The runoff from an office building parking lot at the Maryland Department of Transportation on Broening Highway, Baltimore, needed to be managed. Due to the size of the site there was inadequate space for a landscape-based stormwater application that could encompass the required water quality flow rate. The entire drainage area is approximately ¼ acre, making it a very tight site.
There were various pollutants of concern in the runoff from the parking lot, including trash and debris, sediments (fine, medium and coarse), oil and heavy metals (Cu, Zn, Pb and Cd).
The engineer divided the drainage area into 3 sections. The individual areas were elevated and the runoff was conveyed by sheet flow into 3 swales using parking blocks. The swales were designed to capture and temporally hold the pretreatment and recharge volume.
The swales direct runoff into linear precast StormFilter units designed with 3 cartridges in a trench vault (10' x 3') to treat the remaining water quality flow. Each cartridge has a capacity of 15-gpm. The cartridges are equipped with CSF® leaf media to remove sediments, oil, and metals typically found in urban runoff. When the runoff volume is greater than the volume of the system, overflow is directed to a catch basin to drain through an outfall. The system provides the same function as a sand filter, but provides the same level of treatment in a much smaller space.
The site designed was based on low impact development (LID) principles. Curbs and gutters were eliminated and surface runoff from the parking lot was treated close to the source. The StormFilter provided appropriate treatment for overflow from the swales, which eliminated standing water.
This project is an example of how to integrate a structural BMP with a landscape-based BMP to achieve the desired stormwater management goal and follow low impact development principals. Stormwater runoff is managed close to its source using a multi-benefit natural feature system, in this case vegetated swales.
The StormFilter is used to provide polished effluent quality within a tight footprint, so the water quality flow from the design storm was adequately treated. Meanwhile the integrated system was able to convey the peak flow, avoiding the potential problems (water ponding, scouring) that can be associated with swales