SPILLWAY CREST DESIGN TO SUIT DIFFERENT OPENINGS OF GATES AND MODES OF OPERATION

Abstract

The ogee spillway has been the subjact of more research than perhaps any other hydraulic structure. However,there are still certain functions of spillway# which need deeper probe. In the past, tendency had been to fix the co-ordinates of the profile in such a way that they corresponded, more or less , to the under side of a fully ventilated nappe flowing over a sharp crested weir., for the maximum head of water likely to occur over the crest of the spillway. Generally speaking a curve of this nature gives positiu pressures practically for all conditions of operations. But when this design head is exceeded-, negative pressuresoccur on the spillway face. Also the present day trend is to fix the design head less than the maximum head that is likely to come over the crest. Therefore, at this time of the maximum flow over the spillway, negative pressures are developed, and this is done to increase the hydraulic efficiency of the spillway i.e. the co-efficient of discharge is increased. Ill the same, if this negative pressure is allowed to approach vapour pressure of water, cavitation takes place and the spillway Is damaged. Sven though the flow profile is designed in such a way that negative pressure in the cavitating range is not developed, unless proper care is .. taken during construction also, cavitation may develop due to surface Irregularities. Also obtaining a smooth, well Q igned flow surfaces during initial constructiot is.only part of the problem. Retaining the smooth surfaces over a period of years is also necessary which naturally depends on the type and •'~ strength of concrete used. An attempt has bean made in this dissertation to analyso the cause of cavitation and remedies to obtain a cavitation fre3 spill ay ,profile-& comprahensivo review Is made on various standard crest profiles# Thought a little out of place, a brief mention has also been made of some of the profiles developed earlier. The occurrence and the mechanism of cavitation In a general way has been do olt In Chapter 3. 'bt causes of cavitation on spillways such as crrong design, wrong location of the gate seat, wrong type of gate slots, badly finished construction surfcc3s etc. have bean outlined In the dissartatton in Chapt3r 4. To the fifth chapter some possible remedies to avoid cavitation have bean suggesged. ! lso, a brief revlou is made on th3 specifications of construction finishes, types and strength of concrete to be used etc.