Water exchange and hydrodynamic responses to multi-phase restoration of tidal flat in Luanhe river estuary

The tidal flat restoration has been carried out to reconstruct the natural habitats from large-scale aquaculture reclamations in the Luanhe River Estuary. However, the impacts of the project on water exchange has not yet been fully established, which further limits the understanding of the environmental diffusion in the estuary. This study aims to investigate the evolution of hydrodynamics and water exchange due to multi-phase restoration projects, from the perspective of water age. The results indicate that the riverine discharge provides more dominance than the tidal flow in this typical micro-tidal estuary. The discharge directly determines the intensity of water exchange, while the tidal flow only cause minimal fluctuations in the water age. The first-phase restoration projection widens the river channel to facilitate the interaction between riverine discharge and tidal flow. The implementation of the restoration decreases the water age, indicating an enhancement in the water exchange process. This effect is particularly noticeable during the dry season when the riverine discharge is weaker to restrict the water exchange, as evidenced by a 5.8% and 16.3% increase in lower water age area in the flood and dry seasons, respectively. The upstream flow of tidal currents is strengthened by the restoration to significantly improve the exchange intensity of runoff and tide. The evaluation of multi-phase projects show that the spatial layout of the restoration also makes diverse effectiveness. In such a weak tidal estuary, the restoration significantly changed the runoff, can effectively promote water exchange by improving the interaction between runoff and tide with the area of lower water age increasing by 51.1%. On the contrary, the improvement of water exchange by seaside restoration is limited with the area of lower water age increasing by only 9.6%. While, thorough restoration is crucial to further enhance the water exchange process for a long term. The results can provide valuable insights for the planning and implementation of future restoration to achieve greater ecological benefits.