Long-term morphological evolution and restoration of dredged pits under sand mining in a funnel shaped Estuary

Globally, the demand for shipping and construction-grade sand is increasing at a very high rate; many large estuaries are affected by large-scale dredging, which dramatically impacts estuarine hydrodynamics and geomorphology. Lingdingyang Estuary (LE) has been subjected to large-scale sand mining in recent decades with scattered deep mined pits in the middle shoal. Based on a state-of-the-art modeling tool (TELEMAC), the long-term morphologic evolution of the LE in 1998–2015 and 2016–2021 was reproduced to explain dredging-induced bathymetric changes. Then, the influences of massive dredging on geomorphic evolution were mechanistically analyzed. Finally, geomorphic restoration of the sand dredged pits in the Middle Shoal was evaluated using the long-term morphologic model. Large-scale sand mining changed the sedimentation condition of the Middle Shoal from a slight net deposition pattern to a net erosion pattern in 2012–2015. Overall, both channel dredging and sand mining significantly increased erosion in the inner LE (ILE) from 1998 to 2015; sand mining in 2012–2015 contributed 64% for these erosion, whereas channel dredging accounted for 28% in 1998–2012. In contrast, both channel dredging and sand mining increased deposition in the outer LE (OLE) from 1998 to 2015; sand mining in 2012–2015 contributed 44% for these deposition, whereas channel dredging accounted for 55% in 1998–2012. Strengthened tidal forcing, induced by massive dredging in the ILE from 1998 to 2015, was the dominant factor driving sediment entrapment in the ILE (around huge sand pits in the Middle Shoal) during this period. Additionally, channel dredging induced an increase in flow diversion rate in the Western Channel, which led to sediment transport and deposition in the OLE; it also supported sedimentation in the transition zone between the Middle Shoal and Western Channel. Finally, massive sand mining induced variations in geomorphology in the Middle Shoal from 2016 to 2021 that will require ≥50 years to recover under natural conditions; by 2100, the morphologies of these sand dredged pits in the Middle Shoal are not expected to be restored into their morphologies in 2012. The results of this study comprehensively elucidate the responses of long-term estuarine hydrodynamics and morphology to dredging and provide scientific basis for estuarine management.