The Research On Flow Characteristics Of Coral Reef Atoll Cross Section Under Tidal Flow

The rational development and utilization of coral reefs in the South China Sea is the key to the implementation of China’s national strategy of "Maritime Power" and "Maritime Silk Road" Atoll reef is one of the most common types of coral reef.It is characterized by a ring or horseshoe shape with enclosed or semi-enclosed lagoons in the middle.According to the survey,about one-third of the world’s coral reef hydrodynamic characteristics are dominated by tidal currents,which play a crucial role in the transport of coral sand and nutrients across the atoll,the effect of tidal current on the erosion of coastline can’t be underestimated,and the hydrodynamic conditions or the change of topography have some influence on the velocity variation.Therefore,the investigation on flow characteristics of atolls section under tidal flow can provide a reference for prevention and control of coastal erosion and the protection of ecological environment.In this paper,based on the field observation data and combined with the range of reef topographic parameters,a two-dimensional physical experiment was carried out by cutting a section of atoll reef to research the flow characteristics of the atoll section under different tidal flow and reef-flat submergence,focusing on the variation of the flow velocity along the reef.The results show that both the shoreward flow and seaward flow are affected by the frictional resistance at the bottom of the reef.The bottom velocity is small,the function of velocity and water depth is logarithmic distribution,then with the increase of water depth,the velocity tends to be stable,and the function of velocity and water depth is exponential distribution.The average water level and velocity of the front reef flat increase with the increase of tidal discharge and decrease with the increase of reef-flat submergence.Under the action of the shoreward flow,the mean water level of front reef is higher than zero except the individual position when the tidal discharge is small,and the initial mean water level of the lagoon is higher under the action of seaward flow,as seaward flow is prone to water jump near the reef edge,the mean water level suddenly increases and then begins to decrease near the slope in front of the reef.The variation amplitude of the mean water level under the action of shoreward flow and seaward flow is smaller than that of the shallow reef-flat submergence.When the seaward flow acts only on the atoll reef section with lower reef-flat submergence on the reef flat,a supercritical current occurs near the reef edge.Then,based on the incompressible Navier-Stokes Equations,a numerical flume is established using the k-ω SST turbulence model to verify the experimental mean water level and velocity.Finally,the effects of reef flat width,lagoon width,slope gradient in front of reef,tidal discharge and reef-flat submergence on the flow characteristics of the atoll cross section are studied,the distribution law of velocity and mean flow field on the reef flat is analyzed.And the empirical formula of maximum velocity on the reef edge is fitted by considering the influence of hydrodynamic and topographic factors.The results show that the tidal current propagates from the far sea to the near shore at spring,firstly,the velocity increases with the shallow water depth near the front slope of the reef,and tends to be stable on the front reef;At ebb,the tidal current flows from the lagoon to the back reef,the flow velocity and direction remained stable on the back reef flat,the tidal current velocity decreased again with the increase of water depth on the front slope of the back reef flat.During spring and ebb,the tidal current velocity increases with the increase of tidal discharge and decreases with the increase of reefflat submergence.The change of slope gradient in front of reef has little effect on tidal current velocity on reef flat.When the width of the lagoon changes,the tidal mean velocity on the reef flat is almost unchanged,and the mean velocity at ebb tide is larger than that at spring tide.When the reef flat exists,the velocity at the front reef flat edge decreases slightly with the increase of the reef flat width,and the velocity at the back reef flat edge hardly changes.