Wet Pond / Retention Basin Franny Ritchie and Christophe Chung

Transcription

1 Wet Pond / Retention Basin Franny Ritchie and Christophe Chung Water, Landscape & Urban Design (4.214 / ) Anne Whiston Spirn / James Wescoat definition of wet pond / retention basin: a stormwater basin that includes a substantial permanent pool for water quality treatment and additional capacity above the permanent pool for temporary runoff storage. key functions: (tss), total phosphorous (tp), and nitrates (no3) elements commonly found in agricultural and lawn run- off. temperature sensitive water bodies should be accounted for. key components: a drainage area of at least 10 acres a length to width ratio of at least 2:1 a bottom with infiltration levels that will not allow a wet pond to dry out by a dense stand of emergent wetland vegetation outlet control devices 5 to 7 feet deep safety and aquatic benches permanent routes that are stabilized for vehicles and allow access to forebay, outlet, and embankment areas difference with constructed wetlands: - 3% of the site is allocated to drainage area as opposed to 3-5% of constructed wetlands. inflow, but constructed wetlands can be dry for up to a month. construction and maintenance (Stormwater Wet Pond and Wetland Management Guidebook, February 2009, EPA): A key issue involving wet ponds / retention basins is maintenance. While construction has been widely accepted as a cost- effective way in safeguarding water quality and mitigating against stormwater influxes, little attention has been paid to long- term maintenance. The reason can be attributed to the fact that while wet pond construction is many times mandated by zoning code, federal regulation does not require continued maintenance of such ponds. Improper maintenance (or the lack thereof) is especially an issue, given the fact that it lends wet ponds to pose public safety, public health, and financial hazards to the surrounding

2 community. Consequences of poor maintenance include: over- accumulation of nitrogen and phosphorous that result in degraded conditions (i.e. low dissolved oxygen, algae blooms, over- concentration of pollutants, odors), increase in vermin, mosquito infestations, as well as unsightly conditions. These issues are especially troubling considering the fact that wet ponds (when managed well) can add aesthetic, quality- of- life and financial value to the surrounding community. Studies carried out in urban areas across the country- - ranging from Wichita, KS to Alexandria, VA- - attribute an increase in land values of properties abutting wet ponds. The key elements of maintenance are meant to ensure that the pollutant removal, channel protection and flood control capabilities of ponds are at optimal levels. Some key objectives of wet pond maintenance are as follows: prevent accumulation of sediment, which reduces storage volume (i.e. dredging and muck removal). monitor and maintain safe water levels (i.e. inspect for and address blockage and/or seepage). prevent debris blockage of outlet structure (i.e. unclogging, control rodents, horticultural upkeep). maintain pipes and risers (regularly inspect and repair pipes). protect against invasive plants (horticultural upkeep). maintain slope stabilizing vegetation (maintain grasses, wetland plantings, and trees). ensure structural integrity of the embankment, weir or riser (control rodents, regularly inspect and maintain structural components). prevent pest infestations (trap rodents, introduce predators, addle eggs, prevent fluctuations in water level, as well as maintain a minimum depth of 6-8 ft which supports aquatic predators of mosquito larvae and inhibits vegetal growth that prevents surface water agitation from wind). maintain water quality (introduce bacteria to counteract algal growth, introduce air through diffusers and surface aerators like fountains, and apply flocculants to cause suspended particles and algal growth supporting phosphorous to settle). Pond maintenance requires varying levels of expertise and frequency. For example, mowing of turf grass and removing of debris can be done by citizen volunteers multiple times per year, while de- dredging is less frequent but requires more trained staff and specialized equipment. And, while maintenance of wet ponds have been successfully pursued in suburban communities where homeowner associations are robust and able to collect dues from local residents, this lends the question of whether such maintenance can be pursued in big cities as well where such associations do not exist. Whose responsibility is it then to maintain wet ponds in larger, more urban, more anonymous communities? its implementation in other cities: Typically, retention ponds are associated with suburban or industrial settings, but Portland and Seattle have both introduced urban stormwater mitigation systems modeled on retention ponds that are appropriate for a more dense setting. Even in cities, retention ponds may be a viable option for parks, schools and other systems of open space (many communities, however, are concerned about the dangers of ponds where children are present; ponds may therefore pose a liability issue). Just because retention ponds are most commonly depicted in fields does not mean that they cannot be bordered by roads, houses, etc. In Seattle, the Pinehurst Green Grid serves as a series of retention basins in a residential area. Drainage ditches were enlarged, landscaped and terraced. The project, designed to eliminate spot flooding,

3 creates a series of pocket ponds along roadways. Future applications: wet ponds are designed to reduce runoff by about 7% in areas of five to ten acres historically, they have been used in rural and suburban contexts, as well as to mitigate runoff near interstate highways their large scale and depth make them a difficult match for urban areas, although (as seen above), some cities have done projects that take inspiration from retention ponds they are probably not as good a fit for urban areas as a combination of other strategies discussed in class (rain barrels, rain gardens, green roofs, etc).

4 Wet Ponds & Retention Basins

5 What is a wet pond?

6 The importance of maintenance of Wet Ponds (EPA Report) Well-maintained wet pond

7 Suitable for Cities? Pros: - Effectively manages stormwater overflow - Good for wildlife - Low maintenance after installation Cons: - Potential liability issue in densely populated places, especially near children - Space- consumptive - Expensive to construct - If poorly designed or maintained, danger of mosquitos

8 BMPs from Portland, Seattle, etc. Infiltrate Planter, Portland Pinehurst Green Grid, Seattle

9 What are the possibilities for wet ponds in Philadelphia?