Impacts of Impervious Cover

ENSC 202 – 2004Impervious Cover

Impacts of Impervious Cover

Lecture 6 – ENSC 202


Urban Sprawl

Sprawl is dispersed, automobile-dependent development outside of compact urban and village centers along highways and in rural countryside.

http://www.vtsprawl.org/index.htm


Sprawl &Impervious

Cover

0 20 40 60 80

Commercial

Light Industry

Residential (multifamily)

Residential (townhouse)

Institutional

Residential (1/8 acre)

Residential 1/4 acre)

Residential (1/2 acre)

Residential (1 acre)

Residential (2 acre)

Open urban land

Parks, golf courses, cemetaries

Agriculture

Impervious area (%)

MeanSE

Center for Watershed Protection 2003


Sprawl and Water Quantity• Higher highs/lower lows• Intensification/flashiness

Center for Watershed Protection (2003)


Runoff as a function of Imperviousness

Center for Watershed Protection (2003) after Schueler (1987)


Stream Enlargement due to IC


Fine Sediment Deposition due to IC



Impacts of Deposited Sediments



Impacts of Suspended Sediments


Stormwater contains

more than sediments

State of Maine (1995)

…note the wide ranges


Pollutant concentrations differby land use

Burton and Pitt (2002) Stormwater Effects Handbook

A measure of variance in the data.


National Event Mean

Concentrations



The ‘Simple Method’ for calculating runoff loading from stormwater

Step 1 – What portion of the ‘effective’ rainfall becomes runoff?

Rv = 0.05 + (0.9 * IA)

where Rv = runoff coefficientIA = impervious area (%)

from Impacts of Impervious Cover on Aquatic Systems, p. 61 (CWP 2003)



Step 2 – How much total runoff is there in a typical year?

R = P * Pf * Rv

where R = runoff (inches)P = precipitation (inches)

Pf = fraction of rain events that produce runoff (~0.9)Rv = runoff coefficient (Step 1)

Note unfortunate mix of English and Metric units!



Step 3 – How much total loading occurs due to this runoff?

L = U * R * C * A

where L = loading in lbsU = unit conversion = 0.226R = runoff (inches, Step 2)

C = concentration (EMC, mg/L)A = area (acres)

Can be modified for bacteria


Loss of LWD due to IC


Biotic Impacts of

ImperviousArea

Center for Watershed Protection (2003) as noted.


Sprawl, impervious area, & impairment

Center for Watershed Protection 2003


Impaired Rivers



Impaired Lakes



Imperviousness and habitat qualityVariation within a watershed

0 10 20 30 40 50 Percent watershed impervious cover

Inde

x of

Bio

tic I

nteg

rity

(IB

I)100

80

60

40

20

0

Fairfax County (2001) in CWP (2003)


Variation among watersheds

Total watershed impervious cover

Inde

x of

Bio

tic I

nteg

rity

(IB

I)

Horner and May (1999) in CWP (2003)


Measuring Impervious Cover

Total versus Effective or Net IC

• Direct measurements• Inferred measurements

– from land use– from road density– from population


Stormwater management matters


Mission of the VermontWater Resources Board

“To ensure that rules which guide the management of Vermont's water resources and wetlands are adopted and (on appeal) are interpreted, by a citizen board which is independent of the Agency of Natural Resources (ANR).”


The WRB Stormwater ‘Docket’

“The purpose of the Docket is to create a forum for discussing the technical issues related to developing clean up plans for waters of the state impaired by stormwater in Vermont.”

“The intended outcome of the Docket is …[to summarize] technical information…and to …provide recommendations for developing clean up plans for Vermont’s stormwater impaired waters…”


Key Questions in the WRB Docket• Is it feasible to use ‘source control’ as a primary option?

• Can improvements be achieved in 5 years?

• Can we separate and deal with natural vs man-made sources of pollutants?

• Is a TMDL approach the best way to address clean up streams impaired by stormwater?

• Are stormwater ‘offsets’ a reasonable approach to stormwater management?

• Can we predict how ‘indicators’ of impairment will respond to stormwater treatment strategies?

• What is the best way to evaluate progress toward desired goals?

• What are the best ‘targets’ to judge when we have attained desirable goals?

• If we can’t attain the standards we want, what should we do?


Extra


Redesigning the American Neighborhood

Developing an Ecological & Socioeconomic Framework for Effective Stormwater Management

Purpose: To develop tools that will allow stakeholders, regulators, and researchers to visualize alternative future environmental states that they imagine collectively and then to optimize the mix of interventions at various scales, that will best balance environmental and social, as well as economic, criteria.

http://www.epa.gov/


Why focus on scale?

Effectiveness: uncertainOrientation: local protectionCost: known - mediumRisk: uncertain - medium

Effectiveness: unknownOrientation: source controlCost: uncertain – low?Risk: unknown – medium/high

Effectiveness: known – dependsOrientation: downstream protectionCost: known - highRisk: known - low


Why focus on scale?

Clearly, a mix of interventions is desirable. But what mix? For what purpose? Located where?


Primary Goal

Quantify the balances among environmental, economic, and social costs and benefits for storm water management at whole-watershed, neighborhood, and individual house scales in a typical New England landscape and climate.


Key Objectives

• Assessment: What are the opportunities for intervention?

• Evaluation: What are the comparative cost/benefits of these interventions?

• Participation: How can we better involve community stakeholders to devise successful solutions?

• Implementation: Can we demonstrate the these approaches work?


Project Focus Area


ButlerFarm

Subdivision


Key Collaborations

• US-EPA/SNR-UVM (McIntosh, Bowden, Todd, Voinov)

• Partnership with South Burlington (JB Hoover)

• Collaboration with key consultants (PEC, J Nelson)

• Advice from key stakeholders (Project Working Groups)

• EPA Demonstration grant (JB Hoover)

• NRCD implementation grant (A. Willard, B. Gabos)


National and Vermont StandardsComparison to selected VT Water Quality Standards

No TSS causing impairment

0.010 – 0.054 mg/L total phosphorus

2 and 5 mg/l nitrate for Class A and B waters

10 ug/L acute, 8 ug/L chronic*

23 ug/L acute, <1 ug/L chronic*

57 ug/L acute, 52 ug/L chronic*

<<1 mg/L as pesticides and PCB’s

No oil or grease causing impairment

18 E. coli cfu/100ml 3x in 30days (A1 and A2)

33 E. coli cfu/100ml once (A1 and A2)

77 E. coli cfu/100 ml (B)

•at hardness of 50 mg/L

VT Stormwater Manual, Vol 2 VT Water Quality Standards

Documents

Impacts of Impervious Cover