Flood frequency by regional synthesis

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Abstract
Design of highway bridges in California has been predicated on waterways required for 30‐yr to 100‐yr floods. Determination of such floods is highly speculative because of relatively short stream‐flow records. Longer records for an average stream of a hydrologically homogeneous region have been synthesized by combining nondimensional peak discharges, so as to derive factors for extrapolation of frequency curves for each stream of the region.Reliability of the method, depending upon definition of homogeneity of a region, was tested by a graphical comparison. The test was devised by hypothesizing a ‘perfect’ 1000‐yr flood record and dividing it by chance distribution into 20 50‐yr records. Superimposed frequency graphs of these 20 hypothetical records gave a visual measure of disparities to be expected in natural records of comparable streams.The paper, illustrated by charts, briefs the development of the graphical test, describes the application to one region in California, and shows similar results in a second region of entirely different character.

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