Variable discharges control composite bank erosion in Zoige meandering rivers

This study investigated the roles of variable daily discharges in controlling processes of composite bank erosion in alpine meandering rivers located in the Zoige basin on the Qinghai-Tibet Plateau of China using Bank Stability and Toe Erosion Model (BSTEM). BSTEM was calibrated from compiled daily discharges, field measurements, and pre-determined model parameters for modeling fluvial erosion (FE) and bank collapse (BC) in the lower layer of a conceptualized composite bank profile. Modeling was based on theoretically designed five sets of competent hydrographs, each of which contains seven hydrographs with the same mean daily discharge (Q_a) and variable shapes represented by different combinations of the initial daily discharge, the peak daily discharge, and its occurrence time. The design was guided by 77 real competent hydrographs representing daily discharges recorded in 14 years. Results showed that shapes of these hydrographs (1) had discernable impact on FE and BC, but this impact is secondary to that of Q_a; and (2) affected FE in magnitude, but BC in both magnitude and frequency. After identifying a proportion of the designed hydrographs that are realistic in the existing flow regime, hydrograph-based rating curves for both FE and BC were established and used to create hydrograph-based effective discharges (Q_e) for FE and BC, respectively. Finally, Q_e was correlated to the width of overhanging arm for explaining the dominant processes controlling composite bank erosion and linking the nature of these processes to lateral migration of the associated meandering rivers. This success demonstrates a great potential of using Q_e to reveal many complex processes in fluvial systems.