Hydrodynamic influences on water exchange capacity of a coastal lagoon after phasic restoration projects

Abstract

Engineering solutions are widely conducted to restore coastal lagoons against environmental deterioration for climatic and anthropogenic impacts. To investigate hydrodynamic responses to phasic restoration projects, Qilihai Lagoon is set as a representative object for its typical riverine and marine hydrological regimes. The numerical modelling is based on three engineering stages under real-world conditions. The numerical results demonstrate that the improvement of local connectivity strengthens tide currents, and enhances local water exchange effectively. In adjacent water areas of tidal inlet and streams, local water exchange capacity turns out to be superior because of intensive inflows. The first-stage project optimizes system-wide water exchange capacity by 43% in terms of the half-life exchange rate for dredging basin, while the system requires much more time for water exchange for excessive water volume at the second stage after removing aquaculture ponds. Moreover, based on influence mechanisms of system-wide water exchange capacity, a fast-estimation method is established by initial water volume, tidal prism and stream discharge, where flow re-entrainment and hydrodynamic effects are included. The method can be applied to optimize subsequent engineering implementations and risk managements for coastal lagoons and lagoon-like systems in a morphodynamic equilibrium state.