scholarly journals Formation of rhythmic sorted bed forms on the continental shelf: an idealised model

2011 ◽  
Vol 684 ◽  
pp. 475-508 ◽  
Author(s):  
Tomas Van Oyen ◽  
Huib de Swart ◽  
Paolo Blondeaux
Keyword(s):  
Sediment Transport ◽  
Coarse Grained ◽  
Qualitative Comparison ◽  
Fine Sediments ◽  
Bed Elevation ◽  
Steady Current ◽  
Bed Forms ◽  
Current Shift ◽  
Idealised Model ◽  
Sediment Mixture

AbstractAn idealised model is presented to study the formation of sorted bed forms generated by a wind-driven along-shore current. The study employs a linear stability analysis to describe the time development of perturbations of both bottom composition and bed elevation, superimposed on a flat bed composed of a sediment mixture homogeneously distributed in space. The model considers both bed and suspended loads and takes into account the averaged influence of waves on the flow field and the transport of sediment. The results show that the positive coupling between waves, along-shore current and the erodible heterogeneous bed leads to the amplification of two modes, which exhibit distinct characteristics. A first mode is found to be dominant when moderate hydrodynamic conditions are considered and is primarily amplified by the convergence of sediment transport induced by the changes in the bed elevation. This mode has wavelengths of the order of hundred metres and has coarse (fine) sediments in its troughs (crests). By increasing the height of the waves and/or the strength of the steady current, the second mode can become dominant. This mode is characterised by shorter wavelengths and results from the interaction between the convergence of sediment transport related to changes in the bottom composition and that induced by perturbations of the bed elevation. These bed features can have an up-current or a down-current shift between the centre of the coarse-grained bands and the trough of the bottom wave. Typical growth times of the amplified features are of the order of hundreds of days and the migration rates, in the direction of the along-shore current, range between 0.1 and 10 m per day. A qualitative comparison of the model results with field observations indicates that the generation of two distinct modes provides a possible explanation for the broad range of characteristics of the natural bed features.

scholarly journals SURVEY TECHNIQUES/PROCEDURES AND DATA PROCESSING FOR MONITORING NEARSHORE SEDIMENT TRANSPORT

1982 ◽  
Vol 1 (18) ◽  
pp. 97
Author(s):  
J. Zacks
Keyword(s):  
Sediment Transport ◽  
Surf Zone ◽  
Survey Techniques ◽  
The Past ◽  
Bed Elevation ◽  
Innovative Methods ◽  
Survey Error ◽  
Nearshore Sediment Transport ◽  
The Many ◽  
The Cost

The cost of many coastal projects is often increased by the expensive beach repair and maintenance required to remedy the destabilising effects of structures on the adjoining coastline. Physical and/or mathematical models have been developed for use in planning these projects in order to predict and quantify the effects of marine sediment transport on the coastal topography. Such models need to be calibrated against prototype data and one method of gauging volumetric sediment movement is by successive bathymetric/ topographic profiting surveys which are performed seasonally and annually. Since large quantities of sediment are related to small changes in bed elevation it is clear that this profiling needs to be done with the utmost precision* The areas most affected extend from the beach through the surf zone to water depths of about 25 metres. The surf zone in particular is a dynamic and hostile area which falls outside the traditional activities of both the hydrographic and land surveyors. Consequently innovative methods, deficient in sound survey principle and practice, have often been pursued in this area without any attempt being made to assess the tolerance on the data. This paper attempts to show that it is possible to produce reliable and verifiable results to the required accuracy by using conventional survey equipment and techniques, also by taking the necessary precautions against the many possible sources of survey error. The procedures and techniques described have evolved from NRIO's involvement over the past decade in major projects at Richards Bay, Durban, Koeberg and in False Bay. The results of a recent verification investigation are fully reported in this paper.

