Calculation of Total Conveyance in Natural Channels

Abstract

The evaluation of total conveyance using the summation approach over section elements is reviewed for one‐dimensional, uniform, steady state flow in simple concave channels. The subdivision of a concave section into elements, the introduction of a lateral velocity gradient, the neglect of lateral shear between the elements, and the computation of the nonlinear conveyance as a linear summation of its components all contribute to the overestimation of total conveyance computed by the summation approach. The overestimation is quantified for six selected section shapes. The degree of overestimation is a function of section shape and increases with the number of section elements used in the summation. Based on the integral form of the summation approach, rectangular and trapezoidal section with width‐to‐depth (W/D) ratios above 20 have a computed total conveyance value within 5% above the value of the undivided section. For W/D ratios less than ten, the summation approach produces significantly higher total conveyance values for five of six section shapes. In this low range of W/D ratios, the summation approach should be avoided. In cases where the summation approach is required, the overestimation can be controlled by minimizing the number of section subdivisions. Differences in total conveyance computed by the summation approach are illustrated for a natural section.