Effect of Different Roughness Heights of Horizontal aApron and Bed on Downstream Scour

Horizontal aprons are usually used at the downstream end of hydraulic structures. In most cases، because of the roughness difference at the apron and channel bed، scour occurs at the river bed downstream of the pron. As it deepens it damages the apron itself and ultimately leads to hydraulic structure failure. In order to reach the aim of this study، 24 experimental tests were carried out on four different aprons with (0، 2، 5، 10 and 14. 28 mm) heights، and two different bed material sizes (0. 8 and 1. 4 mm). The effects of different roughness heights on scour was investigated using the densimetric Froude numbers ranging from 2. 37- 3. 91. The results indicated that an increase in apron height and bed grain size significantly reduce the maximum scour depth. In this study، an experimental relationship was also developed for predicting the maximum scour depth at the downstream of an apron. The results were compared with those obtained from previous studies.