Living landscapes: Muddy and vegetated floodplain effects on fluvial pattern in an incised river
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Cohesive floodplain sediment and vegetation are both thought to cause meandering river patterns. Our aims are to compare the isolated and combined effects of mud and vegetation on river planform and morphodynamics in the setting of intermediate-sized valley rivers. We use a numerical model for century-scale simulation of flow, sediment transport and morphology coupled with riparian vegetation settlement, growth and mortality as functions of species traits on which flow resistance depends. Mud fluxes were predicted by excess shear stress relations in combination with the active layer formulation. We found that valley-flooding water levels increase with vegetation density, causing a higher braiding intensity rather than meandering tendency. The shear stress during floods carves channels through the muddy floodplain surface. Higher mud concentration, on the other hand, increases floodplain aggradation, reduces the overbank flow frequency and ultimately causes formation of a single-thread channel. Vegetation causes mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation settling mutually enhance floodplain formation. However, mud and vegetation counteract in two ways. First, vegetation enhances floodplain accretion, which ultimately increases plant desiccation for high mud concentrations. Second, vegetation increases the tendency of periodic chute cutoffs in valleys. The chute cutoffs locally reset the landscape and create new windows of opportunity for the vegetation. Surprisingly, in systems with a high mud concentration this causes hysteretic loops of vegetation cover and delayed mud deposition. Ramifications for the interpretation of Palaeozoic fluvial facies are that even rootless vegetation, capturing cohesive mud closer to the river channel to form thicker floodplain on the point bar, can enhance the tendency to meander and, under high mud supply, form stable channels. However, meandering is more unlikely in narrower valley rivers with higher vegetation density.Keywords:
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Abstract A robustly calibrated and validated hydrodynamic model depicts flow patterns over a topographically complex floodplain with a heterogeneous main channel levee. Simulations highlight floodplain inundation dynamics for two conditions, the passage of sub‐bankfull and overbank flood waves. Sub‐bankfull inundation commences with the passage of the flood wave crest beyond the lower elevation levee breaches, and floodplain wetting is guided by the channel network. Hence, the upstream sub‐bankfull inundation area expands while much of the downstream floodplain remains dry. The onset of overbank flow is spatially variable but becomes continuous, and the through‐bank channels persist as preferential pathways that produce higher velocity flows several kilometers inland. Meanwhile, near‐stagnant zones develop between through‐bank channel mouths, where water is temporarily stored and routed to the channel network. Also, 48% of the inundation water is from the river while 52% enters the study area from the upstream floodplain. Overall, floodplain wetting and draining processes significantly influence flow direction, and characteristics of the flood wave over the floodplain surface such that a single stage in the main channel does not uniquely define floodplain flow hydraulics. Given these findings we propose that assessments of floodplain hydraulic connectivity account for the effects of heterogeneous levee structure and intra‐floodplain exchanges, as well as the typical flow thresholds associated with submergence and emergence of topography.
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Abstract Floodplain sedimentation is one of the most dynamic geomorphic processes within plain and lowland landscapes. There is generally a good understanding of longer term floodplain evolution, but quantitative information on overbank deposition rates for recent shorter timescales is lacking. This paper describes the application of Chernobyl‐derived 137 Cs to quantify floodplain aggradation rates for the River Plava (a small river draining a severely contaminated part of the upland region of Central European Russia), based on detailed sampling of four representative floodplain study sites. Two approaches have been employed for estimating post‐Chernobyl (1986–2009) floodplain accumulation rates. The first was based simply on locating the Chernobyl fallout‐associated 137 Cs peak in overbank sediment sections. The second involved quantification of the increase in the total 137 Cs inventory at individual sampling points associated with the post‐Chernobyl deposition of contaminated suspended sediment. It has been shown that considerable local‐scale variability of overbank deposition rates exists, with aggradation rates on the low level floodplain (6 ± 1.2–14 ± 2.8 mm year −1 ) exceeding by 1.5–3 times the values for the middle level floodplain (2 ± 0.4–9 ± 1.8 mm year −1 ) and by 3–6 times the values for the upper level floodplain (1 ± 0.2–5 ± 1.0 mm year −1 ) floodplain levels. Combining these estimates with information on the areas occupied by different floodplain levels within the 54 km long valley section, derived from detailed geomorphic surveys of the selected reaches, it has been estimated that about 9700 ± 1950 t of sediment have been stored on floodplain during the 1986–2009 period. The role of floodplain storage in the overall basin sediment budget and conveyance losses within the main channel system have been evaluated. Copyright © 2012 John Wiley & Sons, Ltd.
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Abstract River avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges, and the associated increase in cross‐valley slope relative to either the down‐channel slope or the down‐valley slope (the latter is termed the slope ratio in the present paper). Therefore, spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion‐dominated models of alluvial architecture. Detailed evidence on centennial‐ to millennial‐scale floodplain deposition has, to date, been largely unavailable. New data on such long‐term overbank aggradation rates from the Rhine–Meuse and Mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto, and can be similar to observations for single overbank floods. This leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios, potentially resulting in increased avulsion frequencies. A revised input parameter for overbank aggradation rate was used in a three‐dimensional model of alluvial architecture to study its effect on avulsion frequency. Realistic patterns of avulsion and interavulsion periods (≈1000 years) were simulated with input data from the Holocene Rhine River, with avulsions occurring when the slope ratio is in the range 3–5. However, caution should be practised with respect to uncritical use of these numbers in different settings. Evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value, irrespective of whether critical slope ratios are used, as in the present study, or superelevation as has been proposed by other investigators.
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River floodplains have been widely recognized as important sinks for storing suspended sediment and associated contaminants transported by river systems. The grain size composition of floodplain deposits exerts an important influence on contaminant concentrations, and commonly exhibits significant spatial variability in response to the dynamic nature of overbank flow and sediment transport. Information on the spatial variability of the grain size composition of overbank deposits is therefore essential for developing an improved understanding of the processes controlling sediment transport on floodplains, and for investigating the fate of sediment-associated contaminants. Such information is also important for validating existing floodplain sedimentation models. This paper reports the results of a study aimed at investigating the spatial variability of the grain size composition of floodplain sediments at different spatial scales, through analysis of surface sediment samples representative of contemporary floodplain deposits collected from frequently inundated floodplain sites on five British lowland rivers. Significant lateral and downstream variations in the grain size composition of the sediment deposits have been identified in the study reaches. An attempt has been made to relate the observed spatial distribution of the grain size composition of the overbank deposits to the local floodplain geometry and topography. The importance of the particle size characteristics of the suspended sediment transported by the rivers in influencing the spatial variability of the grain size composition of the overbank sediments deposited on these floodplains is also considered. © 1998 John Wiley & Sons, Ltd.
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