The effect of sediment concentration on bedload roughness
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Abstract:
Bedload roughness is the roughness produced by sediment transported near the bed. While it is well established that the magnitude of this roughness is proportional to the thickness of the moving sediment layer, the effect of sediment concentration remains largely unknown. This paper presents the results of a flume experiment that was designed to investigate the effect of sediment concentration on bedload roughness. The experiment consisted of creating flow conditions where bedload transport is supply-limited and injecting gravel size particles (D50=7·4 mm) at the upstream end of the flume in order to produce bedload transport. While keeping flow conditions constant, sediment concentration was varied by successively increasing the injection rate of gravel in the flow. For each injection rate, flow velocity profiles were measured in order to evaluate changes of mean flow velocity U, shear velocity u*, roughness length zo and resistance to flow f. The results indicate that low sediment concentration affects mainly the near bed portion of the flow where it causes a reduction of mean flow velocity and an increase of shear velocity and roughness length. For larger sediment concentration, the whole flow velocity profile becomes affected by bedload roughness but the importance of this effect always remain larger near the bed. As sediment concentration is augmented, mean flow velocity is consistently reduced but shear velocity and roughness length increase until a plateau is reached where these two variables become constant. Copyright © 1999 John Wiley & Sons, Ltd.Keywords:
Flume
Hyperconcentrated flow
Hydraulic roughness
Bedform
Shear velocity
Flow conditions
Diverse bedforms are observed in river beds. Pools and riffles occur in the bed due to the nonuniformity of flow, cross-section and the straight path of rivers and natural channels. From a hydraulic point of view, the variation in roughness is due to variations of flow depth and flow cross-section in pools and riffles, leading to energy losses and local erosion. Vegetated banks have a significant effect on the sustainability of banks and flow resistance, increasing the roughness coefficient in banks and floodplain. Due to the adjustment of environmental conditions and energy loss caused by vegetated banks and bedforms, their interaction plays a significant role in river restoration projects. Consequently, it is necessary to investigate the interactions of bedforms, flow, and vegetation on the stability and fluvial processes. The present study was conducted in Plusjan River in the central Iran to investigate the influence of vegetated banks, and accelerating and decelerating flows (non-uniform flows) on turbulent flow characteristics in the gravel-bed river. Results show that interactions between vegetated banks, flow nonuniformity, and bedforms lead to irregular patterns of velocity, Reynolds stress, and turbulence intensity distributions.
Bedform
Hydraulic roughness
Hydraulics
Flow conditions
Reynolds stress
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Bedform
Shear velocity
Acoustic Doppler velocimetry
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In this study, the large eddy simulation (LES) under the Eulerian method is used to solve the Navier-Stokes equations for turbulent flow simulation. The Lagrangian point-particle model is applied to track particle trajectories and to calculate the forces exerted by the flow on the particles, and the particle–wall and particle–particle collisions are also accounted for. Nine simulations cases were carried out along the line of previous experiments that considered different bedform regimes, namely, ripples and dunes. The resulting bedload intensity parameter and the simulated bedforms for all the cases agree with the results obtained from the existing classical formulas. The three-dimensionality of sediment transport randomly occurs due to the turbulent flow. Coherent structures are formed as the near-bed low-speed fluid streaks entrain into the mainstream over the stoss-side of the ripples, and the high-speed fluid streaks from the mainstream rush toward the bed over the leeside. As a result, kolk–boil and hairpin vortices develop nearby. Ejection and sweep prevail near the bed, where the particles transport. The phenomenon disappears as the flow intensity increases. The presence of bedload particles also modifies the propagation angle and range of velocity fluctuation, especially in the streamwise direction. To conclude, a logistic regression formula for bedload intensity parameters, accounting for the fluid rotation, deformation, and translation terms that signify the fluid vortical motions, is obtained. It reveals that as long as these three terms are accurately quantified, the bed shear stress and bedload transport rate can be effectively estimated.
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Hyperconcentrated flow
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The flow resistance in terms of the friction factor for bank-full or high in-bank flows for 120 high-gradient gravel-bed rivers are examined. The friction factor f was divided into grain and form components and it was shown that the flow resistance due to the grain roughness can be readily calculated by assuming k s = D 50 . The flow resistance due to bars, bedforms, and other channel irregularities can be large in gravel rivers even at bank-full flow.
