Bar and channel evolution in meandering and braiding rivers using physics-based modeling

Abstract

Rivers are among the most dynamic earth surface systems. Some rivers meander, forming bends that migrate, reshape and have inner-bend bars. Other rivers form a complicated braided pattern of branches, islands and mid-channel bars. Thorough understanding of their morphodynamics is important for efficient and sustainable river management. Although field and flume data provide important insight into river behavior, their applicability is limited due to their small spatial and temporal coverage, or scale issues. Therefore, we applied generic state-of-the-art physics-based morphodynamic models to gain insight in the autonomous morphodynamics of braiding and meandering rivers, and their response to perturbations.

First, this study examined the formation and evolution of bars and branches in braided rivers, applying simplified initial and boundary conditions in the state-of-the-art physics-based model Delft3D. The results show that the chosen set of boundary conditions and physics in the numerical model was sufficient to produce many morphological characteristics and dynamics of a braided river, but insufficient for long-term modeling. Furthermore, the results were sensitive to constitutive relations for sediment transport, bed roughness and bed slope.

Second, the bar and branch dynamics within the braided rivers were studied in terms of the stability of bifurcations and its coupling with bar and bifurcation dynamics within the entire network. Asymmetrical migration and reshape of mid-channel bars occurred in response to bifurcation asymmetry. In particular, bar-tail limbs grew in response to local prevailing flow, closing recessive branches and merging bars into large compound bars. Next, water level differences between parallel branches caused dissection of the large bars by cross-bar channels. This cycle of bar growth and dissection formed the basis for a conceptual model of bar and branch dynamics within sand-bed braided rivers.

Third, the effect of upstream inflow variation on meander planform and sustained meandering, which theory and flume experiments indicated as being significant, was studied using three physics-based models. Each models had specific processes, such as floodplain construction and bank erosion, important for meandering and making them complementary. The results showed that initiation and sustaining of high-sinuosity meandering were highly dependent on inflow perturbation, whereas inflow perturbations in low-sinuosity meandering only had a small local effect. Furthermore, high-sinuosity meandering required channel-floodplain conversion in the inner-bends that prevented chute cutoffs and enhanced flow asymmetry.

Fourth, the effects of human-induced perturbations on the morphodynamics in large braided sand-bed rivers, for example discharge variation attenuation, channel confinement, river training works and sand mining, were considered. The results showed that the local morphological response amplified while propagating downstream through the entire channel, instead of being damped. Furthermore, discharge variation attenuation and channel confinement appeared to have limited influence on the general bar pattern and morphodynamics.

This study demonstrated the interaction between bars, branches and bifurcations within braiding rivers, and the response of meandering and braided rivers to external perturbations. It also demonstrated how we can enlarge understanding of the generic behavior of braiding and meandering rivers, and their response to perturbations using physics-based morphodynamic models.