The impact of channel deepening and sand mining on estuarine tidal dynamics

Lingding Bay (LDB) is a part of the Pearl River Estuary in south China. It is surrounded by large growing cities and has been subjected to heavy waterways traffic over the past 5 decades. The tide propagation pattern has been greatly modified ever since. It has widespread morphological and ecological impacts on the LDB system. However, a systematic study on the response of tide propagation pattern to channel deepening and sand mining is currently lacking to provide future management guidelines for the Lingding Bay. Based on a state-of-the-art modeling tool (Delft3D Flexible Mesh), we explore the tidal propagation pattern (tidal amplitude, tidal phase, residual current, and tidal-energy flux) of the LDB over the last century (1901-2016). Three stages of bathymetry and tidal dynamics variation are divided by our investigation. Stage I (1901-1964) is a natural evolution process, the LDB is manifested as gradually filling by fluvial sediment, the tidal amplitude decline, and tidal dynamics decrease. Stage II (1964-1989) is characterized by a slow increase in water depth and tidal dynamics, which affected by channel dredging activities. While stage III (1989-2016) is influenced by channel deepening and sand mining, shown an abrupt increase of water depth in a short time and the rapid enhancement of tidal dynamics.

The investigation indicating that the channel deepening and sand mining activities amplified the tidal dynamics distribution difference between the channel and shoal. The increased tidal dynamics in the channels may increase saline intrusion and coastal flooding risks. Spatially, these two activities may also lead to contrasting morphodynamic patterns between the inner and outer LDB. The morphology state of erosion in the inner LDB and deposition in the outer LDB reported by other studies are consistent with the hydrodynamic variation in our study. Most likely, channel deepening and sand mining in inner LDB cause the sea bed to appear to lose，lead to larger SSC levels in the vicinity. This resulting in the inner LDB formed a new sediment source, under the stronger runoff in inner LDB, the depocenter is moved southward, and outer LDB developed to a sedimentary area. Our findings conclude that channel deepening and sand mining can greatly change the tidal dynamics distribution in an estuary, thereafter affect the sediment transport pattern. We suggest that coastal engineering planning should pay more attention to sand mining activities. The insights obtained from this study are of value to the future management of the LDB and other estuaries that are also under similar stress.