The Study Of The Deposition And Erosion Change Base On The Field Observation Of RTK-GPS In Eastern Chongming Intertidal Wetland

Intertidal wetlands usually refers to the accumulation of sediment between the Mean high high water spring tide and Mean low low water springs tide. A whole intertidal wetland, with salt marsh up and unvegetated flat down. It not only has important ecological values and environmental functions, but also plays an important role in the wave dissipation revetment. In the background of the watershed runoff and sediment runoff condition change, the sea level fluctuations, tidal, wave and other factors, the delta intertidal wetland is always in the process of erosion and deposition. Monitoring the erosion and deposition change of the intertidal wetlands is important for the coastal management. However, usually the intertidal wetland is broad and it’s difficult to walk (soft muddy marsh surface, tidal creeks), high-dense salt marsh vegetation always obstructs the vision. So the traditional method is also difficult to timely and accurate access to the erosion and deposition change information of the intertidal wetlands. The development of RTK-GPS technology in recent years has brought new opportunities for the intertidal wetland height measurement. Series of elevation data will be helpful to cognize the erosion and deposition change of the intertidal wetlands.We survey three sections in Eastern Chongming Intertidal wetland by RTK-GPS, collected the elevation data from September2009to February2012, combined with early accumulation data to explore the spatial and temporal change of the erosion and deposition. The main results and conclusions include:(1) The seasonal erosion and deposition cycle. Generally:erosion trend appears in October to next April, and deposition trend appears in May to October, the maximum marsh surface elevation is in October. This law is basically the same performance in the north, middle and south section. Seasonal erosion and deposition cycle is mainly due to strong wave action in winter, the Yangtze River sediment runoff less and salt marsh vegetation withered-beach bare.(2) Sedimentation rate rise in decline year by year. In the middle section of the most representative, the lower boundary of the salt marsh toward the sea drop from250m/a (1982-1990) to30-50m/a (2006), and in recent years, the lower part of the unvegetated flat appears erosion; since2005, the north and south sections also present slow deposition and fast erosion. The rate such as long-term downward trend mainly attributed to the decline of the sediment runoff in recent Yangtze, No significant relationship with the dynamic conditions. Also the erosion and deposition change maybe connect with the evolution of the ditch beside the sea (between Eastern Chongming Intertidal wetland and North Port North Sand), but the relationship is unclear. In view of the precipitation in the Yangtze River basin slightly decline in recent years, it is estimated that the sediment transport rate-flow decreased due to reduced in10%. In other words, in recent years, the deposition variation is only about10%that can be attributed to climate change in Eastern Chongming Intertidal wetland, while human activity is the main reason.(3) The differences of the process of erosion (Perpendicular to the coastline direction). Generally, the short-term erosion and deposition change (years), compared to the salt marsh (especial the upper part of the salt marsh), the process in the unvegetated is more frequent and intense. One side, it is because the hydrodynamic forces’time and intensity acting on the flat decrease, with the decrease of the submerged probability and the Submerged depth reduce; on the other hand, because the vegetation could slow down the tide and weaken waves. The characteristics of erosion and deposition trend in perpendicular to the coastline direction:the unvegetated flat appears erosion. Little change in the upper part of the unvegetated flat and clearly change in the lower part of the salt marsh. While deposition is rare in salt marsh upper.(4) The differences of the process of erosion (Coastline direction). The average erosion and deposition rate can be decomposed into the vertical rate and the horizontal rate of change. By comparison, the vertical rate size as follow:South section>Middle section>North section, the horizontal rate size as follow:Middle section>South section>North section. Location hydrological conditions and beach slope are the factors affecting the vertical rate and the horizontal rate, usually, it’s believed that the big slope is not conducive the sediment to settle, on the contrary, the slope is small, the deposition rate is big.