Cohasset Rain Garden Project

Cohasset is a South Shore community in which approximately 90 percent of the drinking water supply comes from the Lily Pond surface water treatment plant. Lily Pond is shallow and currently eutrophic. The reduction of nutrient loads and control of in-lake vegetation were identified as goals in the development of pond and water supply management strategy. Low Impact Development (LID) Techniques were instituted to treat roadway runoff within the water supply watershed prior to discharge to the pond. The stormwater Best Management Practices selected include bioretention cells (rain gardens) and vegetated bioretention swales. As a first step a demonstration rain garden was constructed at the Water Department parking lot – to serve as an educational tool for the community. Twenty rain gardens and two vegetated bioretention swales have been constructed so far within roadway rights-of-way. Twenty-nine additional rain gardens will be constructed in Spring of 2008. The runoff filters through specially engineered soil reducing pollutant and nutrient levels, reducing elevated temperatures of stormwater during summer months, and attenuating peak flows. The treated stormwater is collected by under-drains and returned to the stormwater system or infiltrated back into the ground. The Cohasset Rain Garden Project is an example of applying LID techniques to retro-fit existing drainage systems in a developed watershed. LID is an innovative approach to stormwater management involving site design that duplicates the hydrologic features of an undeveloped watershed. Instead of conveying, managing and treating stormwater in large, costly facilities located at the bottom of drainage areas, and often outside of the natural watershed, LID simulates natural hydrologic cycles by addressing stormwater through small, cost-effective landscape features located at the lot level. These landscape features include permeable paving, bio-retention cells / rain gardens, grass swales, filter strips, disconnected impervious areas, and cistern collection systems. Rain gardens represent a low-cost, low-maintenance technique to improve the quality of stormwater that enters the water supply