Influence of the Cross-Section on the Erosion Criteria for Partly Cohesive Sediments

1994 ◽  
Vol 29 (1-2) ◽  
pp. 103-111 ◽  
Author(s):  
Hilde Torfs ◽  
Marc Huygens ◽  
Limbaya Tito
Keyword(s):  
Sediment Transport ◽  
Cross Section ◽  
Cross Sections ◽  
Cohesive Sediments ◽  
Material Content ◽  
Rectangular Channels ◽  
Side Walls ◽  
Mixed Sediments ◽  
Sediment Mixture ◽  
Cohesive Material

Erosion of sediment mixtures, comprising cohesive as well as non-cohesive sediments, has been studied in straight laboratory flumes of circular and rectangular cross-sections. Erosion of mixed sediments depended on the mixture composition (cohesive material content), the type of cohesive sediment, and the shape of the flume cross-section. By increasing the cohesive material content, the erosion resistance of the sediment mixture also increased and the erosion pattern changed from ripples and dunes (noncohesive) to a groove or wavy surface (cohesive). In circular channels, the influence of the “side” walls on sediment transport is important, but the criteria for incipient motion seem to be the same as in the rectangular channels. Hence, sediment transport models developed for rectangular channels cannot be readily applied to circular channels.

scholarly journals Clast imbrications in coarse-grained sediments suggest changes from upper to lower flow regime conditions

2018 ◽  
Author(s):  
Fritz Schlunegger ◽  
Philippos Garefalakis
Keyword(s):  
Sediment Transport ◽  
Flow Regime ◽  
Shear Force ◽  
Foreland Basin ◽  
Coarse Grained ◽  
Late Oligocene ◽  
Incipient Motion ◽  
Material Transport ◽  
Bed Roughness ◽  
Large Flow

Abstract. Clast imbrications are presumably the most conspicuous sedimentary structures in coarse-grained clastic deposits. In this paper, we test whether the formation of such a fabric is related to changes from lower to upper flow regime conditions in streams. To this extent, we calculate the Froude number at the incipient motion of coarse-grained bedload for various values of relative bed roughness and stream gradient. We then compare the results with data from modern streams and stratigraphic records. The calculations show that upper flow regime conditions most likely establish where average stream gradients exceed c. 0.5 ± 0.1°, and where relative bed roughness values are larger than ∼ 0.06 ± 0.01. Similarly, data from modern streams reveal that imbricated clasts are found where channels are steeper than c. 0.5 ± 0.2°, and where relative bed roughness values exceed ∼ 0.07. Likewise, imbricated conglomerates are encountered in late Oligocene foreland basin sequences where paleo-slopes were greater than 0.4°. We use these relationships to propose that clast imbrications occur where channel gradients exceed a threshold, which appears large enough for upper flow regime conditions to establish. We finally relate the formation of an imbricated arrangement of clasts to a mechanism where material transport occurs through rolling, or pivoting. This process requires a large shear force and thus a large flow velocity upon transport, which is likely to be associated with shifts from the lower to the upper flow regime. Our results thus suggest that clast imbrications are suitable recorders of upper flow regime conditions upon sediment transport.

Review of Research Related to Sediment Disaster Mitigation

2007 ◽  
Vol 2 (1) ◽  
pp. 11-18 ◽  
Author(s):  
Shinji Egashira ◽  
Keyword(s):  
Flow Characteristics ◽  
Disaster Mitigation ◽  
Annual Number ◽  
Erosion And Deposition ◽  
Sediment Erosion ◽  
Bed Elevation ◽  
Numerical Predictions ◽  
Sediment Research ◽  
Sediment Mixture ◽  
Structural Methods

The annual number of deaths resulting from landslides and debris flow in Japan over the past 40 years has shown a decreasing tendency as corresponding countermeasures are implemented. The countermeasures constituted of structural and non-structural methods are introduced, focusing on their phenomenological function to clear up roles of sediment research on them. In addition, governing equations which are employed currently in numerical predictions are shown, including formulas for bed shear stress and sediment erosion and deposition in order to illustrate differences between the flow of water and sediment mixture in steep areas and the flow of usually observed floods in rivers. It is recommended that equations which are able to distinguish rapid changes of flow characteristics due to internal solid type friction, sediment erosion and deposition and the corresponding changes of bed elevation should be applied in the numerical computations.

