Uniformity Effect on Scour hole Dimensions due to Free Falling Jets

Prediction of scour due to outflows from hydraulic structures is an important field in hydraulic engineering. There have been many studies concerning this subject which have invocated field analyses or laboratory experiments. As scouring is a complex process and many parameters affect it, there is not any equation that contain all parameters and limited for predicting scour hole dimensions. One of an important parameter that affect the scour hole dimensions is sediment gradation. In the previous researches, the sediment non-uniformity effects on the scour hole dimensions due to free falling jets, did not study completely. In this paper the effect of this parameter is studied. The experiments were performed in the hydraulic laboratory of Tarbiat Modares University. The results show that by using d90 instead of d50 in the densimetric Froude number as a sediment size characteristic better correlation between the densimetric Froude number and scour hole dimensions can be obtained. The previous research data along the data that was obtained in this research were used for checking the accuracy of the previous equations and presenting new equations for predicting the scour hole dimensions. By using the Frd90 and omitting the (d50/Hc) and sediment non-uniformity coefficient, the accuracy of the equations do not changed significantly. The results show that the new equation predict the scour hole dimensions in the field more accurately in compare to the previous equations.