WET POND

Table 1: Summary of Stormwater Functions Provided by Wet Ponds 3
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction1	0%	0%
Total Phosphorus Removal 2	50%	75%
Total Nitrogen Removal 2	30%	40%
Channel Protection	Yes, CPv can be provided above the permanent pool
Flood Mitigation	Yes, flood control storage can be placed above the permanent pool
1 Runoff Reduction rates for ponds used for year round irrigation can be determined through a water      budget computation. 2 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load    removed is the product of the removal rate and the runoff reduction rate. 3 Note that EMC removal rate is slightly lower in coastal plain if the wet pond is influenced by    Groundwater.  See Section 6.2 and CSN Technical Bulletin No. 2 Sources: CWP and CSN (2008), CWP (2007)

SECTION 2:  LEVEL 1 AND 2 DESIGN TABLES

The major design goal for Wet Ponds in Virginia is to maximize nutrient removal and runoff reduction. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced Level 2 design that maximizes nutrient and runoff reduction. The two design levels for wet ponds to maximize nutrient reduction are shown in Table 2. At this point, there is no runoff reduction credit for wet ponds.

Table 2:  Level 1 and 2 Wet Pond Design Guidance 5
Level 1 Design (RR:0 1; TP: 50; TN:30)	Level 2 Design (RR:0; TP: 75; TN:40)
Tv = [(1.0)(Rv)(A)/12] – volume reduced by upstream BMP	Tv = [1.5 (Rv) (A) /12] – volume reduced by upstream BMP
Single Pond Cell (with forebay)	Wet ED 2 or Multiple Cell Design 3
Length/Width ratio OR Flow path = 2:1 or more	Length/Width ratio OR Flow path = 3:1 or more
Length of shortest flow path/overall length4 = 0.5 or more	Length of shortest flow path/overall length4 = 0.8 or more
Standard aquatic benches	Wetlands more than 10% of pond area
Turf in pond buffers	Pond landscaping to discourage geese
No Internal Pond Mechanisms	Aeration (preferably bubblers that extend to or near the bottom or floating islands
1 Runoff reduction can be computed for wet ponds designed for water reuse and upland irrigation 2 Extended Detention provided to meet the treatment volume 3 At least three internal cells including the forebay 4 In the case of multiple inlets, flow path is measured for the dominant inlets (that comprise 80% or more of total pond inflow) 5 Due to groundwater influence, slightly lower TP and TN removal rates occur in coastal plain (see Section 11.2 )

• Space Required: The proposed surface area for a wet pond should be at least 1-3 % of its contributing drainage area, depending on the pond’s depth.

• Contributing Drainage Area: A contributing drainage area of 10 to 25 acres is typically recommended for wet ponds to maintain constant water elevations. Wet ponds can still work on drainage areas less than 10 acres, but designers should be aware that these “pocket” ponds will be prone to clogging, experience fluctuating water levels, and generate more nuisance conditions. A water balance should be conducted to assess whether the wet pond will draw down by more than 2 feet after a 30-day summer drought (see equations in Section 5.2).
• Available Head: The depth of a wet pond is usually determined by the head available on the site. The bottom elevation is normally set by the existing downstream conveyance system to which the wet pond discharges. Typically, a minimum of 6 to 8 feet of head are needed to construct a wet pond.

• Minimum Setbacks: Local ordinances and design criteria should be consulted to determine minimum setbacks to property lines, structures, and wells. As a general rule, wet ponds should be setback at least 20 feet from property lines, 25 feet from building foundations, and 100 feet from septic system fields and private wells.
• Depth to Water Table:The depth to the water table can be a design concern for wet ponds. If the water table is close to the surface, it may make excavation difficult and expensive. Groundwater inputs can also reduce the pollutant removal rates.

• Soils: Highly permeable soils make it difficult to maintain a constant permanent pool in many parts of the Bay watershed. Therefore it is important to directly address fluctuating water levels in design. Soil infiltration tests need to be conducted at proposed pond sites to determine the need for a pond liner, or other methods that address water level fluctuation. Underlying soils of Hydrologic Soil Group (HSG)“C” or “D” should be adequate to maintain a permanent pool.  Most group “A” soils and some group “B” soils will require a liner. Geotechnical tests should be conducted to determine the infiltration rates and other subsurface properties of the soils underlying the proposed pond.
• Karst: If a proposed site is located in or near active karst terrain, wet ponds are not recommended. An alternative practice or combination of practices should be employed at the site. See CSN Technical Bulletin No.1 for guidance on wet pond design in karst terrain (CSN, 2008).

