Thresholds in the morphodynamic resilience of river deltas to sand extraction

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.

Results show that sand extraction produces system-wide effects, with severity increasing with both extraction frequency and volume relative to the upstream sediment supply. We found that when intensive sand mining occurs at a small number of sites, mined reaches accumulated sediment and were able to recover after mining ceased, whereas unmined zones continued to erode due to sediment-depleted flows. This results in a long-term destabilisation of the delta and indicates a sensitivity to upstream perturbations. In contrast, less intensive sand mining spread over a larger number of sites results in an overall lower riverbed that continues to erode and export sediment after sand mining ceases. System recovery occurs only when sediment supply exceeds removal. These findings highlight that localised sand extraction can induce long-lasting geomorphic change, emphasising the need to constrain extraction volumes to maintain morphological stability.