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 95 th Percentile Rainfall Event: Technical definition: the measured precipitation depth accumulated over a 24-hour period for the period of record that ranks as the 95 th 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 95 th 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 2.00 Design Rainfall Depth, Inches 1.50 1.00 0.50 Pre-Post 2-yr/24-hr Storm (PA DEP) Section 438 Technical Guidance (95th Percentile) 1.0-inch 0.00 Dauphin Erie Philadelphia Allegheny Hydrologic Soil Group B Soils * 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.
120.00% Harrisburg, PA - TSS (Residential) 120% Philadelphia, PA - TSS (Residential) 100.00% 100% 80.00% 80% 60.00% 60% 40.00% 40% 20.00% 20% 0.00% 0.1 1 10 0% 0.1 1 10 120% 100% Harrisburg, PA - TSS (Commercial) 120% 100% Philadelphia, PA - TSS (Commercial) 80% 80% 60% 60% 40% 40% 20% 20% 0% 0.1 1 10 0% 0.1 1 10 Rain Depth to Generate 80% of the TSS Mass Loading
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 Commercial Residential 1.0-inch PA Average 0.20 0.00
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)
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