A retention pond contains a permanent pool of water and is also called a “wet” pond. It is designed to catch and release the WQCV over a period of 12 hours. Additionally, it has additional capacity above the permanent pool. It is expected that stormwater runoff mixes with the water in the pool during every runoff event, allowing for a reduced residence time compared to an EDB. It is claimed that the 12 hour drain time allows for better replication of pre-development flows for frequent events and reduces the potential for short circuiting treatment in smaller ponds. Ponds can also be designed for full spectrum detention (Urban Drainage and Flood Control District, 2010).
The USEPA has named two approaches to the design of retention ponds. The first is based on the assumption that all pollutants settle out with sediment. The second treats the pond like a lake and considers eutrophication processes to remove pollution. In practice most designs rely heavily on the assumption of pollutant removal via sedimentation (United States Environment Protection Agency, 2004b).
The following design procedure is recommended by the Urban Drainage and Flood Control District (2010) and is amended with design recommendations from other design guidelines:
Step 1: Consider baseflow
A perennial baseflow must be physically and legally available if the pool is not established by groundwater. Use conservative net influx calculations to account for significant annual variations. Low inflow in relation to the pond volume can result in poor water quality. Evaporation, evapotranspiration and seepage account for losses. A liner is recommended for ponds that lie above the groundwater table (Urban Drainage and Flood Control District, 2010).
Addendum pg.12 Step 2: Calculate the surcharge volume
The Urban Drainage and Flood Control District (2010) recommends that the surcharge volume above the permanent pool should be based on a 12 hour drain time. The USEPA recommends that a surcharge volume be provided above the permanent pool whenever the VB/VR is less than 2.5. It is recommended that the maximum event based volume with a drain time of 12 hours be used. This should be checked with the sedimentation and chemical routing equations (United States Environment Protection Agency, 2004b).
Step 3: Determine the basin shape and depth
It is recommended by the UDFCD that the distance between the inlet and outlet be maximised with a length to width ratio of at between 2:1 and 3:1. This will minimise short circuiting and improve the reduction of TSS (Urban Drainage and Flood Control District, 2010) (United States Environment Protection Agency, 2004b). The USEPA (2004b) also recommendes a mean peremanent pool depth of 1-3 m. The SUDS manual recommends a length to width ratio of at least 3:1 and a wedge plan shape so that entering flow can spread out and in so doing improve the sedimentation process. A maximum depth of 2m is recommended to avoid stratification and anoxic conditions (Woods-Ballard et al., 2007).
Step 4: Calculate the volume of the permanent pool
This can be done with the water quality volume approaches previously discussed, or with sediment or chemical routing techniques. It is recommended by the UDFCD that two depth zones be included in the permanent pool:
Safety wetland bench: This area should be located around the perimeter of the pool with a depth of 6 to 12 inches and a minimum width of 4 feet. This allows aquatic plant growth that helps to strain surface flow, stabilizes the banks and provides a safety foothold for people who fell into the pond (Urban Drainage and Flood Control District, 2010). The USEPA recommends that this bench be 10 feet wide and 1 foot deep (United States Environment Protection Agency, 2004b). The SUDS manual recommends a maximum depth of 45 cm and a minimum width of 1 m (Woods-Ballard et al., 2007).
Open water zone: This is an open volume of water, which provides for sedimentation and nutrient uptake by organisms. It should not be deeper than 12 feet to prevent anoxic conditions (Urban Drainage and Flood Control District, 2010). The USEPA recommends a depth of 6 to 8 feet (United States Environment Protection Agency, 2004b).
The USEPA (United States Environment Protection Agency, 2004b) and SUDS (Woods-Ballard et al., 2007) approach philosophises that the permanent pool provides treatment during dry periods to runoff. A portion of this is displaced during subsequent storm events and the new influent is treated
Addendum pg.13 during the following dry periods. It is recommended that the pool be sized for a specific hydraulic retention time, where after the effluent quality is checked and the volume be modified in subsequent iterations.
T = VB / nVR (A.29)
Where: T = hydraulic detention time in years VB = volume of the permanent pool n = number of runoff events per year VR = volume of runoff for an average storm
Ponds with values of T greater than 2 to 3 weeks have a greater risk of thermal stratification and anaerobic bottom waters. Municipalities in the USA often require specific values of T or, alternatively, VB/VR or minimum total suspended sediment removal rate. It has been shown that ponds designed with T ≥ 2 weeks and VB/VR = 4 achieve TSS removal rates of 80% to 90% (United States Environment Protection Agency, 2004b).
The USEPA recommends a pool depth of 3 to 6 feet to prevent thermal stratification and enable aerobic conditions (United States Environment Protection Agency, 2004b).
The SUDS manual recommends that the “treatment volume” be adjusted with a factor for different areas when calculating the storage volume (Woods-Ballard et al., 2007). Applicable factors are given in the literature.
Step 5: Determine the basin side slopes
It is recommended that slopes for the safety wetland bench should be no steeper than 4:1. Below the bench the slopes should be no steeper than 3:1. A deeper pond reduces penetration of sunlight and therefore algae growth (Urban Drainage and Flood Control District, 2010).
Step 6: Design the inlet
It is recommended that inlets should be designed to dissipate flow energy in order to reduce erosion and improve sedimentation. It should also be designed to diffuse the inlet plume (Urban Drainage and Flood Control District, 2010) .
Step 7: Design the forebay
The forebay allows an opportunity for larger particles to settle out in an area that can be easily maintained. It is recommended that, for maintenance purposes, solid linings be installed. The recommended volume is at least 3% of the WQV (Urban Drainage and Flood Control District, 2010).
Addendum pg.14 Step 8: Design the outlet structure
It is recommended that the outlet should be designed to release the WQCV over a 12 hour period (Urban Drainage and Flood Control District, 2010).
Step 9: Provide a trash rack
It is recommended that a trash rack be provided at the outlet to provide sufficient hydraulic capacity when it is partially clogged (Urban Drainage and Flood Control District, 2010).
Step 10: Design the overflow embankment
It is recommended that the embankment must be designed to withstand at least the 100 year storm (Urban Drainage and Flood Control District, 2010).
Step 11: Consider maintenance
It is recommended that a means should be provided for draining the pond for maintenance. Gravity feed is preferred, but it can also be pumped (Urban Drainage and Flood Control District, 2010). Step 12: Provide vegetation
It is recommended that basin berms should be planted with turf grass. The wetland bench should be planted with aquatic species (Urban Drainage and Flood Control District, 2010).
Step 13: Provide access
It is recommended that maintenance access must be provided to the forebay and outlet works (Urban Drainage and Flood Control District, 2010).
Addendum pg.15