Approximate Method for Quick Flood Plain Mapping

Flood hazard must be quantitatively defined to guide individual land use and construction decisions as well as zoning and building code regulations. An approximate method for defining flood risk is presented for the purposes of: (1)Making initial estimates to guide interim decisions prior to the completion of more thorough studies; and (2)checking completed studies for reasonableness. Since most stream cross sections and flow-frequency relationships are smooth curves, the stage-frequency curve can be extrapolated to rarer events from the relatively easier to obtain data on common events. Annual flood peaks and rating curves for 51 USGS stream gages with over 50 yr of record are used to develop curves normalized with respect to the elevation difference between the crest stages of the 10-yr and 25-yr events. Flood stages are estimated for 37 other streams from these normalized curves, and the results compare very favorably with those achieved through more thorough studies. Limitations of the method are considered.