Channel bifurcations can be found in river network systems from high gradient gravel-bed rivers to fine-grained low gradient deltas. In these systems, bifurcations often evolve asymmetrically such that one downstream channel silts up and the other deepens and, in most cases, they eventually avulse. Past analytical and numerical studies showed that symmetric bifurcations are unstable in high and low Shields stress conditions resulting in asymmetric bifurcations and avulsion, while they can be stable in the mid-Shields range, but this range is smaller for larger width-to-depth ratio. Here, using a one-dimensional (1D) numerical model, we show that effects of sediment grain size and of channel slope are much larger than expected for lowgradient systems when a sediment transport relation is used that separates between bedload and suspended load transport. We found that the range of Shields stress conditions with unstable symmetric bifurcations expanded for lower channel slopes and for finer sediment. In high sediment mobility, suspended load increasingly dominates the sediment transport, which increases the sediment transport nonlinearity and lowers the relative influence of the stabilizing transverse bedslope-driven flux.Contrary to previous works, we found another stable symmetric solution in high Shields stress, but this only occurs in the systems with small width-to-depth ratio.This indicates that suspended load-dominated bifurcations of lowland rivers are more likely to develop into highly asymmetric channels than previously thought. This explains the tendency of channel avulsion observed in many systems.
Abstract. In river-dominated deltas, bifurcations often develop an asymmetrical morphology; i.e. one of the downstream channels silts up, while the other becomes the dominant one. In tide-influenced systems, bifurcations are thought to be less asymmetric and both downstream channels of the bifurcation remain open. The main aim of this study is to understand how tides influence the morphological development of bifurcations. By using a depth-averaged (2DH, two-dimensional horizontal) morphodynamic model (Delft3D), we simulated the morphological development of tide-influenced bifurcations on millennial timescales. The schematized bifurcation consists of an upstream channel forced by river discharge and two downstream channels forced by tides. Two different cases were examined. In the first case, the downstream channels started with unequal depth or length but had equal tidal forcing, while in the second case the morphology was initially symmetric but the downstream channels were forced with unequal tides. Furthermore, we studied the sensitivity of results to the relative role of river flow and tides. We find that with increasing influence of tides over river, the morphology of the downstream channels becomes less asymmetric. Increasing tidal influence can be achieved by either reduced river flow with respect to the tidal flow or by asymmetrical tidal forcing of the downstream channels. The main reason for this behaviour is that tidal flows tend to be less unequal than river flows when geometry is asymmetric. For increasing tidal influence, this causes less asymmetric sediment mobility and therefore transport in both downstream channels. Furthermore, our results show that bedload tends to divide less asymmetrically compared to suspended load and confirm the stabilizing effect of lateral bed slopes on morphological evolution as was also found in previous studies. We show that the more tide-dominated systems tend to have a larger ratio of bedload-to-suspended-load transport due to periodic low sediment mobility conditions during a transition between ebb and flood. Our results explain why distributary channel networks on deltas with strong tidal influence are more stable than river-dominated ones.
