PEAK DISCHARGE FOR SMALL AGRICULTURAL WATERSHEDS

Accurate peak discharge estimates are important when sizing highway cu lverts 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 watersh eds in Nebraska. An analysis of time of concentration t(c) methods was undertaken because of this parameter's importance in many peak-flow m ethods. Seven t(c) equations were compared to recorded t(c) values fro m 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 pea k-flow methods were then compared to the recorded peaks and historical records to find the best estimation method. Based on three storm seas ons of data, a modified form of the Kirpich equation and the U.S. Soil Conservation Service average-velocity equation estimate the time of c oncentration adequately. Peak discharges were most closely predicted u sing statewide regression equations, the Fletcher method, and the rati onal method. Results for the 25-year event (culvert design event) tent atively support the use of statewide regression equations.