Restoring the Hydrologic Cycle by
Controlling Runoff from Smaller Storms
Dr. Andrea L. Welker, PE
Professor
Civil and Environmental Engineering
Point sources
Non-point sources
What happens
to the
hydrologic cycle
when we build?
3
Small storms dominate
400
Philly Wings Airport – 5 years of data (07 to 12)
94% of storms 1” or less
83% of storms 0.5” or less
Number of rain events
350
300
250
200
150
100
50
0
0.25
0.50
0.75
1.00 1.25 1.50 1.75
Amount of rain (in)
2.00
3.00
4.00
Larger storms
Gray infrastructure
Get rid of water
Centralized
Still need flood control!
Smaller storms
Green infrastructure
Hold onto water
Decentralized
Our campus is
a laboratory
1’ 2’
www.villanova.edu/vusp
4’
7
Stormwater Control Goals
Control
Volume of
Runoff
Control
Peak Flow
Rates
Control Pollutants
Promote Evapotranspiration
Establish
Wetland
Structure
and
Function
Infiltration
Trench/Bed and
Pervious
Pavement
Yes
Yes
Yes
A little
No
Rain
Garden/BioInfiltration
Yes
Yes
Yes
Yes
No
Green Roof
Yes
Yes
Yes
Yes
No
Constructed
Wetland
Yes
Yes
Yes
Yes
Yes
Wet Pond/
Retention Basin
Yes
Yes
Yes
Yes
Yes
Type of SCM
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8
Example: car dealership
Will probably need
several SCMS to
significantly reduce
their fees…
Green roof
Captures ~0.5 to 1 in of rain
Promotes evapotranspiration
Increases insulation
Cooling effect
Potential problems – plant health during
droughts, export of nutrients
Pervious pavement
Captures ~1 to 2 in of rain
Promotes infiltration
Potential problems – clogging
Rain gardens
Captures ~0.5 to 1.5 in of rain
Promotes evapotranspiration and infiltration
Adds beauty
Cooling effect
Potential problems – invasive species,
maintenance
On going research…
Longevity
Maintenance
Monitoring
Prediction of
performance
Results from recent work
• Watershed scale effects
of rain gardens
– Link SCM installation to
watershed-wide
restoration goals by
using keystone species
– Use widely available info
(like FEMA flood maps),
estimates of geometry
of stream, and depth
and flow requirements
for fish
– Rain gardens can
improve habitat by
reducing volume,
increasing baseflow
(more constant depths),
and reducing velocity
• Individual scale
% passing
Native soil
100
50
0
10
1
0.1 0.01 0.001
Grain size diamter (mm)
MW-2
MW-4
MW-6
MW-7
0.6
GW EL RISE, METERS
– Our rain garden performs
well: infiltration, ET,
pollutant removal
– Use of “native” soils
– Rain needs to be 1.1 cm or
greater to produce a
measurable rise in gwt of
3.6 cm
– Takes about 3 days to
dissipate
Mixed soil
R² = 0.5704
0.4
0.2
0.0
0
20
40
EVENT PRECIPITATION, MM
60
% passing
• Rain garden soils
– Transparent spec
based on GSD
– Typical soils here
have ~5% organics,
but PA BMP manual
calls for 20-30%
organics
– Lower K = more ET,
higher K = more
infiltration – goals?
100
50
0
10
0.1
0.001
Grain size diamter (mm)
Design
favors
evapotranspiration
Design
favors
infiltration
Our research is supported by: PA DEP, US EPA, The William Penn
Foundation, and the VUSP Partners
THANK YOU!