Prediction of Peak Flows on Small Watersheds in Oregon for Use in Culvert Design1

Using data from 80 Oregon watersheds that ranged in size from 0.54 km2 to 27.45 km2, equations were developed to predict peak flows for use in culvert design on forest roads. Oregon was divided into six physiographic regions based on previous studies of flood frequency. In each region, data on annual peak flow from gaging stations with more than 20 years of record were analyzed using four flood frequency distributions: type 1 extremal, two parameter‐log normal, three parameter‐log normal, and log‐Pearson type III. The log‐Pearson type III distribution was found to be suitable for use in all regions of the State, based on the chi‐square goodness‐of‐fit‐test. Flood magnitudes having recurrence intervals of 10, 25, 50, and 100 years were related to physical and climatic characteristics of drainage basins by multiple regression. Drainage basin size was the most important variable in explaining the variation of flood peaks in all regions. Mean basin elevation and mean annual precipitation were also significantly related to flood peaks in two regions of western Oregon. The standard error of the estimate for the regression relationships ranged from 26 to 84 percent.