Based on Qilihai lagoon, an idealized two-dimensional coupled hydrodynamic and sediment transport numerical model was established to simulate morphological evolution under 7 different widths of tidal inlet. The morphological stability was discussed and main results are that 1) a major tidal creek forms at the lagoon entrance, then divides into multiple bypass tidal creeks; 2) tidal prism increases with the width from 100 m to 350 m, meanwhile the unit width discharge and mean current speed in tidal inlet decrease as tidal inlet widening; 3) the time to reach equilibrium state has a higher correlation with tidal prism.
Coastal lagoon is located in the transition area between sea and land, where the natural environment is complex and changeable. It has multiple values of ecology, tourism and bank protection, etc. However, due to changes of natural environment, e.g. sea level rise and storm surge, and over exploitation and utilization (Ran et al., 2019), ecological balance has been broken, ecological environment has been deteriorating, and lagoon wetland resources have been shrinking. Lagoon system's hydrodynamic environment is controlled by various dynamic conditions such as runoff, tidal current and wind. Tidal inlets connecting lagoon system and open sea directly determine hydrodynamic conditions and control the stability of lagoon system.
Tidal inlet stability was explained in two different ways by Gao (1988). One is the dynamic equilibrium relationship between dynamics and morphology. Escoffier (1940) proposed a relation curve between the maximum bottom shear stress of the channel and its sectional area, which was similar to the P-A (tidal prism - cross-sectional area) relationship proposed by O'brien (1931). Other one is the change rates of geomorphic elements such as plane position and form, section shape and section area of tidal inlets, which depend on the comparison between the factors keeping inlets open and making inlets tend to close (Bruun, 1978).