scholarly journals An Alternative Algorithm for Simulating Flash Flood

2021 ◽  
Vol 930 (1) ◽  
pp. 012076
Author(s):  
M F Khaldirian ◽  
A P Rahardjo ◽  
D Luknanto ◽  
R D R Sondi
Keyword(s):  
Numerical Modeling ◽  
Debris Flow ◽  
Coordinate System ◽  
Flash Flood ◽  
Front Propagation ◽  
Bed Elevation ◽  
Forward Difference ◽  
Lagrangian Coordinate ◽  
The Galerkin Method ◽  
Sediment Mixture

Abstract Most of the approaches in numerical modeling techniques are based on the Eulerian coordinate system. This approach faces difficulty in simulating flash flood front propagation. This paper shows an alternative method that implements a numerical modeling technique based on the Lagrangian coordinate system to simulate the water of debris flow. As for the interaction with the riverbed, the simulation uses an Eulerian coordinate system. The method uses the conservative and momentum equations of water and sediment mixture in the Lagrangian form. Source terms represent deposition and erosion. The riverbed in the Eulerian coordinate system interacts with the flow of the mixture. At every step, the algorithm evaluates the relative position of moving nodes of the flow part to the fixed nodes of the riverbed. Computation of advancing velocity and depth uses the riverbed elevation, slope data, and the bed elevation change computation uses the erosion or deposition data of the flow on the moving nodes. Spatial discretization is implementing the Galerkin method. Furthermore, temporal discretization is implementing the forward difference scheme. Test runs show that the algorithm can simulate downward, upward, and reflected backward 1-D flow cases. Two-D model tests and comparisons with SIMLAR software show that the algorithm works in simulating debris flow.

scholarly journals A New Framework for Modelling Fine Sediment Transport in Rivers Includes Flocculation to Inform Reservoir Management in Wildfire Impacted Watersheds

Water ◽  
2021 ◽  
Vol 13 (17) ◽  
pp. 2319
Author(s):  
Micheal Stone ◽  
Bommanna G. Krishnappan ◽  
Uldis Silins ◽  
Monica B. Emelko ◽  
Chris H. S. Williams ◽  
...  
Keyword(s):  
Particle Size ◽  
Sediment Transport ◽  
Water Resources ◽  
River Sediment ◽  
Numerical Models ◽  
Cohesive Sediment ◽  
Fine Sediment ◽  
Coarse Grained ◽  
Size Distributions ◽  
Particle Size Distributions

Fine-grained cohesive sediment is the primary vector for nutrient and contaminant redistribution through aquatic systems and is a critical indicator of land disturbance. A critical limitation of most existing sediment transport models is that they assume that the transport characteristics of fine sediment can be described using the same approaches that are used for coarse-grained non-cohesive sediment, thereby ignoring the tendency of fine sediment to flocculate. Here, a modelling framework to simulate flow and fine sediment transport in the Crowsnest River, the Castle River, the Oldman River and the Oldman Reservoir after the 2003 Lost Creek wildfire in Alberta, Canada was developed and validated. It is the first to include explicit description of fine sediment deposition/erosion processes as a function of bed shear stress and the flocculation process. This framework integrates four existing numerical models: MOBED, RIVFLOC, RMA2 and RMA4 using river geometry, flow, fine suspended sediment characteristics and bathymetry data. Sediment concentration and particle size distributions computed by RIVFLOC were used as the upstream boundary condition for the reservoir dispersion model RMA4. The predicted particle size distributions and mass of fine river sediment deposited within various sections of the reservoir indicate that most of the fine sediment generated by the upstream disturbance deposits in the reservoir. Deposition patterns of sediment from wildfire-impacted landscapes were different than those from unburned landscapes because of differences in settling behaviour. These differences may lead to zones of relatively increased internal loading of phosphorus to reservoir water columns, thereby increasing the potential for algae proliferation. In light of the growing threats to water resources globally from wildfire, the generic framework described herein can be used to model propagation of fine river sediment and associated nutrients or contaminants to reservoirs under different flow conditions and land use scenarios. The framework is thereby a valuable tool to support decision making for water resources management and catchment planning.

