Numerical Simulation Of Morphological Evolution Of Lagoon Inlets

Lagoon inlet is a common feature of sandy coasts.With good sheltering condition,lagoons are ideal choices for harbor construction and for boats or ships to take shelter from the storms.Tidal inlets are natural waterways connecting lagoons and open seas.Influenced by tidal current,waves,nearshore wave-induced current and other factors,sediment transport and morphological change in lagoon inlets are relatively active.The stability of lagoon inlets has a great influence on harbor construction,navigation and nearshore ecosystem,therefore,the inlet stability is a major topic in the field of coastal engineering.The closure of inlets caused by longshore sediment transport is a major performance of inlet instability which has a great impact on human productive activities.In this thesis a process-based numerical model is used to investigate the morphological change under different wave and current conditions and discuss the trend of morphological change by sediment transport with the combined action of wave-induced current and tidal current.The main contents are as follows.(1)Longshore sediment transport,caused by longshore current which is induced by the breaking of obliquely incident wave,has a great influence on morphological change.The rationality of calculation for longshore current and sediment transport depends on the simulation results of waves.The Goda’s formula is added into SWAN model for better simulation of nearshore waves.Empirical relationship between the wave stepness in deep water and corresponding parameter A is fitted according to field and experimental data.According to validation by relatively independent field and experimental data,SWAN model with modified Goda’s formula can give a good prediction of irregular wave breaking in nearshore zone.(2)Numerical model is tested against experimental data of a physical model test carried out by DELFT HYDRAULICS which aimed at investigating the morphological change of Keta Lagoon.The simulation results are in good agreement with the experimental data,indicating that the numerical model can rationally simulate the trend of morphological changes of lagoon inlets with the combined action of wave and tide.(3)The trend of morphological changes of idealized lagoon inlets under different wave and tidal conditions is investigated.The results show that wave-induced longshore sedeiment transport is the dominant process of the formation of shoal.The shoal will develop faster with the increase of wave height.While the tidal current has a inhibitory effects on the shoal development and contributes to maintain the inlets.With larger tidal range,the development of shoals will be inhibited dramatically.Therefore,limited tidal power and prominent wave condition may cause the closure of lagoon inlets.