Abstract

A structure built over a river is designed based on its maximum flow. It depends on catchment size, roughness, rainfall intensity, and the base course of the river. The structure has to be adequate to resist the pressures, particularly, the water from the upstream drainage area throughout its service life. The runoff generates from the catchment is fused to tributaries, brooks, rivulets, and seams to the river. The speed and depth of the water on these channels are mainly depended on the slope, roughness, and their magnitude. So, the flood and its occurrence time on a structure in a river retard or hustle due to the river-bed characteristics. This study has shown the impacts of these river-bed parameters, mainly slope and roughness, on the hydraulic structures of the river. The hypothetical characters of the sampled river length combination analysis are different. The study has considered the existing river slope and roughness as an initial condition. The maximum, minimum and average slope and roughness are also the considered values in the analysis. The researcher has taken more than five kilometres of sampled river length upstream of Kessem Kebena reservoir and has analysed the flow conditions on these hypothetical river parameters for a constant discharge at the inlet to the river. Nine-character combinations have been produced from the three common roughness and slope values. Hence, these combinations have produced two, unlike results. The one has retards whereas the other group speeds up the discharge on the downstream structures. The extreme discharge variation that has been retarded and has been added from the natural condition to the downstream structure is 5.80 m³/s and 4.20 m³/s respectively. Therefore, the hydrological engineer has to consider the upstream river slope and roughness while estimating the upstream drainage area runoff and design the downstream river structure.

Keywords: Riverbed slope and roughness; Flood; Reservoir; River reaches; Water