Landform Evolution Of Silty Beach Under Coastal Engineering Impacts

The paper studied changes of beach landform impacted by coastal engineering, by taking example of Maojiagang strentch which lies in Lvsi erosion coast Jiangsu province. We started with analyzing the background of Lvsi coast, applied the GIS technology to make the landform map digitization so as to form the DEM of different period. We applied the method such as the section comparison and DEM calculation and analysis and referred the field survey to study the beach landform change before and after the engineering constructing by using both the qualitative and quantitative methods. At the same time, we applied the physical model to accept it. Based on applying the assistant measure such as the change of grain size, susceptibility and spore powder of the beach sediment, we studied the chance and mechanism of erosion-deposition of the beach surface and used mathematical calculation to analyze the trend of erosion-deposition of the beach surface between the two jetties and inside the offshore breakwaters. We drew the main conclusions as following:1. After the construction was completed, the beach between the two jetties had been filled up step by step. The position of sea-route and the depth is steady. The beach on the west of jetty is on the condition of erosion and the beach on the east of jetty is on the condition of deposition. The beach out of the jetty is on the natural condition of erosion-deposition. The sea-route is in a state of natural sway. The natural change of the bank nearby Maojiagang has the characteristic of seasonal change: the sand surface is filled up in spring and summer and is flushed in fall and winter. The sediment of jetty was in the proportion of 62 percent between Nov. in 1991 and Dec. in 1992, and the average rate of sediment was 0.008476m /month. Between Dec. 1992 and Sep. 1993 the proportion was only 21.8 percent, and the average rate was just 0.004439m / month. The beach surface inside the jetty had got a balance of erosion and sediment until September 1993.2. After the construction of offshore breakwater had been completed, the state of the beach surface outside the offshore breakwater turned from flush to sediment. Nearer the distance to the offshore breakwater is. effects of surface sediment inside the offshore breakwater is the better. and formed the new surface that has course incline with initial surface due to deposition.3. The distributing and change of granularity. susceptibility and spore powder distribution accord with the change of surface sediment dynamic erosion-deposition. With the distance from surface to beach increasing, the granularity of deposition is thicker and the susceptibility augment, the granularity of the sediment inside the jetty is less than the outside and the susceptibility is less than the outside. It reflects that the dynamic condition inside the jetty is weaker than the outside. The granularity of the histogram section and the distributing and change of susceptibility and spore powder inside the offshore breakwater show a 3 or 4 gyration, it indicates that the beach surface sediment has got a balance condition until 3 to 4 years after the offshore breakwater was completed.4. The distribution of the beach surface nearby the engineering after the completion of silty coast differs from the distribution that after the completion of sandy and muddy coast, it formed a special configuration of erosion-deposition: Upriver of the jetty has been flushed and the downriver has been filled up after the silty coastal engineering was completed. At the beginning, the sediment headland caused by offshore breakwater was closed to the offshore bank and thus then extended both from land to sea and from sea to land.