Guidelines for Hydrologic Design for Nonhomogeneous Microwatersheds Using the Rational Method

Abstract

The Rational method is frequently used to calculate the peak discharge from a watershed for hydraulic design. However, the Rational method is based on several assumptions that can lead to underdesign and unexpected flood risk. The goal of this research was to improve the understanding of the effects of nonuniformity of land cover and watershed slope on hydrologic design for microwatersheds. The problematic assumptions of the Rational method were challenged through the development of trial microwatersheds with nonhomogeneous runoff coefficients and slopes. The results showed that under certain hydrologic conditions, the traditionally computed peak discharge rates could underestimate the actual maximum discharge that results from a subarea of the watershed. This peak was defined as a premature peak. General guidelines were developed to identify the hydrologic conditions for which a premature peak would be expected to occur. Following these guidelines should reduce flood risk in developing watersheds.