Characteristics of hydrodynamic zone and microgeomorphic adaptation mechanism of the bank failure section in the lower reaches of the Yangtze River
In recent years,the tidal limit of the lower reaches of the Yangtze River has shifted upward,the construction of water-related projects has increased,and the intensity of river bank failure disasters has been on the rise.Previous researchers have found the existence of strong vertical-axial backflow in the bank failure section through field surveys,physical modeling,and numerical simulation,and that there is a strong vertical-axial backflow in the bank failure section;however,the hydrodynamic zone characteristics of the bank-channel interactive zone and the microgeomorphic adaptation mechanism have not been explored.Based on the Doppler acoustic current profiler(ADCP)in the frequent bank failure sections,multibeam system,combined with previous high-resolution underwater topographic data,it is found that the flow velocity and flow direction in the interactive zone of the bank-channel have significant zone characteristics,and according to the backflow strength a,i.e.,the longitudinal flow velocity u and its ratio to the transverse flow velocity v,|u/v|,the zone from the channel to the bank is identified into the main stream zone(α＜1,u＞0),the mixing zone(α≥1),and the near-bank backflow zone(α＜1,u＜0).The mainstream zone develops large and medium-sized dunes,scour grooves,and scour pits,the mixing zone develops accumulators and small dunes,and the backflow zone develops spines and small dunes;the main stream direct flushing in the mainstream zone and the backflow lateral erosion in the backflow zone are the main driving forces for the development of scouring microgeomorphic type and the development of bank failures,and the high flow velocity gradient in the mixing zone near the main stream zone generates scouring geomorphology,and the remaining part of mixing zone remains a low flow-velocity to make the sediment fall and silt to form the accumulation body.The findings can provide important references for the disaster remediation and near-shore engineering design and monitoring.
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