Distinct effects of processes and boundary conditions on fluvial and coastal morphodynamics and stratigraphy (original) (raw)
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Morphodynamic response of a variable-width channel to changes in sediment supply
River channels commonly exhibit downstream variations in channel width, which can lead to the development of alternating shallow and deep areas known as riffle-pool sequences. The response of these channels to variations in sediment supply remains largely unexplored. Here we investigate the morphodynamic response of a variable-width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width. We first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded, armored bed. This sediment-starved bed was subjected to two types of sediment supply increases: a return to the initial constant supply, and the introduction of a well-sorted sediment pulse (analogous to gravel augmentation). Riffles and pools formed in wide and narrow areas, respectively, and the location of and relief between riffles and pools remained the same throughout all experimental runs, regardless of the sediment supply. The primary channel response to changes in supply was adjustment of the overall slope. The sediment pulse evolved primarily through dispersion rather than translation, which contrasts with prior gravel augmentation experiments conducted in constant-width channels and suggests that width variation and resulting riffle-pool topography enhances pulse dispersion. Our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels, and sediment supply changes are unlikely to affect riffle-pool morphology when bank geometry is fixed and water discharge is steady.
Planform changes in large braided sand-bed rivers
1993
As a first step towards the development of a predictive model for channel changes and related bank erosion, morphological .!?rocesses were studied in a braided river with fine sand as bank and bed material. This was done usmg mainly (geometrically corrected) satellite images, the use of which was justified because of the large scale of the river studied: the Jamuna River in Bangladesh with a total width of up to 17 km. The study concentrated on bank erosion, channel shifts, and processes at bifurcations and confluences. It was found that the bank erosion depends on the relative curvature of the curved channels, and that the rate is an order of magnitude larger than predicted by Hickin & Nanson (1984). Several types of channel shifts were identified, and it was observed that cutoffs occur already at very low cutoff ratio's. The main conclusions from the study are that on the one hand much more studies are needed to improve the understanding of these different processes, and on the other hand that there is a clear limit to the period over which predictions can be made due to the observed chaotic behaviour of the channel changes.
2013
1] Braided rivers have complicated and dynamic bar patterns, which are challenging to fully understand and to predict both qualitatively and quantitatively. Linear theory ignores nonlinear processes that dominate fully developed bars, whereas natural river patterns are determined by the combined effects of boundary conditions, initial conditions such as planimetric forcing by fixed banks and the physical processes. Here we determine the capability of a state-of-the-art physics-based morphological model to reproduce morphology and dynamics characteristic of braided rivers and determine the model sensitivity to generally used constitutive relations for flow and sediment transport. We use the 2-D depth-averaged morphodynamic model Delft3D, which includes the necessary spiral flow and bed slope effects on morphology. We present idealized scenarios with the smallest possible number of enforced details in the planform and boundary conditions in order to allow free development of bars driven by the physical processes in the model. We analyze bar and channel shapes and dynamics quantified by a number of complementary metrics and compare these with imagery, field data captured in empirical relations, flume experiments, and predictions by linear analyses. The results show that the chosen set of boundary conditions and physics in the numerical model is sufficient to produce many morphological characteristics and dynamics of a braided river but insufficient for long-term modeling. Initially, braiding intensity with low-amplitude bars is high in agreement with linear analysis. In a second stage when bars merge, split, and increase amplitude up to the water surface, the shape, size, and dynamics of individual bars compare well to those in natural rivers. However, long-term modeling results in a reduction of bar and channel dynamics and formation of exaggerated bar height and length. This suggests that additional processes, such as physics-based bank erosion, or enforced fluctuations in boundary conditions, such as spatial-temporal discharge variation, are necessary for the simulation of a dynamic equilibrium river. The most important outcome is that the modeled pattern of bars and channels is highly sensitive to the constitutive relation for bed slope effects that is used in many morphological models. Regardless of this sensitivity and present model limitations of many models, this study shows that physics-based modeling of sand-bed braided improves our understanding and prediction of morphological patterns and dynamics in sand-bed braided rivers.
2019
Channel bifurcations split flow and sediment around fluvial and tidal bars in rivers and estuaries and at junctions in river deltas and tidal deltas. Their long-term development depends on the balance between sediment partitioned at the bifurcation and the transport capacity in the bifurcated channels. For unidirectional flow, theory predicts that stability depends on channel width-to-depth ratio and sediment mobility, but this has not been tested under high sediment mobility and in tidal conditions. Here we report on nine unidirectional and bidirectional flow experiments on a sand bed with a splitter plate wherein bed development and flow partitioning were monitored. Results show that fluvial and tidal experiments develop unstable bifurcations for intermediate and high sediment mobility and channel width-to-depth ratio in agreement with theory.
