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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)