Analysis Of Mechanism And Influencing Factors Of Coastal Frosion In Fujian Province

Coastal erosion problems are widespread throughout the world. The research on coastal erosion and protection has been one of the hot issues in geosciences. Since 1970s, approximately 60% of sandy coast in Fujian province has suffered erosion, with an average annual recession rate mare than1m yr-1. Based on the field surveys and repeated profile observations assistant with the comparison of many years'topographic maps and previous research, this study shows the spatial variability, varying patterns and mechanism of coastal erosion in Fujian province.Main research includes three objects:(1) Based on the field surveys and the history of coastal development and beach evolution, and the comparison of last 50 years'topographic maps (1961, 1983, 2004), this study illustrates the spatial variability (including location and range) and intensity of coastal erosion according to the recession rate. The impact of coastal geological background on distribution variability has also been discussed.(2) Sandy beaches in the studied area are generally backed by three kinds of backshore landforms such as dunes, seawalls and cliffs, illustrating significant regional variation along the coast of Fujian province. The varying backshore morplogy has an important controlling impact on the processes of profile change and coastal erosion. Three morphological types of beach and backshore combinations can be classified as beach-cliff system, beach-dune system and beach-seawall system.Based on periodic profile measurement and morphological change process analyzing of cliff-beach system, through the application of soil mechanics of soft cliff, impact of precipitation and moisture content of soil on cliff slope stability was analyzed, besides, contribution of precipitation and hydrodynamic during the process of cliff recession was also discussed.Based on the periodic investigation of the morphological change of dune-beach system in different areas and the tracing survey before and after storm surge, the process of erosion and recovery of dune-beach system has been analyzed, the function of driving forces including hydrodynamic and wind in these processes were also discussed.In case of Jinluan Bay in Dongshan Island, Qingshan Bay in Huian and Banyue Bay in Chongwu, shoreline and beach morphological change triggered by coastal structure such as seawall and breakwater was analyzed.(3) Based on the field survey and historical data, by understanding the regional geologic background and coastal zone development activities in recent years, typical cases of coastal erosion mechanism were studied, main influencing factors of coastal erosion in Fujian province were summarized, effect of individual factor on coastal erosion was demonstrated, and contribution of different factor in each typical coastal section was discussed finally.Some conclusions were made in this research, as follows:(1) With respect to coastal erosion intensity, the studied area could be divided into three segments from north to south. The segment located to the north of Min River estuary experienced mild erosion, while severer erosion occurred in the southern region near the Jiulong Jiang River estuary and the severest erosion occurred in the region between the Min Jiang River estuary and the Jiulong River estuary. Regional differentiation in the Meso-Cenozoic coastal tectonics determined the irregular coastline and geological background for large-scale distribution of coastal erosion.(2) The soft cliffs experienced rapid recession, with an average retreat rate of 1.87 m yr-1. Higher cliff suffered more severe recession than lower cliff, revetment building in the bottom of cliff provides effective protection to cliff, as under the same hydrodynamic condition cliff without revetment experiences much more rapid erosion. Cliff erosion was induced by the major external driving forces of precipitation and hydrodynamic and an internal cause of the weakness of cliff. Rainfall infiltration changes the water content of cliff slope soil, thus decreasing the soil strength and resulting slope failure. Cliff recession is a cyclic process which is composed of 4 stages as stable stage, action stage, landslide stage and transporting stage according to morphological change and driving forces.(3) Change in the beach-dune system manifested itself in two different ways. One is the periodic erosion and recovery, which is mainly caused by seasonal changes of wave climate and storms, and the recovery of the beach-dune system, the recovery driving forces including hydrodynamic and wind, the former can compensate the sediment deficit by longshore and cross-shore sediment transport, the latter can restore dune by the accumulation of wind-blown sand. The other is an irreversible erosion process as a result of sediment deficit which was mainly induced by sand mining. Erosion of dune-beach system shows two different ways, the first one is characterized by beach cutting down will stable frontal dune, the second type of erosion in the beach-dune system is characterized by continuous frontal dune retreat and beach erosion.(4) Comparing to natural process, coastal structures make a great impact on the change of coastal morphodynamics in different ways. Seawall can armour the shoreline whereas the monitoring of coastal changes around such structures shows rapid erosion occurred in the vicinity shores during storm surge, resulting a steeper beach slope. Seawall building along the upstream area decreases the sediment supply for downstream beach, thus erosion of downstream beach was induced by sediment deficit. When harbors are established on a coast, by the construction of breakwater to provide shelter for ships, breakwater blocks the longshore sediment transport and wave propagation. Accretion takes place at the upstream side whilst at the downcoast side erosion will occur as a result of sediment deficit and longshore sediment transport.(5) Coastal erosion is resulted by the synthetical effect of various causes, individual cause shows different spatial and temporal effect. Sea level rise and sediment discharge reduction are important background factors. Almost in every beach, sand mining strongly interfere with sediment budgets, and has become the most important cause of erosion along Fujian's coast. The storm surge happens frequently in Fujian province usually induces significant short-term beach erosion, but it makes an irrevisible permanent recession on soft cliff. Many occurrences of destructive coastal erosion have resulted directly from the construction of breakwater and other engineering structures. The low-strength materials of soft cliffs decide the instability of cliff slope.