Experimental Works on Pit Mining Migration and River Bed Changes

Sand and gravel are important resources in infrastructure development and construction. Excessive instream sand mining will have an impact on the river ecosystem, particularly the morphology and hydraulic regime. The dynamic nature of the river system has a significant impact on pit migration and bed profile changes. An experimental study in a laboratory flume using a mobile channel bed was carried out to explain the changes in bed profile in the presence of instream pit mining. In the flume, four different pit sizes ranging from 50 to 70 cm in length and 30–50 cm in width were created. Flow velocities were set to range from 0.6 to 1.5 m/s. Three major findings were observed: (i) the nick point at the downstream were eroded and further migrated before equaling the original bed profile; (ii) the deposition occurred in the middle of the pit, indicating the filling mechanism of pit mining; and (iii) the headcutting at the upstream of pit occurred due to over-steepened nick point, which can migrate further upstream from the original designated pit area. These findings were critical for future instream sand mining guidelines, which should account for the possibility of pit migration in the risk analysis.