• Trout Streams: The use of wet ponds in watersheds containing trout streams is strongly discouraged.
• Use of or Discharges to Natural Wetlands:  It can be tempting to construct a wet pond within an existing natural wetland, but wet ponds cannot be located within jurisdictional waters, including wetlands, without obtaining a section 404 permit from the appropriate state or federal agency. In addition, designers should investigate the wetland status of adjacent areas to determine if the discharge from the wet pond will change the hydroperiod of a downstream natural wetland (see Cappiella et al., 2006b, for guidance on minimizing stormwater discharges to existing wetlands).

• Perennial streams:Locating wet ponds on perennial streams is also strongly discouraged, and will require both a Section 401 and Section 404 permit from the appropriate state or federal permitting authority.

1. Wet Pond (100% permanent pool in single or multiple cells)
2. Wet ED Pond (No more than 50% total treatment volume (WQv) as ED)
3. Pond Wetland Combination (see Stormwater Design Specification No. 13)

• Water Balance Testing

Table 3: Acceptable Drawdown in a Wet Pond, Simple Water Balance

DP - AD  >  MB - ET- INF - RES

Where: DP        =         Average design depth of permanent pool (inches) AD       =         Acceptable Drawdown (24 inches) MB     =           Measured baseflow rate to pond (if any- convert to inches) ET        =          Summer evapo-transpiration rate (inches) (assume 8 inches)               INF      =          Monthly infiltration loss (assume 7.2 @ 0.01 inch/hr) RES     =          Reservoir of water for a factor of safety (assume 6 inches)

Source: Modified from Hunt et al (2007)

• The forebay shall consist of a separate cell, formed by an acceptable barrier. Typical examples include earthen berms, concrete weirs, and gabion baskets.
• A major inlet is defined as an individual storm drain inlet pipe or open channel serving at least 10% of the wet pond’s contributing drainage area.

• The forebay should be at least 4 feet deep and shall be equipped with a variable width aquatic bench for safety purposes. The aquatic benches should be 4 to 6 feet wide and placed 1 to 2 feet below the water surface.
• The total volume of all forebays should be at least 15% of the total WQv (inclusive).
• The bottom of the forebay may be hardened (i.e., with concrete, asphalt, or grouted riprap) to make sediment removal easier.
• The forebay should be equipped with a metered rod in the center of the pool (as measured lengthwise along the low flow water travel path) for long term monitoring of sediment accumulation.

• Internal Slope:  The longitudinal slope through the pond should be greater than 0.5 – 1% to promote positive flow through the pond practice

• Primary Spillway:  Spillway shall be designed with anti-flotation, anti-vortex and acceptable trash rack devices. The spillway shall generally be accessible from dry land. Refer to Introduction Appendix B: Principal Spillways.
• Non-clogging Low Flow Orifice: A low flow orifice shall be provided that is adequately protected from clogging by either an acceptable external trash rack or by internal orifice protection that may allow for smaller diameters. Orifices less than 3 inches may require extra attention during design to minimize the potential for clogging.
• A submerged reverse-slope pipe that extends downward from the riser to an inflow point 1 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.

• Emergency Spillway:  ED ponds shall be constructed with overflow capacity to pass the 100-year design storm event through either the Primary Spillway or a vegetated or armored Emergency Spillway. Refer to Introduction Appendix C Emergency Spillways.

• Pond Drain: Except for flat areas of the coastal plain, each wet pond should have a drain pipe that can completely or partially drain the permanent pool. In cases where a low level drain is not feasible (such as in an excavated pond), a pump well may be provided.
• The drain pipe should have an elbow or protected intake within the pond to prevent sediment deposition, and a diameter capable of draining the pond within 24 hours.
• The pond drain shall be equipped with an adjustable valve, and be located within the riser where it will not be normally inundated and can be operated in a safe manner.

