Tacitly Maximized Small Watershed Flood Estimates

A simple graphical method is presented for predicting the maximum flood peak on ungaged rural watersheds ranging in size from 1/5 to 5 sq miles. The procedure involves the use of two charts, the use of infiltration tables for soils and their vegetal covers, and a knowledge of the length of the longest collector, the height-drop across the watershed, and the 30-min rainfall extreme. Design values involve a latent maximization of peaks from longer or shorter rainfall durations. Results obtained from applying this method are compared to eighty-three floods observed on twenty-nine watersheds in eleven locations of the continental United States. Achievable accuracy appears compatible with enhanced speed of estimation. Because no hydrographic data besides rainfall are required, the method should be applicable in less-developed countries.