Morphological evolution of bifurcations in tide-influenced deltas
2019
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 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 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 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 distributary channel networks deltas with strong tidal influence are more stable than river-dominated ones. 1 Introduction Deltas often consist of distributary channel networks. In these systems, water and sediment are divided at the bifurcations and distributed over the delta. The shape of the delta and the number of active channels depends on many factors like the forcing by rivers, tides and waves (Galloway, 1975; Rossi et al., 2016; Shaw and Mohrig, 2014), sediment availability and sediment type (Geleynse et al., 2011). Bifurcations tend to develop differently in river-than in tide-dominated systems, because tides influence the mouth bar formation processes of active river-dominated deltas (Edmonds and Slingerland, 2007; Leonardi et al., 2013; Shaw and Mohrig, 2014). In tidal deltas tides propagate upstream and can induce bi-directional flows. This unique characteristic may lead to a different morphological evolution of the bifurcations than would occur in the river-dominated zone
Earth Surface Processes and Landforms, 2020
This Special Issue collects 17 selected contributions from participants to the 10th edition of the RCEM (River, Coastal and Estuarine Morphodynamics) Symposium, organized in Padova-Trento (Italy) in September 2017. The series of biennial RCEM Symposia has the key goal of enhancing interaction and promoting integration among the scientific communities focused on the morphological dynamics of river, coastal and estuarine environments, though various combinations of theoretical, observational, experimental and modelling approaches. The 17 contributions to this Special Issue contain 4 state of science reviews and overall offer a broad view of the cross-cutting perspective adopted when addressing morphodynamics. Such perspective accounts for the mutual interplay between morphology, fluid dynamics and other environmental factors, and has presently become a widespread paradigm to address landscape evolution.
Journal of Geophysical Research: Earth Surface, 2015
This paper provides novel observations linking the connections between spatially distributed bed load transport pathways, hydraulic patterns, and morphological change in a shallow, gravel bed braided river. These observations shed light on the mechanics of braiding processes and illustrate the potential to quantify coupled material fluxes using remotely sensed methods. The paper focuses upon a 300 m long segment of the Rees River, New Zealand, and utilizes spatially dense observations from a mobile acoustic Doppler current profiler (aDcp) to map depth, velocity, and channel topography through a sequence of high-flow events. Apparent bed load velocity is estimated from the bias in aDcp bottom tracking and mapped to indicate bed load transport pathways. Terrestrial laser scanning (TLS) of exposed bar surfaces is fused with the aDcp surveys to generate spatially continuous digital elevation models, which quantify morphological change through the sequence of events. Results map spatially distributed bed load pathways that were likely to link zones of erosion and deposition. The coherence between the channel thalweg, zone of maximum hydraulic forcing, and maximum apparent bed load pathways varied. This suggests that, in places, local sediment supply sources exerted a strong control on the distribution of bed load, distinct from hydraulic forcing. The principal braiding mechanisms observed were channel choking, leading to subsequent bifurcation. Results show the connection between sediment sources, pathways, and sinks and their influence on channel morphology and flow path directions. The methodology of coupling spatially dense aDcp surveys with TLS has considerable potential to understand connections between processes and morphological change in dynamic fluvial settings. The first issue is the need for distributed measurements of flow and sediment transport to support the interpretation of morphological change. Pioneering studies that identified erosional and depositional WILLIAMS ET AL.
Long-term evolution and morphodynamic equilibrium of tidal channels
Journal of Geophysical Research, 2002
1 ] This contr ibution inves tigates the morp hodynam ic equil ibrium of funnel -shape d well-mixe d estua ries and/or tida l channel s. The one-di mensional d e Saint Venant and Exner equations are solve d numerical ly for the ideal case of a fric tionally domi nated estua ry consisti ng of noncoh esive sedim ent and with insignif icant interti dal storage of wat er in tida l flats and salt mars hes. This class of estuaries turns out to be invariabl y flood dominated . The resul ting asym metries in surfa ce elevat ions and tidal curren ts lead to a ne t sediment flux within a tida l cycle which is directed landw ard. As a consequ ence, sedim ents are trapp ed within the estua ry and the bott om profi le evolve s asym ptotical ly toward an equil ibrium confi guration, allo wing a vanishing net sedim ent flux everyw here and, in accordan ce with fiel d observ atio ns, a nearly const ant value of the maxi mum flood /ebb speed. Such an equil ibrium bed profi le is characteri zed by a concavi ty increasing as the estua ry co nvergence incre ases and by a unique ly deter mined value of the depth at the inlet section. The final length of the estua ry is fixed by the longi tudinal extens ion of the very shallow area which tends to form in the landw ard portion of the estuary. Note that sediment advect ion is neglec ted in the analys is, an a ssumption appropriat e to the case of not too fine sediment .