Wave Effects On The Storm Surge Height

Storm Surge is a destructive natural disaster.It may hit and overtop the coastal flood control structures violently with the huge movement energy if its height is underestimated,then swarm into coastal inland and swallow residents and buildings,causing massive loss.Hence,the accurate prediction of storm surge height at landfall moment is extremely important to disaster prevention of coastal zone.In general,surge height has four main components:astronomic tide stage,barometric tide induced by the low pressure,wind stress tide caused by the wind drag force,and wave force stage,namely wave setup induced by storm wind waves and swells.Since 1950s,all storm surge models neglected wave force in the numerical simulation process to increase the computation efficiency.After 1980s,with the improvement of computer calculation ability and model optimization,wave forces were taken into consideration by coupling wave model with storm surge model,including wave-induced radiation stress,wave-enhanced wind stress and bottom stress,and vertical turbulent mixing.In order to study the wave effects on storm surge that landing on China coast,this study chose the typical topography of the southeast China coast,offshore and continental shelf of China,and applied the two way coupling Delft3D-Flow and Delft3D-Wave model to simulate the movement and development of Typhoon Khanun.Then,the spatial distributions of typhoon wind field,significant wave height,surge height and wave setup at nine,six,three hours before typhoon landfall and the landfall moment were analyzed.Different from the spatial distributions around typhoon center of surge height and wave height,wave setup originally appears near the shoreline.According to the distribution of wave setup at landfall moment,this study divided the nearshore zone into four regions.Subsequently,we selected typical points from each region,compared and analyzed relations between their locations and magnitudes of surge height and wave setup.Finally,we obtained three reasons that affecting the surge height and wave setup in the nearshore region:distance to the typhoon center,distance to the coastline,and angle of wind direction to shoreline.The maximum wave setup of above specified points is up to 0.36 m,and the maximum ratio of wave setup to surge height reaches 43.3%,indicating that wave setup has a significant contribution to the total surge height,should not be neglected.In designing the height of coastal defense structures,taking wave setup and wave height into consideration not only keeps the maximum surge height in check,but also remains additional height,which is important to disaster prevention of coastal zone.In addition,eight numerical experiments of ideal typhoon conditions were simulated to analyze the sensitivity of wave setup under different boundary incident wave and storm intensity conditions.It is found that wave setup is mainly generated by the storm wind wave,while the contribution from the boundary incident wave under natural conditions is negligible.Nevertheless,under intensive storm wind,wave setup and surge height increase simultaneously,with their ratio at the observation station Haimen keeping around 10%at the landfall moment.