Page 125 - California Stormwater Workshop Handouts

Industrial General Permit Fact Sheet

new or retrofitted treatment control BMPs to meet the TMDL implementation
requirements.

To arrive at these design storm standards, the State Water Board has relied heavily
on previous Water Board decisions concerning treatment efficacy for municipalities,
published documents, stakeholder comments, and reasonableness. In 2000, the
State Water Board issued State Water Board Order WQ 2000-11, which upheld Los
Angeles Regional Water Board's permit requirements which mandated that all new
development and redevelopment exceeding certain size criteria design treatment
BMPs based on a specific storm volume: the 85th percentile 24-hour storm event.
This design storm standard was based on research demonstrating that the standard
represents the maximized treatment volume cut-off at the point of diminishing
returns for rainfall/runoff frequency. 8 On the basis of this equation, the maximized
runoff volume for 85 percent treatment of annual runoff volumes in California can
range from 0.08 to 0.86 inch depending on the imperviousness of the watershed
area and the mean amount of rainfall. This design storm standard is referred to as
the Standard Urban Storm Water Mitigation Plan’s volumetric criterion and there are
multiple acceptable methods of calculating this volume. For more information, see
the California Stormwater Best Management Practices Handbook.9

The San Diego Regional Water Board first established both volumetric and flow-
based design storm criteria for NPDES MS4 permits. It is generally accepted by civil
engineers doing hydrology work to use twice the peak hourly flow of a specific storm
event to use as the basis for flow-based design of BMPs. This General Permit
therefore establishes the flow-based design storm standard to be twice the peak
hourly flow of the 85th percentile 24-hour storm event.

The primary objective of specifying a design storm standard is to properly size BMPs
to, at a minimum, effectively treat the first flush of run-off from all storm events. The
economic impacts of treating all storm water from a facility versus the minimal
environmental benefit of complete treatment justify the design storm approach. It is
unrealistic to require each facility to do a cost benefit analysis of their treatment
structures. To simplify the requirements for design, the State Water Board reviewed
research from the City of Portland10 and the City of San Jose11 to determine the
volume of each rain event compared to the amount of events that occur for that
volume. The results of their findings show an inflection point that is typically found at
approximately the 80 to 85 percentile of recorded storm events.

8 California Regional Water Quality Control Board Los Angeles Region, Standard Urban Storm Water Mitigation Plans and
Numerical Design Standards for Best Management Practices - Staff Report and Record of Decision (Jan. 18, 2000)
<http://www.swrcb.ca.gov/rwqcb4/water_issues/programs/stormwater/susmp/susmp_final_staff_report.pdf>. [as of February 4,
2014].

9 California Stormwater Quality Association, Stormwater Best Management Practice New Development and Redevelopment
Handbook (2003) <http://www.casqa.org/>. [as of February 4, 2014].

10 City of Portland Oregon. Portland Stormwater Management Manual Appendix E.1: Pollution Reduction Methodology E.1-1
(August 1, 2008). <http://www.portlandoregon.gov/bes/article/202909>. [as of February 4, 2014].

11 California Stormwater Quality Association (CASQA). CASQA BMP Handbook (January 2003) New Development and
Redevelopment (Errata 9-04) <http://www.casqa.org/>. [as of February 4, 2014].

Order 2014-0057-DWQ 41