Characteristic analysis of underwater topographic erosion and deposition in the Shanghai Section of the north Hangzhou Bay
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Abstract:
Special underwater topography formed in the estuary area makes it abundant for available resources. The Shanghai section of northern Hangzhou Bay is located on the north side of the middle and rear sections from the top to the mouth of the bay. It is affected by the Yangtze River and Qiantang River runoff and ocean. Thus, it is of great guiding significance for the construction, development, and maintenance of the northern edge of Hangzhou Bay to study the evolution law and change the reason for the underwater topography erosion and deposition. Despite previous analysis, there is a lack of accurate and continuous topographic data. With the advancement of development and construction in recent years, overall analysis and detailed research in key areas are needed.The annual square grid digital elevation model in the north edge of Hangzhou Bay from 2005 to 2020 was generated based on the measured CAD topography data combined with the isobath data of some offshore isobaths according to the analysis requirements of underwater topography scouring and silting in the north edge of Hangzhou Bay. Then for further research, the focal areas were selected with local evolution and used the contour line and the morphological change of the typical section profile to describe the evolution process of terrain over time. To analyze the influence of hydrological and hydrodynamic factors and human activities on the evolution of erosion and deposition along the northern edge of Hangzhou Bay, hydrological and hydrodynamic factors such as water and sediment, tidal current data, and the reclamation were studied. The results show that the northern part of Hangzhou Bay along the Shanghai section showed a weak erosion trend, with a total erosion of 148.2 million m3 and an average annual erosion depth of 2.886 cm in recent 16 years. The Jinshan deep trough is the lowest area, and the terrain elevation dropped in front of Luchao Port. The study area suddenly deposited heavily in 2011. The highest scour happened between 2005 and 2008, and 2008 to 2011 was the highest siltation period. Although the Jinshan deep trough was affected by strong tide and encirclement projects, it contained strong erosion and strong deposition, but the overall performance was a slight erosion with a balanced erosion and deposition, and the erosion intensity was only 12.1% of that in the study area. There were many deep scour pits and steep slopes in Section # 2 and Section #3. The decrease of incoming water and sediment upstream made the north edge of Hangzhou Bay maintain the scouring situation. The supplement effect of tidal current-carrying sediment increased with the tide difference, and the scouring intensity was weakened. It is the equilibrium state formed by the synergy of various influencing factors that will last for a long time. Reclamation engineering is the main influencing factor of topography and erosion-deposition in the Jinshan deep trough. Some areas were blocked by reefs. The shoreline of the surrounding area was stable, but the nearshore scouring was strong, and the topographic change was more intense. Whether long-term scouring will bring risks such as foundation instability and bank collapse, the evolution of scouring and silting terrain characteristics, and the feedback relationship between beach and port development and construction need to be continuously observed and further studied.
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