Morphodynamics of braided rivers under environmental change: controls on the evolution of channel flood conveyance capacity

Abstract

Braided rivers are often characterised by dynamic behaviour that is driven by both internal process-form feedbacks and external variations in water and sediment supply. Such behaviour can involve river bed aggradation or degradation, significant channel widening or narrowing, and changes in planform morphology (e.g., braid intensity). Moreover, such dynamics have the potential to drive changes in the flood conveyance capacity of the river, and to propagate downstream over time. These effects have been observed in numerous field case studies. However, as of yet it has proven difficult to develop a general theory or quantitative understanding of how braided rivers respond to environmental change, or how the morphodynamic sensitivity of such channels is controlled by factors such as valley morphology, flood regime or lateral channel stability.The current study seeks to investigate these phenomena by performing a series of 2D physically-based morphodynamic model simulations of braided river evolution over periods of multiple centuries. Simulations were carried out to model the development of equilibrium channel morphologies, following which environmental perturbations were applied to investigate the effects of: (i) climate change; (ii) increased sediment delivery from hillslopes; and (iii) the impact of dam construction. For each environmental scenario, multiple simulations were conducted to investigate different combinations of variables that control the river morphology. For example, we examine varying degrees of channel confinement (valley width), differences in hydrologic regime, and changes in vegetation dynamics that control floodplain development and river width adjustment.Model results demonstrate that long-term (decadal to centennial) variations in flow and sediment supply can drive significant changes in channel flow conveyance capacity, stage-discharge relationships and the frequency of overbank flooding in braided rivers. Width adjustment represents a dominant mode of river response to environmental change. For example, braided rivers tend to accommodate downstream increases in discharge primarily through adjustments in total flow width. In contrast, constraints on adjustment in channel width lead to the concentration of floodwaters within a narrower channel belt, thereby amplifying vertical channel responses to change while potentially creating laterally stable channel nodes. While concern over future changes in flood regime tend to focus on increases in flood magnitude and frequency, model results illustrate that flood duration may also exert an important influence on channel morphodynamics and hence flow conveyance capacity.