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??