Fitting and interpretation of sediment rating curves

A large part of the sediment in lowland rivers is transported as wash load. As wash load is a non-capacity load it often is modelled using empirical relations, such as the rating curve technique. Sediment rating curves in the form of a power function are derived for several locations along the river Rhine and its main tributaries, using different fitting procedures. Inaccuracies in estimated sediment loads are analysed, and spatial differences in the shapes of the fitted rating curves are related to watershed characteristics. Rating curves obtained by least squares regression on logarithmic transformed data underestimate long-term sediment transport rates by 10–50%. Better estimates are obtained when nonlinear least squares regression is applied. The steepness of the fitted rating curves decreases along the main channel of the Rhine in a downstream direction. Contrary to what is generally believed this is not related to changes in the sediment transport regime. A better indication of the sediment transport regime is obtained when the slope/intercept pairs of the fitted rating curves are plotted in a graph. All locations that plot on the same line have a common discharge-concentration value and appear to be characterised by a similar sediment transport regime. At locations of which the slope/intercept pairs plot on higher lines, a larger part of the annual sediment load is transported during high discharge.