This paper describes an evaluation of the assumption commonly employed in drainage design that the return period of the rainfall used to design a system is the same as the peak flow produced by that rainfall. Specifically, the sensitivity of the frequency response of three catchments to design storm parameters is examined. Parameters include, hyetograph shape, antecedent soil moisture and rainfall duration. A continuous simulation model is used to compute simulated historical frequency responses for three different long term rainfall records. Design storms are also developed from depth-duration-frequency analysis of the rainfall data. Comparisons are made on frequency graphs. It is concluded that significant parameter sensitivity exists but that an appronriate choice of design storm parameters can produce a design which yields peak flows of the desired return period.

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