Adjustments in reach-scale bankfull geometry of a braided reach undergoing contrasting channel evolution processes

Abstract

A braided reach of the Lower Yellow River (LYR) underwent contrasting processes of channel aggradation (from 1986 to 1999) and channel degradation (from 1999 to 2012) due to altered water and sediment regimes. Therefore, remarkable adjustments in bankfull channel geometry occurred in the braided reach over the past 27 years. In this study, the reach-scale bankfull channel dimensions in the braided reach were calculated during the period 1986–2012 using a reach-averaged method, based on the surveyed post-flood profiles at 28 cross sections. Calculation results indicate that the reach-scale bankfull area decreased gradually from 3525 m2 in 1986 to 1482 m2 in 1999 during the channel aggradation stage and increased quickly to 4499 m2 in 2012 during the channel degradation stage, and the main-channel geometry in the braided reach generally tended to become narrower and deeper over the study period. It is also found that the variation in the reach-scale bankfull channel dimensions of the braided reach was closely related to the previous 5-year average discharges and incoming sediment coefficients during both flood and non-flood seasons, with the corresponding empirical functions being developed for these relations. The proposed relations can well reproduce the temporal changes in bankfull channel geometry of the braided reach undergoing contrasting channel evolution processes in response to the altered flow and sediment regime.