• Adequate Outfall Protection:  Design for a stable outfall for the 10-year design storm event. The channel immediately below the pond outfall shall be modified to prevent erosion and conform to natural dimensions in the shortest possible distance. This is typically done by installing appropriately sized riprap placed over filter fabric that can reduce flow velocities from the principal spillway to non-erosive levels (3.5 to 5.0 fps).  Flared pipe sections, which discharge at or near the stream invert or into a step pool arrangement, should be used at the spillway outlet.

• Inlet Protection:  Inlet areas should be stabilized to ensure that non-erosive conditions exist during events up to the overbank flood event (i.e., Qp10). Inlet pipe inverts should generally be located at or slightly below the permanent pool.

• Dam Safety Permits:  Certain classes of wet ponds with deep pools, high embankments or large drainage areas require a small pond or dam safety review from the local review authority, soil and water conservation district or state dam safety safety agency. Dam safety regulations should be strictly followed during stormwater pond design to ensure that downstream property and structures are adequately protected.

• Side Slopes: Side slopes for the wet pond should generally be 4:1 to 5:1 (H:V)  The mild slopes promote better establishment and growth of vegetation and contribute to easier maintenance and a more natural appearance.

• Long Flow Path: Wet pond designs should have an irregular shape and a long flow path from inlet to outlet to increase water residence time and pond performance.  In terms of flow path geometry, there are two design objectives: (1) the overall flow path through the wetland, and (2) the length of the shortest flow path (Hirschman et al., 2009):
• The overall flow path can be represented as the length to width ratio OR the flow path ratio (see the Specs Introduction chapter for diagrams and equation).  These ratios shall be at least 2:1 for Level 1 designs and 3:1 for Level 2.  Internal berms, baffles, or vegetated pennisulas can be used to extend flow paths and/or create multiple pond cells.
• The shortest flow path represents the distance from the closest inlet to the outlet (see Specs Introduction chapter).  The ratio of the shortest flow to the overall length shall be at least 0.5 for Level 1 and 0.8 for Level 2.  In some cases -- due to site geometry, storm sewer infrastructure, or other factors -- some inlets may not be able to meet these ratios; however, the drainage area served by these “closer” inlets should constitute no more than 20% of the total contributing drainage area.

• Water Quality Storage: The total Tv storage may be provided by a combination of permanent pool, shallow marsh and/or extended detention storage. Permanent pool storage can be provided in multiple cells.  Performance is enhanced when multiple treatment pathways are provided by using multiple cells, longer flow paths, high surface area to volume ratios and/or redundant treatment methods (combinations of pool, ED, and marsh). A berm or simple weir should be used instead of pipes to separate multiple pond cells.

• Maximum Extended Detention Level: The extended detention associated with the TV shall not extend more than 5 feet above the permanent pool or basin floor at its maximum water surface elevation.
• Stormwater Pond Benches:The perimeter of all pool areas greater than 4 feet in depth shall be surrounded by two benches:
• A Safety Bench is a flat bench located just outside of the perimeter of the permanent pool to allow for maintenance access and reduce safety risks. Except when stormwater pond side slopes are 5:1 (h:v) or flatter, provide a safety bench that generally extends 8 to 15 feet outward from the normal water edge to the toe of the stormwater pond side slope The maximum slope of the safety bench shall be 5%.
• An Aquatic Bench is a shallow area just inside the perimeter of the normal pool that promotes growth of aquatic and wetland plants. The bench also serves as a safety feature, reduces shoreline erosion and conceals floatable trash. Incorporate an aquatic bench that generally extends up to 10 feet inward from the normal shoreline, has an irregular configuration, and extends a maximum depth of 18 inches below the normal pool water surface elevation.

• Safety Features
• The principal spillway opening shall not permit access by small children, and end walls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.
• An emergency spillway and associated freeboard shall be provided in accordance with applicable local or state dam safety requirements. The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges.
• Both the safety bench and the aquatic bench may be landscaped to prevent access to the pool.
• Warning signs prohibiting swimming may be posted.
• Fencing of wet ponds is discouraged. The preferred method to reduce risk is to manage the contours of the stormwater pond to eliminate drop-offs or other safety hazards. Fencing is required at or above the maximum water surface elevation in the rare situations when the pond slope is a vertical wall.

