Abstract: The characteristics of bank collapse of Jingjiang Reach and the physical and mechanical properties of cohesive soil in the upper part of the riverbank are studied based on field investigation, indoor geotechnical test and collapse model test in different hydrological periods for three consecutive years. The characteristics and influencing factors of the bank collapse process of composite structure are also examined. Results reveal that bank collapses occurred more frequently in lower Jingjiang Reach than those in the upper reach, and on left bank more than those on the right bank. The composite structure of riverbank undergone a process generalized as follows: the slope toe becomes steeper due to scouring, cracks occur and develop on bank top, the bank slope is eroded progressively, and bank instability leads to collapse, then the bank slope shape tends to be stable, and finally enters the next collapse cycle. The moisture content of cohesive soil responds more quickly to water level change than that to soaking time. The incipient shear stress of cohesive soil is 0.531 N/m², and the critical moisture content affecting the cohesion value is about 16%. Affected by longitudinal flow and soil moisture content, the bank slope is of high stability in dry season, but is subjected to local collapse during water level rising; both the flood season and recession stage will see intense slope toe scouring and bank collapse.

Key words: composite riverbank, collapse test, mechanical properties, shear strength, moisture content, Jingjiang Reach