SELBY STORM DRAIN RETROFIT CONCEPT PLAN
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Transcript SELBY STORM DRAIN RETROFIT CONCEPT PLAN
Anne Arundel County
Department of Public Works
Presented by:
Shannon Lucas, P.E.
Brightwater, Inc.
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
Responsibility
Residents of Chesapeake Bay
Watershed
Residents of the South River
Watershed
Impairments
Nutrients (Phosphorous and
Nitrogen)
Total Suspended Sediment
Fecal Bacteria
Fish and Benthic Bio-assessment
Doing our part – taking the
opportunity
Photo from southriverfederation.net
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
Old School Approach
Get the water off the roads and into
pipes as quickly as possible
Size pipes to carry large storm events
Concentrates runoff and moves it
quickly to downstream
Exacerbates problems downstream
more water all at once
erosive velocities
CURRENT APPROACH
Design for the frequent flooding events (2-year storm)
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
Light touch
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
Swales support an attractive, functional landscape plan
LID APPROACH (continued)
Swales 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
Location
Existing
Proposed
Fontron
to Severn
Segments of 15 and
18” CMP; swales
Bioswales, 15” RCP
Severn to
2nd
Swales, 12x15 and
18” CMP, sump at I-6
18” RCP
2nd
None
2-18” RCPs
2nd to
Beach
2-12x18” CMP, swale
(sump at I-1)
2-21” RCPs, enhance
swale (deeper)
Beach
2-15x18 CMPs
Extend swale and
eliminate portion of pipe,
headwalls and 3-18”
RCP
First
15x20” CMP
Headwalls and 3-18”
RCPs
QUESTIONS??