scholarly journals A 2-D hydro-morphodynamic modelling approach for predicting suspended sediment propagation and related heavy metal contamination in floodplain: a sensitivity analysis

2014 ◽  
Vol 11 (2) ◽  
pp. 1741-1776 ◽  
Author(s):  
R. Hostache ◽  
C. Hissler ◽  
P. Matgen ◽  
C. Guignard ◽  
P. Bates
Keyword(s):  
Sensitivity Analysis ◽  
Suspended Sediment ◽  
Heavy Metal Contamination ◽  
Qualitative Comparison ◽  
Fine Sediments ◽  
Morphodynamic Modelling ◽  
Transport And Diffusion ◽  
Set Up ◽  
Informatic System ◽  
Semi Empirical

Abstract. Fine sediments represent an important vector of pollutant diffusion in rivers. When deposited in floodplains and riverbeds they can be responsible for soil pollution. In this context, this paper proposes a hydro-morphodynamic modelling exercise aiming at predicting transport and diffusion of fine sediments and dissolved pollutants. The model is based upon the Telemac hydro-informatic system (dynamical coupling Telemac-2D-Sysiphe). As empirical and semi-empirical parameters need to be calibrated for such a modelling exercise, a sensitivity analysis is proposed. In parallel to the modelling exercise, an extensive hydrological/geochemical database has been set up during two flood events. The main sensitive parameters were found to be the hydraulic friction coefficient and the sediment particle settling velocity in water. Using the two monitored hydrological events as calibration and validation, it was found that the model is able to satisfyingly predict suspended sediment and dissolve pollutant transport in the river channel. In addition, a qualitative comparison between simulated sediment deposition in the floodplain and a soil contamination map shows that the preferential zones for deposition identified by the model are realistic.

scholarly journals Development of an Interdisciplinary Prediction System Combining Sediment Transport Simulation and Ensemble Method

Water ◽  
2021 ◽  
Vol 13 (18) ◽  
pp. 2588
Author(s):  
Hao-Che Ho ◽  
Yen-Ming Chiang ◽  
Che-Chi Lin ◽  
Hong-Yuan Lee ◽  
Cheng-Chia Huang
Keyword(s):  
Sediment Transport ◽  
Numerical Modelling ◽  
Ensemble Method ◽  
River Morphology ◽  
River Channels ◽  
Ensemble Size ◽  
Transport Simulation ◽  
Bed Elevation ◽  
The Ensemble Method ◽  
Simulation Time

The change in movable beds is related to the mechanisms of sediment transport and hydrodynamics. Numerical modelling with empirical equations and the simplified momentum equation is the common means to analyze the complicated sediment transport processing in river channels. The optimization of parameters is essential to obtain the proper results. Inadequate parameters would cause errors during the simulation process and accumulate the errors with long-time simulation. The optimized parameter combination for numerical modelling, however, is rarely discussed. This study adopted the ensemble method to simulate the change in the river channel, with a single model combined with multiple parameters. The optimized parameter combinations for a given river reach are investigated. Two river basins, located in Taiwan, were used as study cases, to simulate river morphology through the SRH-2D, which was developed by the U.S. Bureau of Reclamation. The input parameters related to the sediment transport module were randomly selected within a reasonable range. The parameter sets with proper results were selected as ensemble members. The concentration of sedimentation and bathymetry elevation was used to conduct the calibration. Both study cases show that 20 ensemble members were good enough to capture the results and save simulation time. However, when the ensemble members increased to 100, there was no significant improvement, but a longer simulation time. The result showed that the peak concentration and the occurrence of time could be predicted by the ensemble size of 20. Moreover, with consideration of the bed elevation as the target, the result showed that this method could quantitatively simulate the change in bed elevation. With both cases, this study showed that the ensemble method is a suitable approach for river morphology numerical modelling. The ensemble size of 20 can effectively obtain the result and reduce the uncertainty for sediment transport simulation.

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