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Hydraulic roughness
Flow conditions
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Hyperconcentrated flow
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To maintain the function of rivers such as irrigation, drinking water supply, navigation and to protect against flooding, it is of great importance to gain more insights in the behaviour of rivers. An element of central interest in the behaviour of rivers are bedforms, especially dunes on the bottom of these rivers. The flow in rivers which generally flows in one direction, results in an asymmetrical dune with a stoss and lee side. Due to flow separation and associated energy dissipation, dunes form the main source for hydraulic roughness on the riverbed. The roughness in turn, is a key element in predicting flow conditions and corresponding water levels. The generally non-uniform unsteady flow in rivers causes occurrence of different types of bedforms with varying hydraulic roughness. This research only considered ripples, dunes, washed-out dunes and upper stage plane bed bedform types.
In particular, water levels may increase due to increasing hydraulic roughness associated with rapid growth of dunes during high river discharge. However, due to increasing flow intensities, dunes may also evolve towards upper stage plane beds. In this case, water levels will decrease due to a decrease in hydraulic roughness associated with the transition of dunes to upper stage plane beds (Naqshband, 2014). Previous studies from Karim (1999), Paarlberg et al. (2007), Van der Mark (2009), Van Rijn (1984) and Yalin (1964) demonstrated the direct relationship between the hydraulic roughness due to the presence of bedforms and the bedform height
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Hydraulic jump
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Research on bedforms, especially river dunes with focus on the Dutch river Waal, and the effect of these bedforms on the hydraulic roughness of the river. Morphological descriptions, prototype measurements and flume experiments. PhD thesis Utrecht University.
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Flume
Hydraulic roughness
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River dunes are important bedforms. Problems associated with the development and evolution of dune bedforms include increased flood risks, channel erosion, and damage to fish habitats. The purpose of this paper is to investigate the near-bed flow structure and bedload transport along gavel dune-like bedforms. The velocity field is computed using a relatively simple multi-layer hydrodynamic model, with a parameterization of flow separation in the leeside of dunes. The computation is of high efficiency and avoids uncertainties caused by flow separation. Fractional transport rates for a sediment mixture of sands and gravel are calculated using surface-based techniques. The computed flow velocities and bed shear stresses are in good comparison with acoustic Doppler velocimeter measurements. Bedload transport is shown to increase non-linearly with distance toward the dune crest and reach the maximum at the crest. This implies that dune-length averaged bed shear stress is not suitable for bedload calculations. At low discharges, the bed shear stress is the limiting factor, resulting in insignificant bedload. At high discharges when the bed shear stress exceeds a threshold, the effect of sediment-grain hiding and sediment-size availability are important for bedload calculations. The discharge–transport relationship is highly non-linear. This paper has demonstrated selective transport and potential dune surface coarsening. The simplified modelling approach has a good potential for application to field conditions.
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Hyperconcentrated flow
Crest
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This paper presents a study on the relationship between supply-limited bedform formation and the hydraulic roughness of the riverbed. The results of several new sets of flume experiments with supply-limited or partial transport conditions with bimodal sediment are presented. The results show that both the bedform and grain-related roughness show a clear dependency on the degree of supply limitation. This variation can be attributed to the reduction of the bedform dimensions with a decreasing availability of mobile sediment and an increasing exposure of the usually coarser sublayer (armour layer or pavement) that causes the supply limitation. In the present study, we present a new method to predict the exposure of the immobile sublayer, as well as the bedform and grain-related roughness under supply-limited conditions.
Bedform
Flume
Hydraulic roughness
Deposition
Flow conditions
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Bed roughness influences the hydrodynamic behaviour of the river, for example, elevating water levels for a constant flow discharge. In most hydrodynamic river models, the roughness is assumed as a function of the grain size. However, natural riverbeds are characterized by a wide variety of bedforms at different scales, from the small-to the largescale bedforms. Hence, bed roughness should be considered as a complex expression of topographical variability of a bed surface at multiple scales rather than being simply related to the grain size. Neglecting these important aspects may lead to high errors in the river water level results, inducing dramatic consequences on the flood risk and land use planning. To explore the bed roughness, an in-depth statistical analysis is applied, in this study, to three different surfaces simulated in an experimental laboratory flume. In fact, if the description of bed roughness structures is limited to second-order statistics, then some important information may remain unidentified. Higher-order structure functions are computed to provide information on the bed roughness structures and on their orientation and multifractal characteristics.
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Flume
Hydraulic roughness
Multifractal system
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