Page 44 - California Stormwater Workshop Handouts

Industrial General Permit Order

6. Design Storm Standards for Treatment Control BMPs

All new treatment control BMPs employed by the Discharger to comply with
Section X.H.2 Advanced BMPs and new sediment basins installed after the
effective date of this order shall be designed to comply with design storm
standards in this Section, except as provided in an Industrial Activity BMP
Demonstration (Section XII.D.2.a). A Factor of Safety shall be incorporated
into the design of all treatment control BMPs to ensure that storm water is
sufficiently treated throughout the life of the treatment control BMPs. The
design storm standards for treatment control BMPs are as follows:

a. Volume-based BMPs: The Discharger, at a minimum, shall calculate13
the volume to be treated using one of the following methods:

i. The volume of runoff produced from an 85th percentile 24-hour storm
event, as determined from local, historical rainfall records;

ii. The volume of runoff produced by the 85th percentile 24-hour storm
event, determined as the maximized capture runoff volume for the
facility, from the formula recommended in the Water Environment
Federation’s Manual of Practice;14 or,

iii. The volume of annual runoff required to achieve 80% or more
treatment, determined in accordance with the methodology set forth
in the latest edition of California Stormwater Best Management
Practices Handbook15, using local, historical rainfall records.

b. Flow-based BMPs: The Discharger shall calculate the flow needed to be
treated using one of the following methods:

i. The maximum flow rate of runoff produced from a rainfall intensity of
at least 0.2 inches per hour for each hour of a storm event;

ii. The maximum flow rate of runoff produced by the 85th percentile
hourly rainfall intensity, as determined from local historical rainfall
records, multiplied by a factor of two; or,

iii. The maximum flow rate of runoff, as determined using local historical
rainfall records, that achieves approximately the same reduction in
total pollutant loads as would be achieved by treatment of the 85th
percentile hourly rainfall intensity multiplied by a factor of two.

13 All hydrologic calculations shall be certified by a California licensed professional engineer in accordance with the
Professional Engineers Act (Bus. & Prof. Code § 6700, et seq).

14 Water Environment Federation (WEF). Manual of Practice No. 23/ ASCE Manual of Practice No. 87, cited in
chapter 5 (1998 Edition) and Cited in Chapter 3 (2012 Edition) .

15 California Stormwater Quality Association. Stormwater Best Management Practice New Development and
Redevelopment Handbook. < http://www.casqa.org/ >. [as of July 3, 2013].

Order 2014-0057-DWQ 36