Morphodynamic impacts of sand mining in river deltas

In recent decades, sand extraction from rivers has accelerated to meet the needs of economic development. Locally, this results in river bed and bank erosion, but it is unknown how these local disturbances affect the larger scale morphodynamic feedback and whether sustainable sand-mining strategies can be designed to minimise impacts. Our objective is to test dredging strategies in a river-estuary Delft3D model and to quantify the resulting morphodynamic response of the system. We systematically varied the number and intensity of dredging sites along the river, relative to the sediment supply from upstream. The results show that the system equilibrium is disturbed when the amount of mined sediment exceeds the sediment supply from the river. We found that when intensive sand mining occurs at a small number of sites, the dredged area is able to recover over time after mining ceases, while the downstream estuary continues to erode as a result of upstream sand extraction. In contrast, less intensive sand mining, spread over a larger number of sites, results in an overall lower river bed that continues to erode and export sediment after sand mining ceases, while the non-dredged estuary is relatively stable. With our results, we aim to describe guidelines for more sustainable sand mining.