UNCERTAINTY IN FLOOD ESTIMATES ASSOCIATED WITH ROUGHNESS COEFFICIENT

Although paleoflood reconstructions are frequently used to provide dis charge estimates of extreme floods, a concern remains regarding uncert ainty in Manning's n-values. An analysis of this uncertainty relative to a commonly used step-backwater model was performed for channel reac hes in five canyon rivers. Values of n were selected for each study re ach using four methods. Each set of n-values was then varied by +/-10% and +/-25%. The percentage change in discharge associated with varyin g n-values is inversely proportional to channel gradient and to width/ depth ratio, and proportional to roughness. For channels with a gradie nt less than or equal to approximately 0.01, varying n by +/-25% produ ces a maximum change of 20% in discharge. The greatest discrepancy amo ng n-values estimated using the four methods occurred for channels wit h relatively low gradient and low roughness, but associated discharge varied fairly consistently between 13 and 27%. These results indicate that uncertainties in discharge estimation resulting from the roughnes s coefficient in step-backwater modeling of paleofloods are comparable to or lower than uncertainties associated with other methods of indir ectly estimating flood discharges.