Recent bank retreat processes and characteristics in the braided reach of the Lower Yellow River

The braided reach of the Lower Yellow River (LYR) is experiencing severe channel degradation, and significant bank retreat processes have occurred owing to the operation of the Xiaolangdi reservoir. Estimation of bank retreat processes at typical sections can not only help to understand the characteristics of fluvial processes, but also offer parameters relevant to river regulation and planning. In this study, the variation in bankfull widths at typical sections of the braided reach has been investigated based on the observed cross-sectional profiles during the period from 1999 to 2013, with the maximum multi-year average bank retreat rate of 215 m/a being obtained. Several factors influencing bank retreat have been presented, which reveals that the composition and mechanical properties of the bank soil, and the elevation difference between floodplain and main channel can influence the processes of bank retreat, but the incoming flow and sediment regime is the dominant factor in controlling bank erosion. Empirical relationships between the accumulated bank retreat width and the previous 5-year average fluvial erosion intensity were developed at typical hydrometric sections and sedimentation sections, with the minimum correlation coefficients of above 0.85. Therefore, the calculated processes of bank retreat agree well with the measurements at these typical sections, and the proposed empirical relations can be used to estimate bank retreat processes in the braided reach of the LYR.