Anne Arundel County Department of Public Works

OBJECTIVEMINIMIZE OR ELIMIINATE FLOODING

FROM FREQUENT SMALL STORMSStorms of 1- to 2-year frequency are causing

flooding in the community1-year storm = 2.7 inches in 24 hours2-year storm = 3.2 inches in 24 hours

IMPROVE WATER QUALITY OF STORMWATER RUNOFFUse Low Impact Development SWMgt

approachesMINIMIZE PRIVATE PROPERTY and UTILITY

ENCROACHMENTS

CHALLENGESSelby Community has no adequate storm drain

systemLow vertical relief (flat slopes) make it difficult to

move stormwater runoffHigh water table further impedes the

management of stormwater runoffJurisdictional 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 COUNTYEarlier 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 APPROACHUsing bioswales

Use Bioswales in upstream areas Infiltrate first few inches of runoff where soils

will allowUse Swales with underdrains where soils won’t

infiltrateThe swales will drain within 24 hoursThey support an attractive, functional

landscape plan

LID APPROACH (continued)They slow the runoff enough to reduce

downstream flows in two waysStoring 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

Examples of BioSwalesBefore After

Example from Portland OregonThe 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 1a and 1b 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 1a – Before Figure 1b After

Peak Runoff Reduction with Bioretention, Portland Oregon

Figure 2

SEE DISPLAY BOARD