Analysis On Recent Stability Of The Arc-shaped Coast, South China

Arc-shaped coast is one of the major coastal types in the world. The land-ocean interaction process could be dynamically reflected by the equilibrium and stability of the arc-shaped coast. Based on the data including a large-scale hydrological and sediment samples collected from field observation, historical sea charts over different times, and TM remote sensing images along the different arc-shape coasts in South China, the characteristics of the coastal stability in recent period were discussed by various analysis techniques.(1) The arc-shaped coast has been in the dynamic equilibrium state, which is relatively isolated coastal system. However, the dynamic equilibrium shorelines from the different arc-shaped coasts in South China could be obtained by simulation of the visualized software of Model of Equilibrium Planform of Bayed beaches (MEPBAY), and then the stability of the arc-shaped coasts in South China could be determined by comparing of the actual shoreline configuration of the different arc-shaped coasts in recent period with the simulated dynamic equilibrium shoreline. The results showed that the shoreline patterns of the most arc-shaped coasts in South China could be in the dynamic balance of erosion; A few of the studied coasts were attributed to the type of the dynamic balance of accretion. Furthermore, the rates of coastline movement for the different arc-shaped coasts during1960-2000were obtained by the U.S. Digital Shoreline Analysis System (DSAS). The results revealed that most arc-shaped coasts in South China were in eroded condition with the mean erosion rate about3m/a. It is clearly seen that the arc-shaped coasts attributed to the dynamic balance of erosion could be furtherly eroded due to the relative sea-level rise with associated intensified wave action in present.(2) Variation in sediments of the arc-shaped coast could be automatically adapted to the dominant wave dynamical action to a great degree. These meaning foundings could be shown as follows:a) the sediment was mainly composed of sand with cumulative content of over95%, the grain size was relatively coarse, and its sorting was fairly good; b) From backshore to foreshore, the fine sand and medium sand components was increased, but coarse sand component was decreased. Most coarse grain-size and fine grain-size components within beachface and foreshore could be found, respectively. In addition, the sorting of sediments could be gradually worse from landward to seaward direction, c) From shadow zone to tangent zone along the arc-shaped coast, grain-size in sediments was gradually coarsening. The sorting was also gradually worse with the increased trends in skewness and kurtosis coefficients, d) The pattern of sediment transport was eastward direction due to dominant southwest wave action in summer.(3) It is sensitively for responses of the sediments from backshore to shoreface of the arc-shaped coast to the high-energy typhoon action. The present studies could indicate that, the significant change of grain-size in sediments could be found in the backshore of middle zone, followed by the backshore of shadow zone due to the intensive impacts of landward’Prapiroon’ typhoon action. Furtherly, there was an increased trend in fine sand and medium sand components, and decreased trends in coarse sand component. The sediment along arc-shaped coast suffered from the typhoon action could be relatively fine with well-sorted, positive skewness, and high kurtosis. Moreover, there was no suspended load and bed load of sediments distributed over the backshore zone. Bi-directional sediment transport trends within backshore along coast can be obvious observed. However, sediments within beachface and foreshore could be transported from tangent zone to shadow zone.(4) Characteristics of the shoreface stability of the arc-shaped coast in recent period could be subjected from the global environmental change. a) The fractal dimension of shoreface profiles in South China was from complex to relatively simple due to local sea level rise. The types of shoceface profiles changes can be summarized as:overall erosion, up-erosion and down-deposition, basically stable. Among of them the type of overall erosion can be observed in eastern South China coasts, and the types of up-erosion and down-deposition and basic stability was respectively correspond to the coasts adjacent to Pearl River estuary and the western South China coasts; b) It is similar for shoreface geomorphologic change with those of the shoreface profiles:in eastern South China coasts, morphological changes of shoreface appeared erosion with critical water depth from erosion to accretion of13-15m. Arc-shaped coast located in the Pearl River estuary and the western South China showed two modes including shallow water zone erosion, deep water zone deposition with critical water depth of-5m and basic stability due to relatively weak wave action. In addition, there was a coarsening trend for sediments of shoreface in recent decades.