• Delineation of pondscaping zones within both pond and buffer
• Selection of corresponding plant species
• The planting plan
• Sequence for preparing the wetland benches (including soil amendments, if needed)
• Sources of native plant material.
• Some additional guidance on pondscaping includes the following:
• The landscaping plan should provide elements that promote greater wildlife and waterfowl use within the stormwater wetland and buffers.
• Woody vegetation may not be planted or allowed to grow within 15 feet of the toe of the embankment and 25 feet from the principal spillway structure.
• A vegetated buffer should be provided that extends 25 feet outward from the maximum water surface elevation of the wet pond.  Permanent structures (e.g., buildings) should not be constructed within the buffer. Existing trees should be preserved in the buffer area during construction

• The soils in the stormwater 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. 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.
• Avoid species that require full shade, 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. For more guidance on planting trees and shrubs in wet pond buffers, consult Cappiella et al (2006) and Appendix E of the Introduction to the New Virginia Stormwater Design Specifications , as posted on the Virginia Stormwater BMP Clearinghouse web site).

• Maintenance Access: Good access is needed so crews can remove sediments, make repairs and   preserve pond treatment capacity

• Adequate maintenance access must extend to the forebay, safety bench, riser, and outlet and be designed to allow vehicles to turn around.
• The riser should be located within the embankment for maintenance access, safety and aesthetics.  Access to the riser should be provided by lockable manhole covers, and manhole steps within easy reach of valves and other controls.
• Access roads shall be constructed of vehicle bearing materials, or be built to withstand the expected frequency of use, have a minimum width of 12 feet and possess a maximum profile grade of 15%.  Steeper grades are allowable with stabilization techniques such as a gravel road.
• A maintenance right of way or easement shall extend to the stormwater pond from a public or private road.

• Liners: When a stormwater pond is located in highly permeable soils 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: (1) a clay liner following the specifications outlined in Table 4, (2) a 30 ml poly-liner (3) bentonite, (4) use of chemical additives, or (5) an engineering design as approved on a case-by-case basis by the local review authority. Clay liners should have a minimum thickness of 12 inches with an additional 12 inch layer of compacted soil above it, and must meet the specifications outlined in Table 4.
  

• A minimum of six feet of unconsolidated soil material exists between the bottom of the basin and the top of the karst layer.

• Maximum temporary or permanent water elevations with basins do not exceed six feet. Annual maintenance inspections are conducted to detect sinkhole formation. Sinkholes that develop should be reported immediately after they have been observed, and should be repaired, abandoned, adapted or observed over time following the guidance prescribed by the appropriate local or state groundwater protection authority (see Section 5.9).

• A liner is installed that meets the requirements outlined in Table 5.

• Adjustments to Nutrient Removal.  The numerous lines of evidence reviewed indicate that standard designs of coastal plain wet ponds cannot achieve the desired nutrient removal rates in the current Bay-wide design specification for wet ponds, based on current design, detention times, the influence of groundwater and other factors. Therefore, slightly lower nutrient removal rates are proposed for coastal plain wet ponds to reflect real world performance data for phosphorus and nitrogen removal. Specifically, Level 1 and 2 total removal rates for TP are now proposed to be 45 and 65% respectively, and Level 1 and 2 TN removal rates are reduced to 20% and 30%, respectively. These slightly lower removal rates are supported by pond research and the detention time relationships (see CSN Technical Bulletin No. 2).

• Pocket Ponds.  Another issue relates to wet ponds with a small contributing drainage area that are solely supplied by runoff and groundwater, and often create nuisance conditions and fluctuating water levels. There is virtually no data on these “pocket ponds” that are frequently installed on small commercial sites. Rather than mandating an arbitrary minimum drainage area, it is recommended instead that these pocket ponds must meet the minimum design geometry requirements for all ponds (i.e., sediment forebay cell, aquatic benches, maximum side-slopes of 5: 1 and length to width ratio of  2 or 3 to 1). Designers should strictly adhere to the same design requirements that apply to other wet ponds, which should greatly reduce the number of nuisance ponds that are forced into tight sites (i.e., by reducing or eliminating essential pond design elements).

