Bar dynamics in a sandy braided river: Insights from sediment numerical simulations

Abstract

Bar dynamics in sandy braided rivers are keys to better understand the morphological processes of braided rivers and sediment distributions. Limited by insufficient quantitative information and satellite images that only show the larger-scale morphology and do not reveal the sand bars in the deeper channels, most previous studies have qualitatively assessed bar dynamics, but the ways in which the hydraulic parameters of sandy braided rivers control bar dynamics remains poorly understood. Here, we present sediment numerical simulations on sandy braided rivers to investigate the quantitative controls of river parameters on bar dynamics. Three bar dynamic forms (downstream migration, lateral variations and bar tail elongation) are observed. Unit bars are prone to downstream migration following formation, and the migration rate shows a deceasing trend during braided river evolution. This gradually decreasing migration rate is proven to be strongly connected to the decreasing flow velocity of braided channels that confine bar growth. The lateral variation in bar dynamics exhibits narrowing, migration and widening of sectional morphologies. We suggest that braided channel curvature dominantly controls lateral sediment transportation around the bar and thus results in several lateral variation forms of bars. The bar tail elongation difference between the two sides of the bar is associated with bar asymmetry. Strong bar asymmetry can make apparent differences in bar tail elongation.