Sand-blocking Effects Of Fence With Different Types In Macro-tidal And Windy Coast

Prevention and control of aeolian sand disasters in coast area is an important part of marine disaster prevention.This thesis focuses on the unique environmental characteristics of the interaction of sea,land and air.A macro-tidal and windy coast,and also has a large particle source was selected to achieve the above-mentioned purpose.We studied the practical effects of fences with different types against aeolian sand disasters,after the study of uniqueness of backshore aeolian sand transport and the dynamical impacts of macro-tidal and windy coast on initiation and transport of aeolian particles.The morphological evolution process and accumulation efficiency was analyzed.The sand particle distribution at different locations before and after the fence was also analyzed to explore the potential mitigation procedures against coastal aeolian sand disasters.The results are as follows:(1)The backshore sand initiation is controlled by the combined effect of wind dragging force and the sand cohesion effect of surface sediment water content,and those two mechanics intertwine with each other.When the wind speed increases,the initiation mode gradually shifts from the fluid initiation to the splash initiation,and the critical friction wind speed of fluid initiation is greater than that of splash initiation.When the moisture content increases,the initiation wind speed increases linearly.When the surface moisture content is low,the critical initiation friction wind speed threshold is about 0.4 m/s;while when the surface moisture content is high,the critical starting initiation wind speed is nearly twice of that when the moisture content is low.(2)Determine the corresponding calculation of sediment transport rate under the leadership of two different factors,wind force and moisture content,and the difference of sediment transport caused by the change of fetch under tidal action.Sediment transport rate has been determined under the effects of wind force and moisture content,and the difference of sediment transport caused by the change of fetch under tidal influence also has been analyzed.Corresponding parameters at the backshore of Yuandang Bay beach have been determined using Bagnold’s sediment model.The formula of sediment transport rate under the influence of moisture content is obtained using Hotta’s sediment transport model.Fetch and the water level is in reverse periodic variation and cause sediment transport rates differences,but not as prominent as moisture content change impact to backshore in aeolian sand transport process.The beach critical fetch of Yuandang Bay is less than 85 m.(3)As the porosity of the fence decreases,the overall shape of aeolian sand accumulation profile tends to develop towards the windward side of the fence.This is due to the decrease of wind-sand inflow through the pores and the increase of the partition of the flow structure on the leeward side of the fence.The accumulated profiles correspond to three different types of coastal foredune: hemispherical dune,barcan dune,and transverse dune.Based on the deposited sand amount and total sediment deposition efficiency,and the location with maximum sand accumulation height,we conclude that fence with 40% porosity is the most suitable in Yuandang Bay beach.(4)The sand particles at Yuandang Bay beach belong to medium sand,and the sand particles are unimodal distributed,with characteristics of both active dune sands and beach sands.With the decrease of porosity,the fence filter effect increases slightly.However,due to the fact that the particle size distribution is closely distributed,the filter effect is not significant.