• Treat larger runoff volumes in the spring by adopting seasonal operation of the permanent pool (see MSSC, 2005)
• Plant salt-tolerant vegetation in pond benches

• Do not submerge inlet pipes and provide a minimum 1% pipe slope to discourage ice formation
• Locate low flow orifices so they withdraw at least 6 inches below the typical ice layer
• Angle trash racks to prevent ice formation
• Oversize riser and weir structures to avoid ice formation and freezing pipe
• Increase forebay size if road sanding is prevalent in the contributing drainage area

• Pre-construction meeting

• Initial site preparation (including installation of project E&S controls)

• Excavation/Grading (interim/final elevations,)

• Installation of embankment/riser/primary spillway and outlet

• Implementation of pondscaping plan and vegetative stabilization

• Final Inspection (develop punchlist for facility acceptance)

8.3. Inspections and Routine Maintenance Tasks

Maintenance of wet ponds is driven by annual inspections that evaluate the condition and performance of the wetland (see Table 6). Based on inspection results, specific maintenance tasks will be triggered. An annual maintenance inspection form for wet ponds can be accessed at CWP website at www.cwp.org/postconstruction (see Tool #6 of Managing Stormwater in Your Community: A Guide for Building Effective Post-Construction Programs).A more detailed maintenance inspection form is also available from Appendix B of CWP (2004).

Table 6: Suggested Annual Maintenance Inspection Points for Wet Ponds

Activity

Measure sediment accumulation levels in forebay.

Monitor the growth of wetland plant, tree and shrubs planted. Record species and approximate coverage, and note presence of any invasive plant species.

Inspect the condition of stormwater inlets to the pond for material damage, erosion or undercutting

Inspect upstream and downstream banks for evidence of sloughing, animal burrows, boggy areas, woody growth or gully erosion that may undermine embankment integrity,

Inspect the pond outfall channel for erosion, undercutting, rip-rap displacement, woody growth, etc.

Inspect condition of principal spillway and riser for evidence of spalling, joint failure, leakage, corrosion, etc.

Inspect condition of all trash racks, reverse sloped pipes or flashboard risers for evidence of clogging, leakage, debris accumulation, etc.

Inspect maintenance access to ensure it is free of woody vegetation and check to see whether valves, manholes or locks can be opened and operated.

Inspect internal and external pond side slopes for evidence of sparse vegetative cover, erosion or slumping, and repaired immediately

Note: For a more detailed maintenance inspection checklist, see Appendix B in CWP(2004) Stormwater Pond and Wetland Maintenance Guidebook

Maintenance is needed so stormwater ponds continue to operate as designed on a long-term basis. Wet ponds normally have less routine maintenance requirements than other stormwater treatment options. Stormwater pond maintenance activities range in terms of the level of effort and expertise required to perform them. Routine stormwater pond maintenance, such as mowing and removing debris or trash, is needed several times each year (See Table 7).  More significant maintenance such as removing accumulated sediment is needed less frequently, but requires more skilled labor and special equipment. Inspection and repair of critical structural features such as embankments and risers, needs to be performed by a qualified professional (e.g., a structural engineer) that has experience in the construction, inspection, and repair of these features.

The maintenance plan should clearly outline how vegetation in the pond and its buffer will be managed or harvested in the future. Periodic mowing of the stormwater 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. The maintenance plan should schedule a shoreline cleanup at least once a year to remove trash and floatables.

8.4. Sediment Removal

Frequent sediment removal from the forebay is essential to maintain the function and performance of a wet pond. Maintenance plans should schedule cleanouts every 5 to 7 years, or when inspections indicate that 50% of forebay capacity has been lost. Designers should also check to see whether removed sediments can be spoiled on-site or must be hauled away. Sediments excavated from wet ponds are not usually considered toxic or hazardous, and can be safely disposed by either land application or land filling. Sediment testing may be needed prior to sediment disposal if the retrofit serves a hotspot land use.

SECTION 9: COMMUNITY AND ENVIRONMENTAL CONCERNS

Wet ponds can generate several community and environmental concerns that need to be addressed during design: