Experimental Investigation of Bed Scouring Rate in Flood Conditions

The volume of sediment transport rate during floods is one of the important issues in river engineering. This volume of sediments has caused countless damages to the hydraulic structures which are built along the rivers, every year. The purpose of this study was to investigate the effects of hydrograph unsteadiness parameter on bedload transport rate. For this purpose, a real unsteady flow hydrograph was created inside a 15 m long tilting flume by installing an interface board between the computer and the pump inverter. 20 cases of hydrographs with different unsteadiness parameters were tested and the scour rate was obtained during the hydrograph time. The results indicate that the maximum scouring rate always occurs near the peak of the flood hydrograph and the time lag between them was positive. For hydrographs with the same peak and different base time, the maximum scouring rate occurs in the hydrograph with more unsteadiness. The maximum scouring rate of the bed has a direct relationship with the unsteadiness parameter, and it can be said that an 88% decrease in the flow unsteadiness parameter leads to a 76% decrease in the maximum bedload rate. Also, the scour rate in the ascending limb of the hydrograph is higher than the descending limb, which by reducing the unsteadiness parameter, the distance between the limbs in the sediment hysteresis diagram decreases.