Water and Energy International

The current trend in design of an orifice spillway is keeping the crest as low and near the river bed as possible from consideration of flushing of sediment from the reservoir. Height of spillway crest ‘P’ from reservoir bed is found to be one of the important parameters in the design of overflow spillways. However, no specific work has been reported about the effect of ‘P’ in an orifice spillway. In view of this, a basic research has been taken up at Central Water and Power Research Station, Pune, India using physical and numerical model studies. The studies were carried out for two different experimental setups (i) an orifice spillway with solid bottom profile but without solid roof profile and (ii) a full-fledged orifice spillway with solid bottom as well as solid roof profile. Studies were carried out for P/hd ratio 0.33, 0.67 and 1.33, where hd is the design head. Major objective in the first set up was to identify a need of parameter ‘P'in design of roof profile of an orifice spillway. From the study, the effect of ‘P'is found to be insignificant on upper nappe profile and can be neglected while designing of roof profile of an orifice spillway. The model studies carried out on second set up helped to identify the effect of ‘P’ on other hydraulic parameters such as discharging capacity, pressures on bottom and roof profile of a full-fledged orifice spillway. The studies indicated that the height of spillway ‘P’ does not play a significant role in assessing the performance of an orifice spillway in respect of various hydraulic parameters.