Impact of Rainfall Calculation on
Capture Volume
Ruth Ayn Hocker, P.E.
Chesapeake Bay Foundation
Shirley E. Clark, Ph.D., P.E., D. WRE
Penn State Harrisburg
Challenge: What Should The Water
Quality Treatment Volume Be?
80% Runoff Mass Discharge larger
than storm size that causes permit
concentration exceedences.
Figure Reference:
Pitt, R. and S.E. Clark (2008)
“Integrated stormwater
management for watershed
sustainability.” Journal of Irrigation
and Drainage Engineering.
134(5):548-555.
Challenge: What Should The Water
Quality Treatment Volume Be?
Water Quality Storms: associated
with permit pollutant
concentration exceedences (small).
Regulations set goals of 80% or
greater removal of pollutants from
runoff at outfall.
What is the appropriate water
quality storm event?
Figure Reference:
Pitt, R. Small Storm Hydrology
Lectures.
USEPA Definition of Water Quality
Storm for Federal Facilities
• Technical Guidance on Implementing the
Stormwater Runoff Requirements for Federal
Projects under Section 438 of the Energy
Independence and Security Act
(Section 438 Technical Guidance)
– Issued by EPA in December 2009
– http://www.epa.gov/owow/NPS/lid/section438/
Section 438 Technical Guidelines
95th Percentile Rainfall Event:
• Technical definition: the measured precipitation depth accumulated
over a 24-hour period for the period of record that ranks as the 95th
percentile rainfall depth based on the range of all daily event
occurrences during this period
• 24-hour period is typically defined as 12:00:00am to 11:59:59pm
• Minimum 20-30 year rainfall record is recommended [NOTE: All
stations in this analysis had > 30 years of rainfall data]
• Small rainfall events (≤ 0.1 inch) excluded
– absorption, interception, evaporation accounts for losses and these
storms produce little to no runoff.
Removing Small Events: Produce Negligible Runoff
(and Lowers the 95th Percentile Storm)
Using Interevent Period versus EISA Daily Rainfall
Maryland Unified Stormwater Sizing Criteria
and Environmental Site Design (ESD)
ENVIRONMENTAL SITE DESIGN: While Environmental Site Design (ESD) may be used to
address recharge volume requirements and WQv, limiting it to these criteria alone may
not provide sufficient treatment to mimic natural hydrology for wooded conditions or
address Channel Protection Volume.
http://www.mde.state.md.us/programs/Water/StormwaterManagementProgram/
MarylandStormwaterDesignManual/Documents/www.mde.state.md.us/assets/doc
ument/chapter2.pdf
Pennsylvania Regulation Chapter 102:
Delta 2
• 25§102.8(g):
(2)PCSM Plan stormwater analysis: “…PCSM BMPs
will…manage the net change for storms up to and including
the 2-year/24-hour storm event when compared to
preconstruction runoff volume and water quality”
(i) Existing predevelopment nonforested pervious areas
considered meadow in good condition
(ii) When the existing project site contains impervious
area, 20% of the existing impervious area to be disturbed must
be considered meadow in good condition or better, except for
repair, reconstruction or restoration of road- ways or rail lines,
or construction, repair, reconstruction, or restoration of utility
infrastructure when the site will be returned to existing
condition.
Federal versus States’ Design Rainfall
Depth Criteria
2.50
Design Rainfall Depth, Inches
2.00
1.50
Pre-Post 2-yr/24-hr Storm
(PA DEP)
Section 438 Technical Guidance
(95th Percentile)
1.00
1.0-inch
0.50
0.00
Dauphin
Erie
Philadelphia
Hydrologic Soil Group B Soils
Allegheny
* PA DEP Pre-Post 2-yr/24-hr
Storm values were computed
using CN hydrology to
estimate the net difference in
runoff from meadow to
impervious in accordance with
25§102.8(g), assuming HSG-B
soils.
Harrisburg, PA - TSS (Residential)
Philadelphia, PA - TSS (Residential)
120.00%
120%
100.00%
100%
80.00%
80%
60.00%
60%
40.00%
40%
20.00%
20%
0.00%
0%
0.1
1
10
0.1
Harrisburg, PA - TSS (Commercial)
1
10
Philadelphia, PA - TSS (Commercial)
120%
120%
100%
100%
80%
80%
60%
60%
40%
40%
20%
20%
0%
0%
0.1
1
10
0.1
1
Rain Depth to Generate 80% of the TSS Mass Loading
10
EISA and Rainfall Depth Associated with 80% Annual TSS
2.00
1.80
1.60
1.40
1.20
1.00
95th Percentile
0.80
0.60
0.40
0.20
0.00
Commercial
Residential
1.0-inch
PA
Average
Summary
• EISA:
– Simple, data available on internet, set procedure
– But: storm size may be lower than storm that generates
80% of TSS loads, smaller than using inter-event period
• 1-inch:
– Simple and set procedure
– But: storm size may be lower than storm that generates
80% of TSS loads
• Delta 2 and other CN/Rv procedures:
– Simple and set procedure
– But: variability in application (based on decisions of land
use and HSG)
Thank You