ch 8 storm water management and lid
TRANSCRIPT
8/3/2019 Ch 8 Storm Water Management and LID
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Water & Land Use:
Urban Stormwater,Watershed Management, and
Low Impact Development
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The Hydrologic Cycle
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Before 1970
Structural SWM
1970s-1980s
Good Drainage
1990s – 2000s
Low-Impact SWM
2010
Sustainable SWM
Objectives Provide adequate sw
drainage from
developed land; try to
control flood flows
Provide adequate
drainage, manage new
floodplain
development, mitigatestorm flows closer to
the source, apply
erosion and sediment
controls and best
management
practices (BMPs) for
runoff pollution
Adequate drainage by
on-site mitigation of sw
flows; infiltration to
support baseflows andlow flows; runoff
treatment; non-erosive
channel velocities;
protect/restore natural
drainage channels;
floodplain management.
Use watershed and
subwatershed approach
to integrate sw
management, flooddamage mitigation, water
quality, stream restoration
and sustainable and
livable community design.
Control
Measures
Structural methods:
Increase drainage
capacity, gutter
streets, enlarge/line
channels;
pipes/culverts;“armor” natural
channels with
concrete/rocks to
prevent channel
erosion; use sw
detention
Structural methods:
Mitigate storm flows
by on-and off-site
detention; increase
drainage capacity as
necessary.
More effective on-site
and other decentralized
runoff control and
treatment; encouraging
or mandating “low-
impact” development designs and integrated
sw control practices;
infiltration;
bioengineering to
restore natural
channels.
Land use planning and
design to manage
development density,
minimize and mitigate
impervious cover; low
impact and light imprint methods to achieve
desired densities and on-
site controls. Daylight
buried streams.
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Before 1970
Structural SWM
1970s-1980s
Good Drainage
1990s – 2000s
Low-Impact SWM
2010
Sustainable SWM
Design
Methods
Size capacities based
on the Rational Method
and other rudimentarytechniques
Analyze effects of
land use change on
sw quantity andquality and size
capacities using
sophisticated
computer modeling
techniques.
Use of both
computer models
and simpler sizingand design
methods to
estimate land use
impacts and apply
appropriate on-site
measures.
Integrative models
(BASINS, SUSTAIN)
for multipleobjectives;
landscape design,
new urbanist,
smart growth land
development
principles.
Program
Ordinances &
Financing
Public works funded by
tax dollars
SW ordinances
require developers
bear costs in
projects, sw fees
and tax dollars,
SW utilities; more
effective
prescriptive and
performance-based
sw ordinances;
impact fees; citizen
volunteers.
Stronger
federal/state
oversight; SW
integrated with
sustainable
community
planning;
ordinances reflect
best practices for
new and
redeveloped sites.
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The Clean Water ActGoal: ―Fishable & swimable waters‖
Elimination of pollutant discharges
Significant improvement in the quality of natural waters fromreduction of organic and sediment discharges
BUT…
Continued impairment of watersProblem shift from easy clean up of conventional pollution fromdischarge pipes to harder clean up of runoff pollution and toxics
From technology permits to TMDLs
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TMDLsCWA called for discharge limits for eachpollutant violation in order to meet WQS.
Total discharge allocations to regulated sources= WLA + LA = TMDL – Natural NPS
WLA – industrial and municipal waste load
allocationLA – NPS load allocations
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Total Maximum Daily Load (TMDL)
approach:Determining the TMDL to achieve WQS – this oftenrequires sophisticated monitoring data and modeling of discharges, receiving waters, and, for NPS, watersheds.
Allocating TMDL to sources— this requires factoring inequity and economic considerations.
Basing permits of regulated sources on TMDL allocations
Managing unregulated sources to achieve TMDLallocations
NRC report commented on difficulty of incorporating into
MS4 permits
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2007 GAO Report & 2009 NRC
ReviewEPA Role – more vigilant oversight needed,more financial support
More stormwater monitoring is needed.
Watershed models developed.
Convert piecemeal system into watershed-basedpermitting system.
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Water Quality
TradingImpaired for phosphorous
Dischargers: 5 industrial,2 POTW, 1 NPS
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Current
(lb/da)
TMDL
Target
(lb/da)
Reducti
on
(lb/da)
Increme
ntal
Cost/lb
WQ
equival
ent
Feasible?
(1)Herb’sFarm 753 527 225 $5 1:1 (2)5:1 (6) -Uncertainty is 50%
(2)Pleasantville POTW
791 633 158 $20 1:1 (1)1:3(4)5:1(6)
(1): $5 x 1 /0.5 = $10<$20 YES(4): $36 x 1 /3 = $12 < $20 YES
(3)Acme, Inc 547 410 137 $60 1:1(4,5)
(4): $36 x 1 / 1 = $36 < $60 YES
(4)Production Co.
