Revitalizing an Old Cul-de-sac to Improve Stormwater Quality

by Adrienne St. John, Public Works Engineer, Town of Bedford Department of Public Works

 

The Shawsheen River is an important recreational and natural resource, although most reaches of the River are 303d listed as impaired by a multiplicity of causes, including pathogens, nutrients, metals, and toxicity. MassDEP has also set a bacteria TMDL (Total Maximum Daily Load) for the River. The Town of Bedford, Massachusetts recently designed and installed two raingardens in the middle of cul-de-sacs to provide pollutant removal, increase infiltration, reduce impervious cover and eliminate direct discharges of untreated stormwater to the Shawsheen River.

 

 

 

The Bedford Department of Public Works (DPW) chose 2 residential cul-se-sacs located approximately 250 feet from the Shawsheen River and near the Town’s water supply well field; each with one catch basin and a direct 12 inch pipe discharging to the River. Each catch basin collects approximately 20,000 square feet of pavement. The rain gardens were sited to capture and treat the sheet flow before the catch basin, although an underdrain was installed and piped to the catch basin. The gardens were installed by removing the pavement footprint, resulting in a reduction in impervious area. The gardens remove pollutants by filtering runoff through a specified soil mix consisting of 24 inches of 50 percent compost and 50 percent sand, which DPW mixed on site. A variety of native shrubs and perennials including viburnum, dogwoods, witch hazel and sweetpepper bush were selected as recommended for wet basins prone to periods of dry, hot weather. The roots of the plants also assist with pollutant uptake. The DPW also used landscaping boulders along the leading edge of the rain gardens to provide vertical visibility, since each garden is set below grade.

 

Comprehensive Environmental Inc. developed a Loading and Reduction template for the Town to use to estimate the overall function of the gardens. The program uses inputs such as land use, impervious cover, winter sanding practices and runoff values to determine the estimated pollutant loading. The worksheet then calculates the annual pollutant removal percentage to determine efficiency of the rain garden. Using this worksheet, the Town anticipates the following annual removal rates from a typical subdivision roadway of similar size to be:

 

Total Suspended Solids: 900 – 1400 pounds

Total Phosphorus: 1.7 – 3.1 pounds

Total Nitrogen: 10 – 18 pounds

Fecal Coliform: 10 – 40 cfu/100 ml

 

The project was funded through a Non-point Source Section 319 Grant, administered by MassDEP and funded by the US EPA. The grant included the design and construction of the rain gardens, as well as the promotion of proper pet waste disposal and installation of catch basins/door hangers to remind residents of good housekeeping practices to prevent stormwater pollution.

 

The rain gardens were installed in 2009. The total cost of materials (underdrain pipe, area basin, crushed stone, plants, filter fabric) was about $8,500. The work was performed in-house by the DPW. There have been no washouts, short circuiting or overflow problems with the new gardens, even during the recent heavy rain events. The vegetation in the rain gardens is inspected annually to remove weeds and invasive species that tend to grow along the edge of the basin.

 

As a result of this program, Bedford engineering and labor forces are now capable of easily designing and building the gardens. The time needed for engineering staff, modeling, coordination, construction layout, and installation was significantly reduced once the first rain garden was complete. The DPW crew has come to understand the need and function of rain gardens and will be efficient at constructing similar basins in the future.