F.1
As discussed in Vol II–4.2 Conveyance System Design Flow, the Rational Method, the Santa Barbara Unit Hydrograph (SBUH) Method, the Western Washington Hydrology Model (WWHM), and MGSFlood may all be used to determine the conveyance design flow rates provided that the basin limitations for each option are met. The Rational Method is preferred by Kitsap County for design of systems serving smaller contributing basins primarily because it tends to provide higher conveyance design flow rates than hydrograph methods, resulting in a more conservative design with a built-in safety factor.
Only the Rational Method equation is provided below; refer to Volume III, Chapter 2 of the Ecology Manual for instructions on using the SBUH Method and WWHM, and consult with the model user manual for a complete description on how to use MGSFlood. With the Rational Method, peak runoff rates can be determined using Equation F.1:
Q = C * I * A
where:
Q = runoff (cubic feet per second [cfs]
C = runoff coefficient (dimensionless units); see Table F.1
I = rainfall intensity (inches per hour [in/hr]); see Figure F.1
A = contributing area (acres)
Figure F.1. Rainfall Intensity-Duration Curves.
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