Response of braiding channel morphodynamics to peak discharge changes in the Upper Yellow River

Abstract

The braiding intensity and dynamics in large braiding rivers are well known to depend on peak discharges, but the response in braiding and channel–floodplain transformations to changes in discharge regime are poorly known. This modelling study addresses the morphodynamic effects of increasing annual peak discharges in braiding rivers. The study site is a braiding reach of the Upper Yellow River. We estimated the effects on the larger‐scale channel pattern, and on the smaller‐scale bars, channel branches and floodplains. Furthermore, we determined the sensitivity of the channel pattern to model input parameters. The results showed that the dominant effect of a higher peak discharge is the development of chute channels on the floodplains, formed by connecting head‐cut channels and avulsive channels. Widening of the main channel by bank erosion was found to be less dominant. In addition, sedimentation on the bars and floodplains increased with increasing peak discharge. The model results also showed that the modelled channel pattern is especially sensitive to parametrization of the bed slope effect, whereas the effect of median grain size was found to be relatively small. Copyright © 2018 John Wiley & Sons, Ltd.

Living landscapes: Muddy and vegetated floodplain effects on fluvial pattern in an incised river

“…These processes are likely more important than outer‐bank erosion, which is fortunate because the latter is relatively poorly represented in numerical models (Schuurman et al. ). In sum, the Allier is a dynamic, living landscape with such conditions and dimensions that its pattern is sensitive to effects of floodplain formation, allowing us a high degree of control over the most important variables that affect channel pattern.…”

“…The bend migration rate is therefore sensitive to the bed‐slope effect (Schuurman et al. ), but as the bed‐slope effect also determines bar length we chose the value that reproduces the meander bend length of the Allier best (van Oorschot et al. ) and is near the optimal value for bank erosion in Schuurman et al.…”

Growing Forced Bars Determine Nonideal Estuary Planform

“…Physical scale experiments and numerical models complement observations in natural systems because they can provide higher temporal resolution, enabling detailed observation of the morphodynamic evolution of bars. In this study we use physical experiments, because the produced channel and bar patterns in numerical models (e.g., Braat et al, 2017;van der Wegen & Roelvink, 2012) depend on calibration parameters such as the transverse bed slope effect that strongly affect channel-shoal interaction and bar dynamics Schuurman et al, 2018).…”

Critical dependence of morphodynamic models of fluvial and tidal systems on empirical downslope sediment transport

“…Morphodynamic models are therefore widely used tools to study and forecast the development of these landscapes. However, in practice, all large-scale models depend on model choices and need some form of calibration to converge to a stable morphology, for example by the choice in roughness predictor 4,5 , adding coarser grain sizes in the channels 6 or include a non-erodible layer that limits channel depth 7 , and increasing the transverse bed slope parameter, which determines the amount of sediment transported on channel side slopes. The latter has proven to be most effective, since the bed slope parameter linearly increases downslope sediment transport and thereby directly affects channel depth and bar dimensions and therefore has the largest effect on large-scale morphology 8,9 . The problem is that morphodynamic models show severe and unrealistic channel incision and require artificially and seemingly arbitrarily transverse bed slope parameters up to a 100 times higher 8-10 than physically correct [11][12][13] to counteract this incision and obtain realistic bar and channel patterns.…”

“…Here environment means initial and boundary conditions, which determine sediment characteristics, flow conditions, channel pattern, and bar regime. Models of environments with a large-scale balance between erosion and deposition, such as estuaries and rivers, particularly have the tendency to overpredict channel depth and number of channels and required very high slope effects up to a factor of 100 5,17 . In contrast, models of systems with dominant erosion such as a tidal channel network, usually had slope factors <10 [18][19][20] , and depositional systems such as river deltas all used the default value [21][22][23] .…”