Extreme Roughness of Natural River Channel

The paper deals with the hydraulic design of a channel improvement scheme. Special attention is focused on the synthesis of the stage-discharge relationship for the downstream end of the improvement reach. In the absence of more directly applicable information, the design of the tail-water rating curve is based on the roughness characteristics of the river channel. The latter is deduced from a combination of high-water mark and flood flow which was recorded during the occurrence of a hurricane over the watershed. Results of a computer study are presented which suggest a Manning roughness coefficient of n = 0.225 for the channel reach under consideration. This value is larger than the corresponding predictions made on the basis of conventional methods for estimating n. The implications of this finding are explored relative to the configuration of the improvement scheme in the upstream channel.