Velocity and Diffuse Flows These design features generally relate to the hydrologic regime and conveyance of flow in wetlands. An experienced wetland consultant should be involved in the planning, design, and planting processes.

The effective flow path during dry weather and small storm events can be much greater if hi marsh areas are provided to serve as baffles. Pollutant removal is also enhanced due to the longer residence time. In larger storm events, the flow path will be more direct from the inlet to the outlet. Copyright 2000, CWP

Provide Microtopography within Wetland Minimize Open Grass Areas (Geese Loafing) Deeper Water Areas for Greater Retention, Mosquito Control and Outlet Protection Diversity of wetlands can easily be provided by providing microtopography and pondscaping zones. These features also enhance pollutant removal abilities and habitat value.

Complex Wetland Microtopography
Microtopography refers to the contours along the bottom of a wetland system. It creates a variety of environmental conditions that favor the unique requirements of many different species of wetland plants and increases the surface area to volume ratio. This slide presents the concept of microtopography schematically in plan and profile. Copyright 2000, CWP

This slide shows a healthy wetland community with a low flow channel and complex microtopography that allows for a diverse plant community to establish.

Live fascines (brush bundles) can also be used to provide complex microtopography.
Copyright 2000, CWP

Pondscaping Zones Deepwater -1.5 to -6.0 feet
Shallow Marsh to -0.0 feet Shoreline Fringe 0.0 to 1.0 feet Riparian Fringe 1.0 to 3.0 feet Floodplain Terrace 3 to 6 feet Upland Areas 6 feet + Pondscaping zones should be determined based on the inundation frequencies that correspond to the design storm elevations. For example, the deepwater and shallow fringe areas will be located within the permanent pool elevation, while the shoreline fringe may exist within the limits of the extended detention elevation. The next slide presents this concept schematically.

Pondscaping Zones in a Stormwater Wetland (Plan view)
Pondscaping zones in wetlands should be dominated by the shallow marsh and shoreline fringe zones, where emergent vegetation will thrive. Copyright 2000, CWP

Preparing the Wetland Bed - Seven Steps
Prepare grading plan Grade to interim elevations Add topsoil and/or mulch amendments Grade to final elevations (provide microtopography) Allow wetland to fill for a few months to verify planting depths Measure and stake planting depths De-water wetland prior to planting period Preparation of the wetland bed is an often overlooked process. Upfront planning and attention to detail at this phase can increase survival rates of vegetation substantially. Following the above seven steps can help ensure long-term survival of wetland plants. It is particularly critical to ensure that planting depths are correct, because wetland plants can only survive at specific planting depths.

This slide shows a wetland cell being dewatered in preparation for planting.
Copyright 2000, CWP

Wetland depths should be measured and staked prior to planting to confirm design depths of pondscape. In addition, the limits of planting zones should be staked and flagged so that planting crews can easily match the intended species with the proper planting zone. Copyright 2000, CWP

Select 2-3 Dense Growing, Aggressive Species, add 5 other Wetland Species to Promote Diversity (Avoid Cattails, Phragmites, and Loose Strife) Where Possible, Utilize Wetland Mulch/Seedbank to Promote Diversity Establishment of a healthy and diverse wetland community is a critical component in the design and construction process. Recommended guidelines are provided above.

Seedbanks for Wetland Establishment
Wetland mulch - upper six inches of wetland soils Up to 20 species can emerge from seedbank (uncertainty about ultimate community) Donor sites are highly restricted due to 404 regulations Spread mulch 3-6 inches deep in hi marsh Specific guidelines to consider when using wetland seedbanks for establishment of vegetation are provided above.

When Planting Wetland Nursery Stock: Plant in Single Species Clumps, 18” On Center Initial Plantings Should be at least 50% of Wetland Surface Area Follow-up after First Growing Season with Reinforcement Planting Transplanting Window and Post-Nursery Care is Very Important Nursery stock use will likely be more expensive than using a seedbank, but propogation is often faster. Considerations for using nursery stock are presented above.

This slide shows some nursery stock ready for transplanting
This slide shows some nursery stock ready for transplanting. Timing is critical when developing a pondscaping plan, to ensure that nursery stock can be planted soon after arrival. Copyright 2000, CWP

This slide shows some shoreline fringe planting of nursery stock
This slide shows some shoreline fringe planting of nursery stock. In this case, the permanent pool was not drawn down prior to planting. Copyright 2000, CWP

A finished product after a few years of establishment might look like this.
Copyright 2000, CWP

#18 Pond Buffers and Setbacks
Provide buffer that extends 25 feet outward from the maximum water surface elevation of the pond Provide an additional 15 foot setback to permanent structures Preserve trees in the buffer area during construction Plant trees, shrubs, and native groundcovers in the buffer Mow only maintained rights-of-way and the embankment within the buffer Manage remaining buffer as a meadow or forest Where feasible, stormwater ponds and wetlands should be screened from adjacent land uses with vegetated buffers to provide enhanced habitat and to reduce potential exposure to community nuisances such as odors. Recommended buffer guidance is provided above.

This slide illustrates a well designed buffer that is providing good habitat.
Copyright 2000, CWP

Minimizing tree removal during construction can provide mature buffers to ponds and wetlands.
Copyright 2000, CWP

#19 Maintenance Measures
Vest maintenance responsibility for the pond and buffer with a responsible authority using a legally binding and enforceable maintenance agreement Inspect pond annually in wet-weather conditions Remove sediment in the forebay every 5 to 7 years, or after one foot of sediment deposition has been recorded in the forebay Provide a suitable on-site sediment disposal area An often forgotten, but critically important, feature of all stormwater treatment practices is maintenance. Identifying a responsible party and schedule of maintenance measures is necessary for long term performance of the facility. Some maintenance considerations for ponds and wetlands are presented above.

#20 Maintenance Access Provide a 25 foot wide maintenance right-of-way easement, extending to the pond from a public or private road Maintain a maximum slope of less than or equal to 15%, stabilized to withstand heavy equipment Extend access to the forebay, safety bench and riser. Design to allow vehicles to turn around Provide lockable manhole covers and manhole steps In addition to ensuring that maintenance will happen on a regular basis, there also needs to be appropriate access provided to the facilities. Legal access must be provided and sufficient to allow heavy equipment to operate.

Maintenance easements can be as unassuming as this access between two townhomes.
Copyright 2000, CWP

Miscellaneous Considerations
Adjust design to minimize stream impacts caused by STPs (e.g., temperature increases, bank erosion, fish barriers) Re-establish natural stream conditions below the practice in as short a distance as possible Comply with applicable federal/state/local permit requirements In addition to the 20 pond and wetland design features that are presented in this slide show, there are some additional miscellaneous considerations to keep in mind during the design and construction process.

Design of Stormwater Ponds

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