Peak Discharge for Small Agricultural Watersheds

To prevent flooding and ensure economic design, the accuracy of peak discharge estimates are crucial in sizing highway culverts. The Nebraska Department of Roads sponsored this study to identify the most acceptable peak discharge estimation method for use on small agricultural watersheds in their state. An analysis of time of concentration (TC) methods was conducted due to this parameter's importance in many peak-flow methods. Seven TC equations were compared to recorded TC values from four watersheds, each less than 5.2 km (2 mi) in area. Hyetograph-hydrograph analysis provided the field TC values. Next, six peak-flow methods were compared to the recorded and historical values for identification of the best estimation method. Based on three storm seasons of data, a modified Kirpich equation and the United States Soil Conservation Service average-velocity equation predict the TC adequately. The closest peak discharge predictions were generated by statewide regression equations, the Fletcher method, and the rational method. The use of statewide regression equations is also supported by results from the 25-year culvert design event.