Sligo Park Hills Low-Impact Development Stormwater Management Design


With no stormwater or drainage infrastructure in place, one of the oldest developments in Montgomery County, Maryland, was plagued by recurring roadway flooding and the erosion of its steep terrain during heavy rains. To alleviate these concerns and provide as much stormwater quality treatment as possible with minimal construction impacts, Gannett Fleming designed 31 low-impact development (LID) stormwater management features for Sligo Park Hills. Situated along Sligo Creek in Silver Spring, the 50-acre wooded community is part of the Anacostia River watershed.

The features were designed to follow Maryland Department of the Environment criteria to use environmental site design (ESD) practices for treatment to the maximum extent possible (MEP). They provide treatment for the first inch of rainfall, groundwater recharge and runoff storage, and controlled release for one-year storms wherever possible. Construction was contained within existing county right-of-way lines to avoid private property impacts.

What We Did

The team used GIS and field surveys to identify all potential LID feature locations in the community. The type of feature used at each location was selected to provide the most effective management and treatment, based on drainage area, size, slope, and runoff velocity. Vegetated bioswales and rain gardens capture and filter runoff from tiny parcels of land up to larger impervious surfaces, such as parking lots. Streamside gabion-lined biofiltration systems containing drainage media, a bridging stone and separation layer, and an underdrain reduce pollutants in runoff by an estimated 50 to 60 percent and total suspended solids by more than 90 percent. They also alleviate the severe erosion of adjacent streambanks.

Water quality inlets installed along roadways promote sedimentation and remove free oil from runoff. Roadside parking pads made of permeable pavers capture and filter runoff and reduce roadway ponding. Together, the LID features treat runoff from 5.5 acres of impervious area, helping mitigate neighborhood flooding and erosion and providing vital pollution protection for the watershed.

Key Features

  • Customized, vegetated LID features provide natural runoff treatment for a variety of spaces throughout the community.
  • Water quality inlets trap free oils and sediment from roadway runoff.
  • Permeable pavers capture and treat roadside runoff and reduce ponding.


  • Reduced runoff pollutant by 50 percent, protecting Anacostia River.
  • Provided water quality treatment for 5.5 acres of impervious area.
  • Alleviated roadway flooding and streambank erosion.


Montgomery County Department of Environmental Protection and Department of Transportation


Silver Spring, Md.



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