- STORMWATER MANAGEMENT
- CONSTRUCTION OVERSIGHT
Montgomery County, MD
The Department of Environmental Protection in Montgomery County, MD (MCDEP) is retrofitting some of its public facilities with Environmental Site Design techniques to achieve the County’s goal to treat 20% of existing impervious areas. Nobis designed a LID stormwater management facility on a gymnasium addition at this elementary school located in a well-established residential neighborhood in Potomac.
Nobis designed a three- and four-tiered bioretention facility within an existing slope. Grass weirs are provided at each tier where the stormwater collects and percolates, and is treated for pollutants by bioretention media. The overflow runoff is collected at the bottom tier and ties into an existing drainage system. The site was constrained by a 100-year floodplain to the north, limited parking areas that could not be affected, and substandard playing field sizes that could not be reduced. To meet current state and county stormwater management regulations, underground stormwater management facilities were not feasible. The stormwater goal was for post-development hydrology to replicate woods in good condition. In addition to stormwater management design, Nobis obtained NPDES permits through Maryland Department of the Environment as well as a floodplain district permit and erosion and sediment control permits.
This project was met with challenges during design and construction. The site constraints limited the available area for stormwater treatment to an existing 3:1 slope. To limit the facility to a slope area, a series of retaining walls needed to be constructed to create “steps” in the hillside. Three- and four-tiered areas were created to allow for biofilters to be placed on each tier. Designing the system to be stable on the hillside required the use of HDPE liners at each biofilter to prevent saturation issues around the wall footings. The tight constraints of working on the hillside required the retaining wall contractor and the biofilter installer to work in conjunction one tier at a time, starting at the bottom of the hill and working upwards. Construction was completed in 2012 on time and within the budget. Nobis performed construction inspection services for the stormwater management facilities as well as shop drawing review and as-built preparation.
Nobis completed the design of the project within the original schedule and delivered site permits and approvals more than one month prior to project bidding and more than three months prior to the start of construction. The collaborative efforts of Nobis, MCDEP and Montgomery County Public Schools allowed the project to move forward without delay and with a unique solution that met the needs of all stakeholders.