The experimental investigation of the maximum depth and length of the created pit holes due to the bed material removal under subcritical flow condition

Abstract
Background and Objectives: The removal of sand and gravel from the bed and the banks of the rivers cause to change in the hydraulic and hydrology conditions of them as well as the aggradation and degradation processes in the bed and the banks and also the destruction of existing structures along the river. The purpose of the present study is to study the pit migration (maximum depth and length of the holes) due to the material extraction.
Materials and Methods: For this research, a rectangular experimental flume with length, width, and height of (5, 0.3, 0.45) meters was used. The longitudinal slope was fixed to zero. The pit holes with different sizes and different entrance discharges including relative velocities of 0.83, 0.89, 0.94 and 1, have been operated. The experiments have been accomplished with or without downstream gate by using two sediments mean dimensions of the 0.15, 0.6 mm. In this study, the duration of the experiments, were fixed to 1 and 1.5 hours respectively for including and without end gate flow conditions. such that the equilibrium time for both of them was the last 15 minutes. A 3D laser scanner was also used to take data from the bed profile after flow disconnection. Also from the side view of the experimental flume, the digital photos were taken to view the conditions of the pit propagation at different time intervals. Afterward, using the MATLAB, Tec-Plot 360 and Grapher software tried to rearrange the data series for 2 and the 3-dimensional view from time variation of the pits were extracted. During the experiments, the bedforms at the down and upstream of the pit holes were observed and their temporal variations were recorded.
Results: The results of the experiments showed that with decreasing of the Froude number, pit depth and also by creation of the secondary currents inside the holes, the dimensionless ratio of the decreases and reversely, the dimensionless parameters of the and increases. On the other hand, it was concluded that by increasing the Froude number and shear stress, the dimensionless ratio of the would be increased. The longitudinal profiles and 3D view of the pit migration at the all of the sediment and flow conditions were observed and depicted. At the end, the experimental equations of the maximum scouring length and depth of the pit hole were presented. In general, it was shown that the submerged angle of repose of materials ( ) has the highest direct effect and the dimensionless ratio of has the highest reverse effect on the maximum scouring length and depth of the pit hole. The extracted relationships with experimental data were verified by means of the other set of data series and the results were quite satisfactory.

Conclusion: The results of the experiments have been illustrated by presenting the none-dimensional relationships for maximum depth and length of the pits under gated and none-gated conditions. The effects of the variation of the sediment dimension and flow conditions were also discussed. It was proved that the lowest error percent for maximum scour depth was observed in the 0.15 mm mean sediment particles, having the relative error of 2.98% and RMSE of 0.23. Also, the lowest error percent of the maximum scour length is related to a 0.6 mm mean sediment particles with a relative error of 7.45% and an RMSE of 0.5.