New Research Informs Coastal Wet Pond Design
A minor rebellion occurred last month in Tidewater Virginia about CSN’s proposed restriction on the most common practice in the coastal plain – the wet pond (see Stormwater Guidance for the Coastal Plain of the Bay Watershed). The basic proposal was to restrict groundwater – influenced wet ponds such that only the portion of the permanent pool above the seasonally high water table would be considered eligible to meet water quality volume requirements. About a dozen engineers from tidewater Virginia vociferously complained that this would take away one of the few effective stormwater options when the water table is real close to the land surface.
CSN agreed to go back and review the available coastal plain wet pond research to see if such a restriction is really warranted. It turns out that quite a few researchers have looked at coastal plain ponds in recent years, including performance studies in Florida and South Carolina, and an interesting study on harmful algal blooms in brackish wet ponds.
I have summarized some of the key points in a technical memo on coastal plain wet pond nutrient removal performance, and its implication for stormwater design. The main conclusions:
- Nitrogen and phosphorus rates for coastal plain wet ponds need to be reduced slightly to match their observed performance, according to nine studies that examined the influence of groundwater on pond performance.
- Internal design factors such as long flow paths, multiple cells and expanded wetland cover are essential to achieve these slightly lower nutrient removal rates.
- Sizing should be based on wet pond detention time and not the depth to water table. Thus, it is not necessary to exclude the water quality volume below the water table, as long as the pond meets the first two design conditions. A revised design table that eliminates the groundwater and depth exclusion is provided below for comment.
|Table 3- Level 1 and 2 Wet Pond Design Guidance: Coastal Plain|
|Level 1 Design (RR:0; TP:45; TN:20)||Level 2 Design (RR:0; TP:65; TN:30)|
|TV= (1.0)(Rv)(A)/12||TV = 1.5 (Rv) (A) /12|
|Single Pond Cell (with forebay)||Wet ED or Multiple Cell Design*|
|Flow path = 1:1 or less||Flow path = 1.5:1 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|
|Maintenance access to forebay/riser||Maintenance access to forebay/riser|
|Runoff reduction can be computed for wet ponds designed for water reuse and upland irrigation* in addition|
|Runoff reduction can be computed for wet ponds designed for water reuse and upland irrigation* in addition to forebay|
Based on the research, I propose moving wet ponds from the “discouraged” to the “acceptable” list for the coastal plain, and will make substantive changes to the Wet Pond design specification and Technical Bulletin No. 2 to reflect these changes.