| After the Wenchuan earthquake on May 12,2008,the loose deposits from the landslide and other geological disasters caused by the earthquake and the secondary geological disasters after the earthquake entered the river course in the earthquake-affected area,and the siltation of the river bed resulted in the change of the river's topography,and the flood disaster occurred risk increased.However,due to the lack of river monitoring data,the details of the river course changes after the earthquake are still unclear,and the study of the river course in the earthquake-stricken area after the earthquake also faces challenges.It is clear that the river course changes in the earthquake-affected area after the earthquake can provide scientific basis repair the river course in the earthquake-stricken area and flood disaster risk prediction.In this study,the river channel of Qingping Town,Mianyuan River,which was severely affected by the Wenchuan Earthquake,was selected as the research object.We use repeated airborne lidar data,satellite stereo pair data and drone image data to obtain the topography of the river channel in the year,1,5,and 11 years after the earthquake.The channel parameters analyzed include active channel migration,riverbed longitudinal profile,river bed gradient,and riverbed cross profile.The differential DEM method is used to determine the location and volume of sedimentation and erosion,and discussed the influencing factors of river course changes.The main results and conclusions are as follows:(1)The terrain data generated by airborne lidar data and UAV images data is more suitable for river-scale terrain change monitoring.These two types of terrain data describe the shape of the river channel in the most detailed manner,while the terrain data generated by the satellite stereo pair can only describe simple outline.The lidar terrain data is the closest to the measured value in elevation,and the average errors of the two phases terrain data are 0.06 m and 0.17 m,respectively,while the average errors of the satellite stereo image pair terrain data and the UAV image terrain data are-0.54 m and 0.17 m,respectively.The altitude difference of the UAV images terrain data at each checkpoint location is large,and the continuity of the terrain data is not as accurate as the lidar terrain data.(4)After the earthquake,the landslides and other disasters in the Mianyuan River Basin were mainly concentrated on both sides of the main river of the Mianyuan River and in the southeast of the basin.Abundant rainfall can promote the restoration of vegetation on landslides,but it can also trigger new landslides.In this complex process,the mountain environment is slowly stabilizing.During the three years after the earthquake,the vegetation recovery on the landslide was relatively slow,and new landslides appeared,and the mountain environment was still unstable.Three years later,with the gradual restoration of vegetation,the construction of water and soil conservation and disaster prevention engineering facilities,the mountain environment became Relatively stable,there are basically no large-scale disasters in the basin.From an inter-annual perspective,the rainfall situation in the study area is unstable,and the trend of annual rainfall increase and decrease occurs repeatedly,but the annual heavy rainfall period is still concentrated in July-September.(3)In order to explore the morphological changes of river channel after earthquake,the analysis was made from three aspects: active channel,longitudinal section and transverse section of river bed.After the earthquake,the active channel migrates laterally continuously,the width and curvature of the active channel change,and the channel bifurcates and sandbars appear.After the earthquake,the inter-annual riverbed longitudinal profile elevation and the riverbed transverse profile elevation first decreased and then increased over time.After the earthquake,the riverbed has increased year by year,and the added value of the riverbed ratio one year after the earthquake is 1.77 times of the average added value from 5 to 12 years after the earthquake.Field investigations found that the changes in the channel morphology of the Mianyuan River are controlled by mud-rock flow activities,the slope and width of the main channel.The Mianyuan River Basin is rich in loose solid matter and has a high risk of disasters during periods of heavy rainfall.(4)After the earthquake,the erosion and sedimentation of the river course changes every year,and the differential DEM method is used to illustrate the changes over the years.From 2008 to 2009,the average sedimentation depth was 0.37 m,the average erosion depth was 0.91 m,and the percentage SI of sediment imbalance was-38.75%.Erosion dominated the topographic changes in this period.From2009 to 2013,the average siltation depth was 7.19 m,the highest point was 30.28 m,the average erosion depth was 1.61 m,and the percentage SI of sediment imbalance was 46.34%.Sedimentation dominated the topographic changes during the four years.From 2013 to 2019,the average sedimentation depth of the reach in the study area was 2.80 m,the average erosion depth was 0.83 m,and the percentage SI of sediment imbalance was 45.01%.Sedimentation dominated the topographic changes during the six years.The change of river topography in the early stage after the earthquake was mainly controlled by erosion and then dominated by sedimentation.There are 6 kinds of erosion and sedimentation laws in the reach of the study area,erosion-deposition-deposition(-+ +),erosion-erosion-deposition(--+),deposition-erosion-deposition(+-+)and deposition-deposition-deposition(+ + +),the erosion-deposition-erosion(-+-),deposition-deposition-erosion(+ +-),including the area of the erosion-deposition-erosion(-+ +)accounted for 55.66%,and are mainly distributed in the second half of a channel.There is no direct or inversely proportional relationship between the area of erosion deposition and the volume of erosion deposition,which is mainly determined by the depth of erosion deposition.The average depth of erosion and deposition increases first and then decreases in the three periods after the earthquake. |
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