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“ impervious surfaces are responsible for more stormwater runoff than any other type of land use” [1] …. Introduction. 5 Case Studies.

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“impervious surfaces are responsible for more stormwater runoff than any other type of land use”[1]…


5 Case Studies

[1] EPA, 2000: Low Impact Development (LID) - A Literature Review. EPA-841-B-00-005, Office of Water, Washington, D.C. [2]


Four Sites





Stormwater Quantity


Three paving materials

Manning’s n

Runoff Coefficient


Interlocking pavers


Porous asphalt

0.18- 0.29

Porous concrete


Objective: evaluate various surface materials in urban parking areas to reduce stormwater runoff and annual pollutant loads


“Porous pavement is a permeable asphalt or concrete surface that allows stormwater to quickly infiltrate to an underlying stone reservoir”

Pennsylvania Stormwater Management Manual

peak flow
Peak Flow

Q = CiA

  • Rational Method
  • widely used for smallurban drainage area
  • required limited data

As Tt=Tc increases peak runoff decreases

Peak Flow (cfs)

travel time

Length of Flow Path

434.73 ft

Tc= Tt for only one segment

S Congress

695.10 ft

695.10 ft

E 7th Street

632.33 ft

632.33 ft

632.33 ft

E. Riverside Dr

509.67 ft

509.67 ft

Burnet Road

Travel Time

“Travel time ( Tt ) is the time it takes water to travel from one location to another in a watershed”.

Time of Travel (hr)

Depth of precipitation (in)

Recurrence interval 2yr

Ferguson, B. 2005. Porous Pavements. CRC Press

annual pollutant loads
Annual Pollutant Loads

Simple Method

L = 0.226 * R * C * A

  • “No significant relationship was found for total suspended sediment based on impervious cover or development condition”
  • TP and TN: impervious cover ranges: 0-1% and 5-100%

Pollutant concentration: STORMWATER RUNOFF QUALITY AND QUANTITY FROM SMALL WATERSHEDS IN AUSTIN, TX, City Of Austin, Watershed Protection Department, Environmental Resources Management Division, Water Quality Report Series, COA-ERM/WQM 2006-1, November 1, 2006


Water Quality Volume


632,397.80 ft2

S Congress

93,5788.57 ft2

667,667.02 ft2

E 7th Street

1,706,624.16 ft2

658,754.68 ft2

E. Riverside Dr

902,001.86 ft2

621,467.95 ft2

Burnet Road

1,051,130.49 ft2

Average 10% reduction in WQvwith decrease impervious surfaces

An average increase of21% of WQv

New York State Stormwater Management Design Manual


Costs! / Incentives?

  • Regional Stormwater Management Participation Fees
    • “the number of impervious acres…based on the maximum allowable impervious cover”
  • BMP’s
  • Land

on average a 33% increase

note: Average price per ft2 from EPA, Dr. Barrett, City of Seattle Department of Planning and Development and City of New York

;base/storage bed not included in cost estimate.


Preliminary Conclusions

  • Maintaining flow path lengths
    • Regulation
    • Reduce stormrunoff and increase water quality
  • Disconnecting impervious surfaces
    • Porous surfaces
    • END

Simple Method

L = 0.226 * R * C * A

L= Annual load (lbs)

P= Pollutant Concentration

A= Area (acres)

R= Annual Runoff (inches)

R = P * Pj * Rv

P= Annual Rainfall (in)

Pj= Fraction of year rainfall events that produce runoff 0.9

Rv= Runoff Coefficient


Ia= Fraction of impervious cover


WQv=Water Quality Volume (ft3)

A= Total Area (ft2)

P= 90% Rainfall event (inches) 1.4 in for Austin


Ia= Fraction of impervious cover

Ap= Porous Pavement Surface (ft2)

n=Porosity of bed gravel (0.4)

dt=Depth of gravel bed/reservoir (ft)

WQv=Water Quality Volume (ft3)

New York State Stormwater Management Design Manual