Selby Bay Storm Drain Improvement Anne Arundel County, MD

Project Owner’s Name/Address:
Department of Public Works
Anne Arundel County
2662 Riva Road
Annapolis, Maryland 21401
Mr. Ken Weeks
(410) 222-7908

Completion Date:        Sept 2011

Project Features:

• In-situ infiltration tests
• Wetland Delineation
• Performed the H&H analysis
• Computed the  reduced peak discharges due to proposed infiltrating bioswales
• Introduced in-situ soil enhancements to improve infiltration to AACo
• Storm Drain improvements design to elliminate frequent flooding and improve water quality in an old residential community with many exisitng constraints
• Presented design to Community Association and answered questions.
• Prepared and secured all permits
• Prepared all construction plans and documents

The Selby Bay community in Edgewater Maryland in Anne Arundel County was evaluated for opportunities to apply Low Impact Development Stormwater Management practices and ESD techniques in an effort to reduce discharges and gain water quality improvements.  The community has narrow right of ways, many existing utilities, and a high water table.  The methods described below can be applied in many coastal Bay communities to improve drainage and water quality.

A desktop analysis of land use, land cover, soils, and topography was performed.  Field assessments were also conducted to assess drainage problems.  In situ infiltration tests were conducted and surprisingly positive results were obtained.

A detailed hydrologic and hydraulic (H&H) analysis of the study area was conducted with an innovative twist. In accordance with the provisions of Chapter 5 of the Maryland Stormwater Management Manual, the TR-55 model was used to calculate the flows using the Runoff Curve (CN) number method.  However, the traditional TR-55 method is applied innovatively as developed by McCuen and M. Clar to model the benefits of providing storage through use of swales (detailed in “Clar method for LID Design using TR-55/TR-20.”).  In general, this method uses a reduced CN and an

increased Time of Concentration (Tc) to account for the flow attenuation provided by the swales. 

Ultimately, the use of bioswales in the upper portions of the drainage area was chosen to provide the following benefits:  water quality treatment through infiltration and filtration processes, detention storage on-site both in the swales and in the biosoils that will be installed under the swales, increased time of concentration by lagging the initial rainfall, reduced Runoff curve numbers reflecting the infiltration and on-site storage, and a reduction of the downstream peak discharges allowing the use of smaller less expensive pipe systems to convey the runoff.   

The figures from top to bottom show a rain event and subsequent flooding, an in-situ infiltration test, and a bioswale schematic.