Study On The The Variation Of Erosion And Deposition And Evolvement Trend Of Yangtze Estuary Tidal Flat After Three Gorges Project Operation

The Yangtze River carries a huge amount of sediment deposited in the estuary of the Yangtze River every year,forming a wide and vast tidal flat.Under the influence of the tide,the tidal flat is sometimes submerged and sometimes exposed to water,its upper part is often exposed to water,and its lower part is often submerged.Tide flat has various functions,such as reducing waves,maintaining biodiversity and purifying water body,and is also an important land and wetland resource.Since the operation of the Three Gorges Project,the amount of sediment reaching the Yangtze estuary has decreased by more than 80%.In order to stabilize the river regime,maintain the waterway and the utilization of water resources,the Yangtze Estuary has carried out intensive waterway and river regulation projects in the past 20 years,most of which are related to sand fixation and enclosure tidal flats.These human activities have complicated and farreaching influences on the evolution of tidal flat in the Yangtze Estuary.This study analyzes and compares the variation of erosion and deposition in the main Yangtze estuary tidal flats before and after the operation of the Three Gorges Project by using a combination of measured topographic data and mathematical models.This is of great scientific and practical significance for understanding the evolution trend of Yangtze Estuary channel.The main results are as follows:(1)After the operation of the Three Gorges Project to 2019,the overall changes of the tidal flat area in the lower reaches of Xu Liujing in the Yangtze Estuary were as follows:the high tidal flat increased by 73.8%,the middle tidal flat reduced by-30.7%,and the low tidal flat reduced by-9.4%.The period with the largest decrease was from 2010 to 2016,which was the period when the Yangtze Estuary tidal flats were regulated with the greatest intensity,mainly concentrated at Hengsha Dongtan and Nanhui Dongtan in the estuary.Tidal flat in the Yangtze estuary evolution tended to be gentle from 2016 to 2019.(2)The relationship between Yangtze Estuary tidal flat and regulated engineering can be divided into four categories with different evolutions.Tidal flats that have been enclosed,such as Xintonghai sand and Xincun sand,high,medium and low tidal flat area are greatly reduced.The tidal flat with built projects,such as Jiuduan sand and Nanhui Dongtan,generally show the increase of high tidal flat,and the middle tidal flat decreases greatly and the low tidal flat decreases little.The evolution of the tidal flat affected by the surrounding project is related to the relative position of the project and the river regime of the tidal flat and the hydrological and sediment conditions.For example,the northern tidal flat of Chongming,whose upstream "North lake" project protrudes into the river,the high tidal flat increases by 243.1%,while the middle and low tidal flat decrease.Baozhen sand,because of the construction of Qingcao sand reservoir on the right side of the river narrowed the river width,resulting in Baozhen sand scouring overall.Basically in the natural evolution of tidal flats,such as Guyuan sand and Biandan sand,all show the evolution characteristics of increasing high tidal flat and decreasing middle and low tidal flat.(3)The MIKE21 numerical model is used to predict the future evolution trend of Biandan sand without engineering influence.The results show that:under the natural conditions of decreasing sediment coming from the upstream,the sand body of Biandan sand is mainly deposited,the sediment accumulation is concentrated in the high tidal flat which increases by 170.4%,while the area of the middle and low tidal flat decreases by-36.5%and-53.8%respectively.(4)The decrease of sediment inflow in the Yangtze River Basin is the main driving factor of tidal flat evolution in the Yangtze Estuary,the effect is slow but long-term.The overall performance is the area of high tidal flat increased,while that of middle and low tidal flats decreased.The local regulation project is the trigger factor of the tidal flat evolution of the Yangtze Estuary,the impact is dramatic but rapid.For example,the influence of promoting silting project is the rapid deposition of high tidal flat,while the adjacent middle and low are mostly eroded.The drastic impact of the regulation project on the tidal flat usually lasts for 3 years,after which the impact will gradually decreases.