Study On Morphological Change Of Sand Cay Under Regular Waves

In recent years,global climate change has been accompanied by sea level rise and strong typhoons,strong typhoons are prone to trigger large-scale storm surge and typhoon extreme wave events,which will destabilize low-lying reef islands on coral reef flat,and sea level rise may make the residents of low-lying reef islands no longer suitable for living in decades.However,the South China Sea is also a high-risk area for storm surge and tsunami disasters in my country.Compared with the coast of China,the safety of the South China Sea islands and reefs under the influence of storm surge and tsunami is facing greater challenges.Due to the remote location of sand cays in the South China Sea,it is relatively difficult to obtain field information.There has been little information accumulation and related research on the natural geomorphology and evolution process of sand cays,and there is a lack of sufficient knowledge on the topographic evolution of sand cays,coupled with the existence of a large number of sand cays(including artificial sand cays)in the South China Sea,and their existence is related to the core interests of national sovereignty.Therefore,there is an urgent need to strengthen the research on the long-term and short-term evolution of sand cays in the South China Sea under the influence of global climate change and extreme weather to serve the needs of maintaining China’s territorial rights and interests and island engineering facilities.In addition,the study of the balance of sand cays under wave action has important practical significance and reference value for the evolution of shorelands and coastal protection issues.This study is based on a generalized horizontal one-dimensional reef islands model,and the free liquid surface and the profile elevation of the reef islands under the test conditions are mainly measured by wave height meter and LRI-Ⅲ 3D topographic measurement system.The physical model test mainly investigates the effects of six factors,such as incident wave height,incident wave period,reef-flat water level,initial height of sand cay,upper bottom width and distance of sand island from reef edge,on the topographic evolution pattern of sand island under the action of regular waves.The numerical simulation adopts XBeach,a two-dimensional planar coastal dynamic landform numerical program package based on structured grids that has emerged in recent years,which is also beginning to be applied to the simulation of coral reef coastal dynamic geomorphology with its advantages of open source,easy extension,high computational efficiency and advanced solution.In this paper,a regular wave numerical flume is established based on solving the nonlinear shallow water equations in the water depth averaging mode,and the feasibility of the model is verified with the physical model test results,followed by sensitivity analysis of some parameters in the flow,non-hydrostatic correction,sediment transport and topographic evolution modules,respectively,so as to determine the optimal combination of parameters and apply them to typical working conditions.The results of the physical experiment show that the intensity of the overall evolution of the sand cays along the reef and the scouring intensity of the ocean edge increase with the increase of the incident wave height and period.The influence of reef-flat water levels on the overall evolution of sand cays and the scouring of ocean edge is not a single change.The overall migration of sand cays along the reef and the scour elevation of the ocean edge show a reciprocating trend with the increase of the reef-flat water levels.The scour elevation increases with the increase of the initial height of the sand cay,when the distance between the sand island and the reef edge increases,the scour elevation of the ocean ridge of the sand cays decreases,while the upper bottom width of the sand cays has little effect on the topographic of the sand cays.The numerical simulation results show that the bed friction coefficient,maximum wave steepness,layer distribution in the nonhydrostatic model,sediment source term coefficient,coral sand porosity and density have significant effects on the numerical model,and it is found that the rate of sand cay topography along the reef decreases with increasing the bed friction coefficient,because the increase of bed friction coefficient will increase the bed friction and shear,resulting in the reduction of wave height transmitted to the vicinity of the sand cay.This results in a decrease in the degree of variability of the sand cay profile.The scour intensity and the degree of variation along the reef of the sand cay increased with the increase of the maximum wave steepness,the layer distribution in the nonhydrostatic model and the porosity of coral sand;while the opposite was true with the increase of the sediment source term coefficient and the density of coral sand.Finally,the optimally combined sensitivity parameters were applied to the model and compared with the standard conditions of the physical test,and it was found that the numerical calculation results were in good agreement with the measured results of the physical test.