Coastal A-Z [return to Table of Contents] Low Impact Development and Bioretention It is well documented that any increase in impervious surface has a direct relationship to increased runoff into the ocean or other receiving water body by preventing water from naturally infiltrating into the ground. Generally speaking, parking lots, roads and other paved surfaces not only increase the amount of impervious surface, they also collect toxic materials from cars which are then transported to the ocean. These materials include petroleum hydrocarbons, other automotive fluids, and toxic metals such as copper, lead, manganese and zinc. Considering the important marine resources and recreational use of our coasts we believe it is essential that all efforts be made to minimize the impacts of polluted runoff. Fortunately, there are ways to minimize these impacts and still allow for parking and other associated development. Broadly, this approach is known as Low Impact Development (LID) and uses various land planning and design practices and technologies to simultaneously conserve and protect natural resource systems and reduce infrastructure costs. LID still allows land to be developed, but in a cost-effective manner that helps mitigate potential environmental impacts. More specifically "bioretention", an LID technique, can reduce runoff impacts from parking lots. The goal of bioretention is to reduce the impervious surface of parking lots, side walks, etc. and also provide avenues for infiltration and/or natural filtration of runoff before it reaches the ocean. These techniques could also be useful in areas where there is a desire to channel water away from bluffs. Several studies have been conducted to analyze the effectiveness of various LID practices based on hydrology and pollutant removal capabilities. Bioretention areas, grass swales, permeable pavements and vegetated roof tops were the most common practices studied. These techniques reduce the amount of Effective Impervious Area (EIA) in a watershed. EIA is the directly connected impervious area to the storm drain system and contributes to increased watershed volumes and runoff rates. There are documented case studies that conclusively link urbanization and increased watershed imperviousness to hydrologic impacts on streams. Existing reports and case studies provide strong evidence that urbanization negatively affects streams and results in water quality problems such as loss of habitat, increased temperatures, sedimentation and loss of fish populations (USEPA, 1997). In general, bioretention areas have been found to be effective in reducing runoff volume and in treating the "first flush" (first 1/2 inch of rain) stormwater. Results from several studies indicate that removal efficiencies were quite good for both metals and nutrients. Effluent volumes were lower than influent volumes. The effectiveness of grass swales was also quite good for both pollutant removal and runoff volume reduction. Reduction of impervious surfaces can greatly reduce the volume of runoff generated by rainfall. Several methods can be employed to reduce total impervious surface area. Permeable pavements and vegetated rooftops are two methods to accomplish this goal. Vegetated rooftops have been used extensively in Germany for more than 25 years and results show up to 50% reduction in annual runoff in temperate climates. Many opportunities exist to retrofit these systems into older highly urbanized areas of the United States. Permeable pavements can also reduce impervious surfaces. However, they are more expensive to construct than traditional asphalt pavements. Costs of these systems may be offset by the reduction of traditional curb and gutter systems to convey stormwater. Benefits of these alternate pavement types include better infiltration, ground water recharge, reduction in runoff volume and treatment of stormwater for pollutants. A study in Florida indicated that less than 20% of rainfall was converted to runoff when using permeable pavements. Studies done by the University of Washington compared several different treatments of varying permeablility. The study results indicated that the higher the amount of perviousness of the treatment, the greater the reduction of runoff volume and pollutant loadings. Included below are links to some websites that provide information on LID and bioretention. Low Impact Development: LID Info from U.S. Department of Housing and Urban Development Low Impact Development Center EPA Bioretention: EPA, link #1 and link #2 Low Impact Development Center Prince George's County, Maryland University of Maryland There are many useful fact sheets from the Stormwater Manager's Resource Center (SMRC), an EPA sponsored program