Channel expansions are needed in hydraulic structures to provide a necessary increase in cross-sectional area in the direction of flow. The design of channel expansions should be simple and short so that they are easy and less expansive to construct. However, such expansions typically lead to eddy formation, flow separation and undesirable (from the energy conservation perspective) energy head losses. This experimental investigation aims to minimise the energy losses by fitting a simple hump at the otherwise flat bottom. The focus is on subcritical flow that prevails in open channels. Using a re-circulating flume of rectangular shape, experiments of subcritical flow in expansions with and without a hump were conducted for measurements of water pressure and surface elevation. These experiments covered a range of values for the Froude number. From the measurements, the specific energy at the upstream and downstream ends of the expansions and the energy losses were determined. The use of a hump has been shown to reduce the energy loss coefficient by a factor of three or higher. The presence of the hump leads to flow acceleration and suppresses the adverse pressure gradient, which is known to cause flow separation and energy dissipation. Simple humps can conveniently be incorporated into existent expansions without much modification to the hydraulic structures.
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