228 171 57 $36 1:1(3,5)
(5)Widgets,Inc
165 124 41 $49 1:1(3,4) (4): $36 x1 /1 = $36 < $49 YES
(6)HopevillePOTW
62 50 12 $100 1:5(1)1:5 (2)
(1): $5 x 5 /0.5 = $50 <$100 YES(2): $20 x 5 = $100 = $100 ??
(7)AAACorps
195 166 29 $15 None: Lucky Creek compliance
(8)ChemCo. 1645 493 1151 None: reduction > supply
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State SW ManagementMaryland SW Management
Early leader because of Chesapeake Bay initiatives
in 1984.Developed a model stormwater managementordinance to help localities meet requirements.
Washington SW ManagementStrict requirements
Seattle provision requires the use of greenstormwater infrastructure.
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Portland SW Management
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Portland Ordinances & Manual1. Manage runoff as close to source as possible
2. Mimic natural hydrologic functions
3. Integrate runoff into the built environment
4. Design for multiple sustainable benefits
5.Act early to avoid costly mitigation andrestoration
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LID “Integrated Management Practices” (IMPs)
On-site detention, infiltrationConservation & minimization (C&M): conservingnatural conditions on-site and minimizingimperviousness
Landscaping (L): alter the terrain and vegetationcover to minimize runoff.Storage (S): retain or detain rainwater to prevent ordelay the generation of runoff.
Conveyance (C): convey runoff throughout the sitewhile providing infiltration.Infiltration (I): capture runoff and allow it to infiltrateinto the soil.
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Low Impact Development
LID uses landscape design features to mimic thenatural hydrologic regime.
LID measuresminimize impervious surface,
reduce piping and connectivity of impervious sources(reduce Effective Impervious Area),
minimize clearing and grading,
conserve and maintain natural vegetation, and
incorporate on-site detention and infiltration.
Runoff storage is provided throughout a site’slandscape.
Goal: maintain pre-development runoff volume and
time of concentration
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Bioretention Rain Garden
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Vegetated Infiltration Basin & Planter
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Pervious Pavements
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Swales
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Filtration:Sand filters
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Pollutant Removal Capabilitiesn TSS TP TN Carbon Bacteria HC Metals
SW Ponds 44 80 51 33 43 70 81 50-74
SWWetlands
39 76 49 30 18 78 85 40-69
Infiltration 6 95 70 51 54 ND ND 98-99
Filters 19 86 59 38 54 37 84 49-88
Swales 9 81 34 84 69 (25) 62 42-71
Ditches 11 31 (16) (9) 18 5 ND 0-38
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SW Management Practice SizingWQv = [(P)(Rv)(A)]/12 (acre-feet)
Rv = 0.05 x 0.009 (I)
I = Percentage impervious cover
P = 90% rainfall event number (about 1 inch,
may vary)
A = site area in acres
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In Fairfax County,
WQv = 0.5 in / 12 in/ft x Impervious surface ft2
= ft3
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Fairfax CountySizing SMC BMP
Store WQv on site
WQv = ½ inch rainfallover impervious surface
EXISTING CONDITIONS PROPOSED SITE CONDITIONS
Woods = 3,800 s.f. Woods = 1,300 s.f.
Roof + Pavement = 2,500 s.f. Roof + pavement = 5,100 s.f.
Lawn = 5,700 s.f. Lawn = 5,600 s.f.
Site drains to SE corner Roof leaders drain to driveway
Drainage pattern unchanged
WQv = 0.5/12 x 5100 = 213 cubic feet
Option A: add a 213 s.f. bioretention basin in the SE drainage corner.
Option B: use 213 s.f of permeable pavement in the SW drainage corner.
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From the Site to the Watershed Approach
LID and BMPs are essential for stormwater management and stream quality improvement
But ―urban stream syndrome‖ requires a
comprehensive watershed restoration approachincluding retrofit BMPs and reforestation of riparianbuffers and watershed canopy.
Fairfax County
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Fairfax County
Continued impairment despite
Environmental Quality Corridor
(EQC) restriction on riparian
Development since 1990
Virginia’s CBPA Resource Protection Areas:
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Virginia s CBPA Resource Protection Areas:
Our best mechanism for riparian area protection
Fairfax extended RPAs in 2004-05
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Fairfax County Watershed Management Plans
Watershed management plans for 30 watersheds and
800 stream miles below 50-acre catchment scale25-year prioritized list of improvement projectsintegrated into its MS4, TMDL, and CB response plans
Belle Haven Watershed below: 3 of 30 site-scalewatershed improvement projects
Stream restorationExtended detention
Bioretention
Anacostia Watershed
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Anacostia WatershedHistoric loss of 70% forest cover, 6,500acres wetlands; 25% impervious cover
1987 Agreement2007 funding2010 Anacostia Watershed Restoration Plan
SMC retrofits
Stream restoration
Wetland restoration; fish blockage removal
Riparian reforestation
Toxic remediation, trash reduction
Parkland acquisition1972 1985 1997
Reformulated Impervious Cover Model (ICM)
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Reformulated Impervious Cover Model (ICM)