EFFECT OF SCALE ON COEFFICIENT OF DISCHARGE OVER WEIRS AND SPILLWAYS

Abstract

Coefficient of discharge of weirs and spillways 'is to be known before head in order to prepare stage discharge curves, regulation charts and correct fixation of crest levels. In order to find out the coefficient of discharge correctly. the usual pre» cedure is to construct a geometrically similar model of the spi .iwaay satisfying the Froudian law and to observe the coefficient of discharge on the model. It was observed at 1,R.I.Roorkee (tJ.P.) and C.W.P.R.S.Poona that these observations of coefficient of discharge involve certain scale effects. At both the places it was observed that the values o coefficient of discharge decreases with the Increasing R.F. of the scale i.e. smaller models will have lesser values of the coefficient of discharge. It is therefore difficult to predict correct values o the coefficient of discharge on the basis of model observations. Tb usual method followed In the Engineering practice is that the designers adopt suitable value of the coefficient for the geometri-cal and operational condition of the sp .livay from standard curves apdt ion such as U,B.B.R supposed to verify the coefficient of discharge so determined on a model. Suitable safer value is adopted for the prototype and its regulation charts etc. are prepared* It has so far not been possible to determine the discharge unction analytically, and as such the extent of scale effect is not known. On the basis of known parameters entering the discharge equation, a dimensional analysis has been performed to find out the dimension less ratios affecting the coefficient of discharge. The dimensional ratios are Reynolds number, Webers number, K/h or 'f' (roughness parameter), P/h (geometrical parameter),