The Sims Bayou Federal Flood Damage Reduction Project, with one of its detention basins shown here, started in 1990 and is scheduled for completion in 2009. Total project cost is estimated at $345 million; the federal government will pay $220 million and the Harris County Flood Control District will cover the remainder. Photo: Harris County Flood Control District.
Chapel Hill, N.C., is in its final stages of building a large public works operations facility, which includes geothermal systems; rainwater collection systems; active and passive solar, bioretention, and pervious asphalt; with the latter two shown here. Photo: Town of Chapel Hill
The Green Approach
The best management practice (BMP) trend is moving toward more holistic approaches in lieu of structural holding and filtering components, such as underground storage tanks or filters, says Ian Cooke, executive director of the Neponset River Watershed Association in Canton, Mass. The group works to protect and restore the Neponset River, its tributaries, and the surrounding watershed.
Current BMPs that are growing in popularity include bioretention and “country drainage,” or simple ditches by the side of the road. “Public works departments are thinking more holistically, and that construction approach will be cheaper,” says Cooke.
Since there is an unaccounted-for rise in the number of devices to hold and treat stormwater before it's released to the drainage system, reducing the amount that municipalities have to spend on these devices will be welcome. “This rise is mostly due to the implementation of National Pollutant Discharge Elimination System Phase II requirements and other more stringent state and local requirements,” says Andy Hadsell, PE, CFM, stormwater group leader for Chas. H. Sells Inc., an engineering firm based in Mooresville, N.C.
While the jury is still out on the best management practice to use since everyone's stormwater problem is different (see sidebar for info on easier BMP selection), many stormwater utilities are turning to bioretention as one way to manage stormwater. Bioretention cells or sites are vegetated depressions designed to collect, store, and filter stormwater runoff. These sites can include a mix of hard structures (perforated pipe that sends overflow to a storm drain) topped with soil, plants, and microbes.
Stormwater, as it falls from above or flows from nearby impervious surfaces, slowly percolates through the soil of a bioretention cell, which acts as a filter to remove some suspended solids. Some water is also taken up by the plants. Commonly known as a rain garden, they can add aesthetic beauty to a residential area.
Bioretention areas can provide excellent pollutant removal and recharge for the “first flush” of stormwater runoff. Properly designed cells remove suspended solids, metals, and nutrients (nitrogen and phosphorus), and can infiltrate 1 inch or more of rainfall.
Cooke's nonprofit group, for instance, worked with the Milton, Mass., department of public works to design bioretention cells along the Pine Tree Brook. Bacteria levels—especially coliform—were above the total maximum daily load, or TMDL, levels allowed in the brook, plus Milton wanted to be proactive in cleaning its water.
Milton and the Neponset River Watershed Association partnered to acquire a Section 319 grant through the Clean Water Act to pay for three bioretention cells along the brook. The partnership used the grant to pay $110,000 for the engineering, construction, enhanced wetlands, and other structures for these bioretention cells.
“There's no simple way to measure their success,” says Cooke. No direct inlet or outlet data can be gathered around these cells because there is no outlet pipe, but Reva Levin, Milton's department of public works program manager, says the brook's health is improving.