Feedback between tidal hydrodynamics and morphological changes induced by natural process and human interventions in a wave-dominated tidal inlet: Xiaohai, Hainan, China

The feedback between morphological evolution and tidal hydrodynamics in a wave-dominated tidal inlet, Xiaohai, China is investigated through data analysis and numerical model experiments. Historically, Xiaohai Inlet had two openings, located at the north and south of Neizhi Island (a rocky outcrop), respectively. The evolution of Xiaohai Inlet was dominated by the natural process before 1972. In addition to the natural process, human interventions, including the closure of the north opening, 50% of freshwater reduction, and increase of land reclamation, have altered tidal hydrodynamics and morphological evolution since 1972. A series of numerical model simulations were conducted to investigate the influence of morphological changes on the hydrodynamics and the influence of human activities on the inlet evolution. The natural process has caused narrowing and shoaling of the inlet throat, development of the flood-tidal delta, and shoaling of the tidal channel inside the lagoon. Human interventions have accelerated these changes. Consequently, the tidal propagation from the offshore into the lagoon has been impeded and the tidal energy has been dissipated substantially. Tidal current has changed from ebb-dominant to flood-dominant in most parts of the inlet system whereas the inlet throat has remained as ebb-dominant, the tidal prism has decreased consistently, and sediment has continued to deposit inside the inlet. As a result, the changes of morphology, hydrodynamics, and sediment transport show a positive feedback. The human interventions have had both advantageous and adverse influences on the stability of the inlet. The closure of the North Opening has decreased the longshore sediment input to the inlet, and increased the tidal prism, ebb velocity, and sediment transport in the south opening, thus enhancing the inlet's stability. However, reducing the river discharge and landfill of the tidal flats has resulted in a decrease of the tidal prism, the ebb velocity, and the ability to export sediment, thus having the tendency to deteriorate the inlet's stability. A stability analysis based on a closure curve methodology has shown that Xiaohai Inlet is in a state of dynamic equilibrium at present.