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)

http://www.mde.state.md.us/programs/Water/StormwaterManagementProgram/MarylandStormwaterDesignManual/Documents/www.mde.state.md.us/assets/document/chapter2.pdf

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.

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

0.00

0.50

1.00

1.50

2.00

2.50

Dauphin Erie Philadelphia Allegheny

Desig

n Ra

infa

ll De

pth,

Inch

es

Pre-Post 2-yr/24-hr Storm(PA DEP)

Section 438 Technical Guidance(95th Percentile)

* 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.

Hydrologic Soil Group B Soils

1.0-inch

0%

20%

40%

60%

80%

100%

120%

0.1 1 10

Philadelphia, PA - TSS (Residential)

0%

20%

40%

60%

80%

100%

120%

0.1 1 10

Philadelphia, PA - TSS (Commercial)

Rain Depth to Generate 80% of the TSS Mass Loading

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

120.00%

0.1 1 10

Harrisburg, PA - TSS (Residential)

0%

20%

40%

60%

80%

100%

120%

0.1 1 10

Harrisburg, PA - TSS (Commercial)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

EISA and Rainfall Depth Associated with 80% Annual TSS

95th Percentile

Commercial

Residential1.0-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