Measurement And Characterization Methodology Of Riverbed Morphology In Mountain Rivers

Mountain rivers are widely distributed in nature,and are distributed in Hengduan Mountains and Qinghai-Tibet Plateau.The traditional field measurement method is suitable for small-scale measurement of channel morphology.For braided rivers in mountainous areas with wide river channel,traditional measurement methods consume much manpower and material resources,while the application of UAV(unmanned aerial vehicle)low-altitude aerial survey technology can greatly improve the efficiency and precision.By using two different scales of measured method,total station and UAV,the topographic data of three field surveys in 2017-2018 were obtained.The morphological characteristics and evolution of the Xiaojiang River basin in Dongchuan,Yunnan Province,and the Lancang River source in Qinghai-Tibet Plateau were characterized and analyzed.At the same time,in order to improve the accuracy of UAV aerial topographic data and remove noise,a noise processing technology based on UAV aerial topographic data is proposed,which provides a scientific data basis for the study of large-scale braided river morphological evolution.The main results are as follows:(1)The riverbed structure is an important micro-geomorphological feature in the small watershed channel.The high-precision three-dimensional riverbed topography can be obtained by traditional measured methods.Based on the topographic survey data of seven sections of the main and tributaries of the Xiaojiang River basin in April and September 2017,four new dimensionless parameters(concavity-convexity number,average concavity,average concavity,and concavity-convexity)for the characterization of riverbed structure and their calculation methods are proposed.The new parameter characterization results are consistent with the predecessors' parameter calculation results,and can be used to characterize the development degree of riverbed structure and its local morphological characteristics from many aspects.Moreover,the characterization of the concavity and convexity is more distinguishable if the degree of the riverbed development is relatively lower.(2)Based on the field aerial survey data of Lancang River source regionin May,2018,a noise processing technique for UAV aerial survey data is proposed by using Pix4D mapper,Cloud Compare and ArcMap software.The effect of noise processing is good,and high precision aerial survey topographic data are obtained.(3)Using the high-precision topographic data of 5 surveyed reaches of Zhaqu Xiangda Section of Lancang River source in May,2018,the calculation and analysis of cross-sectional morphological characteristics,water area,number of branches and length-width ratio of the branches were carried out.The relationship between water level and cross-sectional shape,water area,number of braided channels and length-width ratio of braided channels is established.The complex relationship between the number of braided branches and water level is revealed,and the "single peak" is characterized.The "long-thin" degree of the channel shape at each level of the river.