Experimental Study On The Artificial Convex Structure For Beasch Protection And Siltation Promotion

In recent years, the rare sea level rise and the amount of land sediment reduction have caused the day by day strengthen of the marine hydrodynamics, the further expand scope of beach erosion, the narrow beach width, the steeper slope and the material of surface coarsening.These problems become increasingly serious which caused the extensive concern by expert in the Water Conservancy and the Marine. And of course it becomes one of the hotspot issues in the coastal engineering research. The promoting sediment deposition technology in China has achieved good results for promoting sediment deposition. But some types will lack some applicability for the tourism development of the beach.The offshore convex protection measures, like the porous square artificial reefs and sand bar studied in this paper not only have the convenient construction but also have the versatility. They also can play a positive role to the dynamic protection for the beach and the offshore ecological protection. Under the contemporary principle of the coastal defenses, for instance offshore protection and the attention to environmental protection, shoreface nourishment or placing artificial reefs into offshore zone can satisfy people's requirements of the use of beach. The model reefs have been on service in China, whose physical stability can satisfy the overturned and drift of waves. The sand of shoreface nourishment can directly dredge from the deep sea.Under the effect of the wave and current, the longitudinal and transverse sedimentation motion will cause the coastal terrain change. Sediment movement along the coast mainly depends on waveguide. And lateral movement, or the shore offshore movement is mainly caused by the water waves moving. Transverse sediment movement direction determines the beach profile change trend. Therefore, how to weaken or block sediment both horizontal and vertical motion through the engineering measures will be the fundamental of the beach protection engineering. But how to change the flow of such dynamic factors to the direction and intensity of sediment transport will be the key to the beach protection. Shoreface nourishment or setting the artificial reefs, through the weakening transmission wave strength, formatting shield area to provide the conditions of sediment deposition. Sand bar will bring influences through the area of flow field which can weaken the offshore flow carrying capacity. And the block function of artificial reefs to the reef flow can prevent sand offshore losses.Through the analysis of beach erosion mechanism of reason, this paper compares the relevant coast protection engineering home and abroad, which have found that the engineering measures,like as the main means for sand beach, the rigid engineering measures to raise the comprehensive auxiliary measures of engineering and all through the sea power factor to complete maintenance beach itself, for example scraping beach have gradually become the main means for the developed countries.The shoreface nourishment and setting artificial reefs into offshore can not only reducing the waves,forming the sedimentation environments, but also the artificial reefs or inshore sand bar can form a ideal ecological environment, providing security for fish and the seabed.On the basis of previous researches, the flow and scour around the artificial reefs under fixed bed flow field are calculated. Through the contrast analysis, "Realizable" and RNG the two kinds of turbulenandce model both can form a simulation test the flow field of reefs, but the result of "Realizable" are more consistent and close the data of indoor physical model test, thus "Realizable "model has more advantages in the numerical analysis model of the reefs. On the basis of this, I use " Realizable model to further analyze the flow field of artificial reefs,seeking how the factors, such as the velocity of flow,depth of water and the reef angle of facing flow, influence the flow around reefs. The follows are the results:1. The reefs will block the flow of water rang 1.25c to bed, enhancing flow out of the scope of 1.25c, the largest velocity is the water surface after the reefs.2. The hydrodynamic pressure distribution can be seen from the bed of the simulation. The starting position of local scour reefs should locates at crescent regions in front of reefs.3. With the increasing of its velocity, the artificial reefs' surface upwelling and bottom resistance of the longitudinal flow rule range consistent.But behind the reefs vortex flow basically is stable at the bottom of 3-4 times length of the reef. The velocity is close to minimum near here and provide guarantee for sediment deposition.Through the physical model test, the action of flow field around the reefs and the bottom after scouring stability were measured,which showed that the reef local present Longitudinal axis symmetric scour center. No cover porous reefs of scour depth and width are still the smallest in the average flow. So the reefs without cover are used in nearshore protective beach by experimental data.Through the waves of physical model experiment on the beach for selected typical section, the waves and characteristics of the beach respond to artificial reefs or the sand bar servicing in offshore zone are analyzed.The results shows that the artificial reefs and the offshore artificial sand bar can play better effect of reducing the waves energy and promoting sedimentation.Derived by maximum entropy theory to form a stable beach profile, and was validated with experimental data, the analysis of limitations and rationality of the formula is presented.