Bifurcations are common features in fluvial systems, where a larger upstream channel splits into two smaller downstream channels and can be found in steep slope rivers as well as in lowland rivers and deltas. In the fluvial-dominated channel networks, river flow divides at bifurcations and determines the sediment distribution as well as the morphodynamics of the downstream channels. In these systems, one of the downstream channels of a bifurcation is often abandoned, resulting in one dominant downstream channel that conveys the river discharge (Bolla Pittaluga, Coco, et al., 2015;Kleinhans et al., 2008). In the tide-influenced systems, tides cause a change in flow magnitude during ebb and even change the system into a confluence during flood if tides are large enough, suggesting that bifurcations in these systems may develop differently. Hoitink et al. (2017) suggested that such channel abandonment rarely occurs in tide-influenced systems, keeping both downstream channels open to convey the water. In this paper we investigate the effect of tides on the stability and asymmetry of bifurcations as found in tide-influenced river deltas.Morphodynamics of river-dominated bifurcations have been thoroughly studied for a range of settings in river systems, from low sediment mobility in gravel bed rivers (
In river-dominated deltas, bifurcations often develop an asymmetrical morphology, i.e. one of the downstream 5 channels silts up while the other becomes the dominant one. In tide-influenced systems, bifurcations are thought to be less asymmetric and both downstream channels of the bifurcation remain open. The main aim of this study is to understand how tides influence the morphological development of bifurcations. By using a 2DH morphodynamic model (Delft3D), we simulated the morphological development of tide-influenced bifurcations on millennial time scales. The schematized bifurcation consists of an upstream channel forced by river discharge and two downstream channels forced by tides. Two 10 different cases were examined. In the first case, the downstream channels started with unequal depth or length but had equal tidal forcing, while in the second case the morphology was initially symmetric but the downstream channels were forced with unequal tides. Furthermore, we studied the sensitivity of results to the relative role of river flow and tides. We find that with increasing influence of tides over river, the morphology of the downstream channels becomes less asymmetric. Increasing tidal influence can be achieved by either reduced river flow with respect to the tidal flow, or by asymmetrical tidal forcing of the 15 downstream channels. The main reason for this behaviour is that tidal flows tend to be less unequal than river flows when geometry is asymmetric. For increasing tidal influence, this causes less asymmetric sediment mobility and therefore transport in both downstream channels. Furthermore, our results show that bedload tends to divide less asymmetrical compared to suspended load, showing a possible stabilizing effect of lateral bed slopes on morphological evolution. In our simulations, the more tide-dominated systems tend to have a larger ratio of bedload and suspended load transport. Our results explain why 20 distributary channel networks deltas with strong tidal influence are more stable than river-dominated ones.
River deltas and estuaries are transition zones where rivers debouch into the sea. Land, river and sea meet in these zones, making them very valuable, both economically and ecologically. Their morphodynamic development is modified by an interplay between external-river and sea-forcing and anthropogenic activities in the area. These processes could affect economic activity and the natural habitat in the delta. Understanding the morphodynamics of river deltas is therefore very important for delta management plans.River deltas are formed by depositional processes driven by the interaction between upstream river forcing and downstream sea forcing. This interaction results in unique patterns based on the forcing's relative importance. Tide-influenced river deltas have a typical distributary-channel landscape with the number of channels increasing downstream. Here, the depositional processes are largely driven by the interaction between river flow conveyed from upstream and tides that come in from the sea along the channels. Most channel junctions in such systems are bifurcations: upstream channels that split into two downstream channels. The sediment distribution at bifurcations determines the sediment flux throughout the entire delta and hence the morphological development of the delta itself. Therefore to understand the morphodynamic development of the delta and its distributary channels, we need to understand the morphodynamics of bifurcations that connect the distributaries.Unlike river-dominated deltas, tide-influenced deltas have a tidally modulated river flow that not only changes in magnitude but can also change in direction. This flow modulation can differ in different deltas, depending on the relative dominance of river discharge over tidal flow. The resulting sediment transport patterns are spatially and temporally complex and so are the morphodynamics. In addition, the morphodynamics of tide-influenced bifurcation are less well understood than those of river-dominated bifurcation. This thesis therefore aims to improve our understanding of the morphodynamics of tide-influenced bifurcations in river deltas.To achieve this objective, a one-dimensional (1D) numerical model was developed to simulate morphodynamics in tide-influenced channel networks under a wide range of conditions. In this model, a novel approach was proposed to solve the sediment distribution at bifurcations under the changing flow conditions in tide-influenced deltas. To confirm the findings of the new 1D model I compared them with a well-established Delft3D model. With the help of these models, I systematically studied the critical factors in nature that influence the morphodynamics of bifurcations in tide-influenced deltas. The results are presented in Chapter 2 through 5 of this thesis, which are summarized in the next four paragraphs. chapter 2 describes how I used the 1D model to study the effect of channel slopes and sediment grain size on the morphodynamics of river-dominated bifurcations in which only river discharge is imposed from ...
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