Investigation of Local Scour Phenomenon Downstream of Stilling Basins in the Presence of Stepped Spillway

Scouring is one of the issues that is important in the design of various types of hydraulic structures. One of these structures is a stepped spillway. The aim of this study is to evaluate the similarity between the profiles of scour holes downstream of the stepped spillway. To do so، results of 67 experiments in two different experimental models were used; these results were in a wide range of particle Froude number، the stilling basin length، downstream depth، sediment particle size distribution and two different slopes of stepped spillway. In this study، 5 different lengths of stilling basin، from 37 cm to 120 cm، were used. Also، the range of utilized discharge was from 8. 43 lit/s to 88. 95 lit/s. Duration of the tests were selected to be 6-hours، 8 hours، 12 hours and 24 hours and، in total، 834 scour profiles and approximately 85000 points to were used derive required relations and to performother analyzes. Based on the experiments results، the similarity between scour-holes located in downstream of stepped spillway was analyzed. In this process، two methods were used to make the profiles dimensionless. Based on statistical parameters، best method was selected and similarity of downstream profiles were shown. Relations that predict maximum scour depth، the distance from maximum depth to the end of the stilling basin and volume of transported sediments are developed based on the dimensionless parameters in different time periods. Furthermore، impact of different parameters on the geometry of scour holes were studied. When the Froude number increases، while all other parameters are constant، the dimensions of the scour-hole increases. When the stepped spillway slope increases، the dimensions of the scour-hole decreases and، moreover، when the tail water depth increases، while all other parameters are constant، a deeper hole with longer longitudinal distance forms. Overall، 8 cross section profiles under different conditions were taken. These profiles indicate transverse development of the scour-hole، in addition to longitudinal development.