Prediction of peak flows for culvert design on small watersheds in Oregon

Graduation date: 1982Forest engineers must frequently make flood frequency estimates for\ud very small watersheds when designing culvert installations. Empirical\ud formulae and simplified rainfall runoff models, the most commonly used\ud techniques to predict floods from very small watersheds, require considerable\ud engineering judgement to give reasonable results. As an alternative\ud to such methods, this study presents equations to predict peak\ud flows on small watersheds in Oregon. The equations were developed from\ud 80 watersheds ranging in size from 0.21 to 10.60 square miles.\ud Oregon was divided into six physiographic regions based on previous\ud flood frequency studies. In each region, annual peak flow data from\ud gaging stations with more than 20 years of record were analyzed using\ud four flood frequency distributions (Gumbel, two-parameter log-normal,\ud three-parameter log-normal, log Pearson type III). The log Pearson\ud type III distribution was found to be suitable for use in all regions of\ud the state, based on the chi-square goodness of fit test. Flood magnitudes\ud having recurrence intervals of 10, 25, 50, and 100 years were\ud related to physical and climatic indices of drainage basins by multiple\ud regression analysis. Drainage basin area (A) was the most important\ud variable in explaining the variation of flood peaks in all regions.\ud Mean basin elevation (E) and mean annual precipitation (P) were also\ud significantly related to flood peaks in two regions in western Oregon.\ud The following equations to predict the 25-year flood were developed for\ud each physiographic region in Oregon: (1) Willamette region Q₂₅ = 156A·⁸⁰\ud (2) Coast region Q₂₅ = 6.31A¹⁰¹E·⁵¹ (3) Cascade region Q₂₅ = .O32A⁴⁴P¹·⁹⁷ (4) Rogue-Umpqua region Q₂₅ = l63A·⁷⁷ (5) Blue-Wallowa region Q₂₅ = 67.6A·⁴