Transcript SELBY STORM DRAIN RETROFIT CONCEPT PLAN

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
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 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
QUESTIONS??