| Since the operation of the Three Gorges Reservoir,the water and sediment situation in the Middle Reaches of the Yangtze River(MRYR)has changed significantly,and the sediment amount has decreased by 70%,which leads to continuous erosion and erosion of the mid-channel bar in the branching channel.Under the Yangtze River Protection,the erosion and shrinkage of the mid-channel bar will have an important impact on river regime adjustment,flood control safety and river ecosystem.However,the research on the evolution law of the mid-channel bar and the difference of scour is still unclear.At the same time,the mid-channel bar is the main morphologic unit of branching channel evolution,which has important ecological service function.There are many negative factors affecting the ecological status of the mid-channel bar,such as continent erosion,human encroachment,etc.,so the ecological health evaluation of the mid-channel bar is a problem worthy of study.In this study,hydrologic data,remote sensing interpretation and uav aerial survey were used to analyze the characteristics of runoff and sediment in the Chenglingji-Hukou reach of the MRYR in recent 30 years,and the morphological changes and evolution process of five typical mid-channel bars in the straight and slightly curved reaches were quantitatively analyzed.At the same time,the ecological health evaluation method of the mid-channel bars was put forward to evaluate the ecological health of Tianxingzhou.The ain results of this study are as follows:(1)After the impoundment of the Three Gorges Reservoir,the annual sediment transport in Chenglingji-Hukou section of the MRYR decreased significantly,74.82%at Luoshan station and 69.88%at Hankou station.However,the change of annual runoff was not obvious,with a decrease of 8.88%at Luoshan station and 6.72%at Hankou Station,respectively.Moreover,under different flow levels,the river bed in the MRYR suffered continuous scouring,which caused the main channel to cut down,and the water level decreased by 1.32 m on average.(2)In the past 30 years,the evolution process and vegetation coverage of different mid-channel bars are significantly different.Mid-channel bars in the slightly curved reach is more stable than the straight section.The annual changes in mid-channel bars in the slightly curved reach are only 1/7 of the straight reach.There are also significant differences in the evolution process of different mid-channel bars in the same type of river reach.The area of Nanyangzhou in the straight curved reaches increased by 30%,the area change of Fuxingzhou was relatively stable,but the area of Baishazhou shrank by 43.83%.The area is related to the flow and water level in a significant power function.The area of Tianxingzhou in the slightly curved reach increased by 6.58%,but it is actually in a state of scouring,the area is significantly related to the elevation.However,the area of Daijiazhou remained stable from 1999 to 2021 with a small change.The Normalized Difference Vegetation Index(NDVI)of Baishazhou and Tianxingzhou was in the range of[0,0.2],showed an increasing trend,and the spatial difference decreased over the years.The vegetation coverage of Baishazhou tail is more than that of other parts,and the vegetation coverage of the head shoal of Tianxingzhou is basically zero.(3)The scour mechanism of the mid-channel bar is different under the influence of different factors.There is a difference in the scour position between the straight reach and the slightly curved reach.In the same mid-channel bar,the scouring force of the main branch is greater than that of the branch.The protection project is beneficial to the stability of the shoreline of mid-channel bar under the influence of scouring.For example,before the implementation of Tianxingzhou protection project,the shoreline was eroded and retreated 1.43 km,and the shoreline was basically stable after the implementation of the project.(4)The continental body between the outcrop line of the mid-channel bar at the highest and lowest water levels is the aquatic-terrestrial ecotone of mid-channel bar.According to the natural and social attributes of mid-channel bar,the ecological evaluation index system of mid-channel bar was established,and the weight of each level was determined by analytic hierarchy process.Using this index system to evaluate the ecological health of Tianxingzhou,the result shows that the comprehensive score is 70.40,which is in a healthy state. |
