Uncertainty in Flood Estimates Associated with Roughness Coefficient

Although paleoflood reconstructions are frequently used to provide discharge estimates of extreme floods, a concern remains regarding uncertainty in Manning's n-values. An analysis of this uncertainty relative to a commonly used step-backwater model was performed for channel reaches in five canyon rivers. Values of n were selected for each study reach using four methods. Each set of n-values was then varied by ±10% and ±25%. The percentage change in discharge associated with varying n-values is inversely proportional to channel gradient and to width/depth ratio, and proportional to roughness. For channels with a gradient less than or equal to approximately 0.01, varying n by ±25% produces a maximum change of 20% in discharge. The greatest discrepancy among n-values estimated using the four methods occurred for channels with 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 roughness coefficient in step-backwater modeling of paleofloods are comparable to or lower than uncertainties associated with other methods of indirectly estimating flood discharges.