American Society of Civil Engineers

Graphical Calculation of First-Flush Flow Rates for Storm-Water Quality Control

by David C. Froehlich, Ph.D., P.E., D.WRE, M.ASCE, (Consulting Engr., 303 Frenchmans Bluff Dr., Cary, NC 27513-5662. E-mail:

Journal of Irrigation and Drainage Engineering, Vol. 135, No. 1, January/February 2009, pp. 68-75, (doi:

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Document type: Journal Paper
Abstract: Regulations for mitigating nonpoint source pollution from small catchments often include requirements for treating a first-flush depth of runoff, either by storing the storm water until it can be treated and released, or by passing it through a filtering device. In either case, the structural measure used to improve water quality needs to be designed or selected to accommodate a flow rate that corresponds to the first-flush runoff depth. An uncomplicated graphical procedure for calculating first-flush design flow rates is presented that is based on standard National Resource Conservation Service rainfall — runoff computation methods in which excess precipitation obtained by applying the runoff curve-number approach to 24-h design storm storms is transformed to runoff using triangular unit hydrographs. The solution is made dimensionless by grouping parameters, and, as a result, can be condensed into a single graph that provides highly accurate flow rate estimates.

ASCE Subject Headings:
Flow rates
Stormwater management
Water quality
Quality control