The hydraulic analysis of side channel spillways as reservoir outlets

Etheridge, M J (1990) The hydraulic analysis of side channel spillways as reservoir outlets. (MPhil thesis), Kingston Polytechnic, .


This thesis presents a methodology for analysing the hydraulics of side channel spillways.Such structures consist of a weir discharging into a receiving channel whose longitudinal axis runs parallel to the length of the weir. This has a unique characteristic of producing spatially varied flow in the channel. Once passed the receiving channel the flow enters the exit channel for the steep descent to the stilling basin before resuming its original course. The thesis produces a mathematical model which removes the need to resort to an expensive physical model to analyse such structures thereby enhancing the quality of service to the Client. The mathematical model produces rating curves which not only describe the relationship between reservoir level and spillway flow but more fundamentally determines the magnitude of the design flood. It concentrates on the performance of the spillway when the overflow weir is subject to heavy submergence as a result of passing extreme floods stipulated by the Reservoirs Act 1975. Published data which defines the relationship between degree of submergence of a weir and the reduction in the weir's coefficient of discharge is used to match the water profile in the channel with that of the induced distribution of flow over the weir. The model identifies changes in the location of the governing hydraulic control section throughout the range of the rating curve. Commonly found associated features which might also affect the rating curve are also investigated, eg arch bridges, tapering channels. The results of the mathematical model are compared to those obtained from scale physical models commissioned by Rofe, Kennard and Lapworth as a result of reservoir inspections. At maximum reservoir levels, flows obtained from the mathematical model are within 8% of those from the physical model. With the spillway passing the design flood the heads required over the weir by the mathematical model are within 15% of those in the physical model.

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