Fuzzy Analysis of Slope-Area Discharge Estimates

When velocity estimates of channel flows are not available, the slope-area method is widely used to make discharge estimates. The accuracy of such estimates depends on many factors but is generally believed to be low. In this study, fuzzy set analyses were used to compute the distribution of slope-area discharge estimates, with the distribution very similar to that assessed using statistical methods used in hydrology. The effects of errors in channel roughness, channel width, channel side-slopes, and flow depth on the accuracy of discharges were assessed. The large errors possible in channel roughness are a primary source of error in discharge estimates. Channel incision and vegetal growth are shown to significantly influence the accuracy of slope-area discharge estimates and the length of time before a slope-area rating curve should be recalculated. Records as short as 10 years can be seriously biased due to a failure to account for the effects of incision or vegetal growth.