- Maryland Department of Transportation, State Highway Administration
- KCI Technologies
- Montgomery County, Maryland
- Our Role
- Assessment, Design, Project Management Assistance.
- 200 swales
- Enhanced accuracy of statewide stormwater treatment database
- Support for state’s targeted reduction of Chesapeake Bay pollutants
- Inventory of existing swales assists stormwater treatment planning.
Committed to protecting the Chesapeake Bay from pollution, the Maryland Department of Transportation’s State Highway Administration (MDOT SHA) is identifying and assessing existing water quality grass swales along rural roadways and in residential subdivisions. The goal is to determine the extent to which swales provide treatment in accordance with National Pollutant Discharge Elimination System (NPDES) permit standards.
The assessment supports state goals of reducing the amount of nitrogen, phosphorus, and suspended solids carried into the bay by untreated runoff. Grass swales remove pollutants through vegetative filtering, sedimentation, biological uptake, and infiltration into underlying soil. They are a common drainage feature along Maryland’s highways, but have not been included in MDOT SHA’s NPDES database of treatment systems until now. Adding them to the database provides a more accurate picture of roadway runoff treatment in the state.
As part of MDOT SHA’s assessment, Gannett Fleming verified, measured, and mapped approximately 200 grass swales identified as potentially effective treatment features in Montgomery County. The team used GIS mapping to verify the accuracy of each swale’s location. It completed field measurements of the swales’ slopes, ditch lengths, bottom widths, and velocities to help determine the extent of stormwater treatment each swale provided. Field analyses verified the size and location of drainage areas larger than 3 acres.
The team also identified factors that would disqualify a swale as an effective feature according to NPDES guidelines, including underlying soil contamination, insufficient infiltration capacity and close proximity to structures. Effective swales were added to treatment totals to help the county achieve state total maximum daily load reduction requirements. Swales that did not meet NPDES standards were identified for future evaluation, to determine if retrofits would improve their treatment capabilities.
- Locations of 200 existing roadside grass swales verified and mapped
- Swale measurements assisted in determining existing treatment capacity
- Identified constraints on treatment effectiveness establish retrofit possibilities.