Peak Discharge for Small Agricultural Watersheds

Abstract

Accurate peak discharge estimates are important when sizing highway culverts to prevent possible flood damages and to ensure economic design. The purpose of the undertaken study was to find the most acceptable peak-discharge-estimation method for use on small agricultural watersheds in Nebraska. An analysis of time of concentration t c methods was undertaken because of this parameter's importance in many peak-flow methods. Seven t c equations were compared to recorded t c values from four watersheds each less than 2 sq mi (5.18 km 2) in area. Field t c values were derived from a hyetograph-hydrograph analysis. Six peak-flow methods were then compared to the recorded peaks and historical records to find the best estimation method. Based on three storm seasons of data, a modified form of the Kirpich equation and the U.S. Soil Conservation Service average-velocity equation estimate the time of concentration adequately. Peak discharges were most closely predicted using statewide regression equations, the Fletcher method, and the rational method. Results for the 25-year event (culvert design event) tentatively support the use of statewide regression equations.