Anne Arundel County Department of Public Works

OBJECTIVE MINIMIZE OR ELIMIINATE FLOODING FROM FREQUENT SMALL STORMS Storms of 1- to 2-year frequency are causing flooding in the community 1-year storm = 2.7 inches in 24 hours 2-year storm = 3.2 inches in 24 hours IMPROVE WATER QUALITY OF STORMWATER RUNOFF Use Low Impact Development SWMgt approaches MINIMIZE PRIVATE PROPERTY and UTILITY ENCROACHMENTS

CHALLENGES Selby Community has no adequate storm drain system Low vertical relief (flat slopes) make it difficult to move stormwater runoff High water table further impedes the management of stormwater runoff Jurisdictional wetlands limit management options Annual cleanout of existing stream channels and wetlands is prohibited by regulations Sanitary Sewer System constrains area for retrofits

PREVIOUS EFFORTS BY COUNTY Earlier Effort by County resulted in design with very large multiple pipes that required raising roads. Interference with existing Sanitary Sewer System became prohibitive. There was no water quality improvement with the earlier design. Large pipes required substantial encroachment on private property

CURRENT APPROACH Use Low Impact Development (LID) or Environmental Site Design approach to reduce runoff volume to downstream areas Improve water quality of runoff using LID techniques Minimize conflicts with private property and utilities

LID APPROACH Using bioswales Use Bioswales in upstream areas Infiltrate first few inches of runoff where soils will allow Use Swales with underdrains where soils won’t infiltrate The swales will drain within 24 hours They support an attractive, functional landscape plan

LID APPROACH (continued) They slow the runoff enough to reduce downstream flows in two ways Storing runoff temporarily in swales and underground in the soils actually reduces total volume of runoff Increasing the time for runoff to concentrate reduces the magnitude of peak flows Bioswales treat runoff and improve water quality

Examples of BioSwales

Before After

Example from Portland Oregon The City of Portland is retrofitting some existing streets with LID/bioretention facilities. One of the retrofit objectives is to reduce the volume of storm runoff to their combined sewer system. Figure 1 a and 1 b show Siskiou Street before and after 2 bioretention cells were retrofitted. Figure 2 presents the results of a 25 year storm simulation that the City conducted to determine the effectiveness of the retrofit. It can be observed that the reduction in peak flow to the storm sewer, which is how we size storm darin pipes, and the total volume of flow to the pipe was dramatically reduced. This is an exciting “fact”!

Peak Runoff Reduction with Bioretention, Portland Oregon Figure 1 a – Before Figure 1 b After

Peak Runoff Reduction with Bioretention, Portland Oregon Figure 2

SEE DISPLAY BOARD

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