Erosion of Layered Sand-Mud Beds in Uniform Flow

Torfs, Hilde

doi:10.1061/9780784400890.243

Cited by 5 publications

(7 citation statements)

References 2 publications

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“…This is very similar to previous mixedsediment research conducted by Torfs (1994), Torfs et al (1996) and Mitchener et al (1996), who used combinations of both naturally mixed sediments and prescribed mixtures. This paper also draws on a natural mixed sediment from Portsmouth Harbour.…”

Section: Potential Data and Algorithm Limitationssupporting

confidence: 83%

“…Laboratory simulations have been previously used to examine many aspects of mud:sand behaviour (e.g. Ockenden and Delo 1988;Williamson and Ockenden 1993;Torfs 1994;Torfs et al 1996;Dankers et al 2007). The laboratory flume study ) examined the flocculation/aggregation dynamics for three different mud:sand (M:S) mixtures of the following ratios: 75:25 (Run A), 50:50 (Run B), and 25:75 (Run C).…”

Section: Overview Of Primary Experimentsmentioning

confidence: 99%

“…When sand is added to a predominantly muddy matrix, Mitchener et al (1996) found that this increased the binding potential between the clay particles, for example the subtidal mud patches off Sellafield in the Irish Sea (Feates and Mitchener 1998). Thus, the physical effect of adding cohesive mud to a sandy environment can create increased bed stability (Kamphuis and Hall 1983;Alvarez-Herandez 1990;Williamson and Ockenden 1993;Torfs 1994;Mitchener et al 1996;and Panagiotopoulus et al 1997), which can potentially lead to mixed-sediment flocs forming when the eroded bed is entrained.…”

Section: Introductionmentioning

confidence: 97%

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The settling dynamics of flocculating mud-sand mixtures: Part 1—Empirical algorithm development

et al. 2011

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European estuaries tend to be regarded as being either predominantly muddy or sandy. In some estuaries, the cohesive and non-cohesive fractions can become segregated. However, recent laboratory tests have revealed that mud and sand from many coastal locations can exhibit some degree of flocculation. A clear understanding of the dynamic behaviour of sediments in the nearshore region is of particular importance for estuarine management groups who want to be able to accurately predict the transportation routes and fate of the suspended sediments. To achieve this goal, numerical computer simulations are usually the chosen tools. In order to use these models with any degree of confidence, the user must be able to provide the model with a reasonable mathematical description of spatial and temporal mass settling fluxes. However, the majority of flocculation models represent purely muddy suspensions. This paper assesses the settling characteristics of flocculating mixed-sediment suspensions through the synthesis of data, which was presented as a series of algorithms. Collectively, the algorithms were referred to as the mixedsediment settling velocity (MSSV) empirical model and could estimate the mass settling flux of mixed suspensions. The MSSV was based entirely on the settling and mass distribution patterns demonstrated by experimental observations, as opposed to pure physical theory. The selection of the algorithm structure was based on the concept of macroflocs-the larger aggregate structures-and smaller microflocs, representing constituent particles of the macroflocs. The floc data was generated using annular flume simulations and the floc properties measured using the video-based LabSFLOC instrumentation. The derived algorithms are valid for suspended sediment concentrations and turbulent shear stress values ranging between 0.2-5 g l −1 and 0.06-0.9 Pa, respectively. However, the MSSV algorithms were principally derived using manufactured mixtures of Tamar Estuary mud and a fine silica sand, which means that the algorithms presented are site-specific in nature, and not fully universal in application. In terms of mass settling flux (MSF) accuracy: at the lower flux range (195-777 mg m −2 s −1 ) most MSSV predictions were within a few percent of the observations, whilst for the largest observed MSFs (1.3-21 g m −2 s −1 ), the MSSV demonstrated a close fit with the data. Even for the highest observed MSF of 33 g m −2 s −1 (produced by a 75M:25S mixed suspension), the MSSV only under-estimated the flux by 18%. The MSSV algorithms indicated a trend whereby a rise in sand content, and a subsequent decrease in mud, favours the microflocs as the dominant flux contributor. Parameter comparison testing indicated that by applying a singlesediment assumption to a mixed-sediment environment, pure mud algorithms under-predicted at each concentration by as much as 25% and did not handle sandy mud sediments particularly well. Slow constant settling velocity (0.5 and 1 mm s −1 ) parameters severely under-predicted MSF (at times dow...

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Section: Potential Data and Algorithm Limitationssupporting

confidence: 83%

Section: Overview Of Primary Experimentsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

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The settling dynamics of flocculating mud-sand mixtures: Part 1—Empirical algorithm development

et al. 2011

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“…However, the increase in s ce is lower when sand grains are added to mud than when mud particles are added to a sandy bottom (Mitchener & Torfs, 1996) The sedimentation processes within mixed sand/mud beds remain poorly understood. Laboratory tests have been performed to observe the simultaneous deposition of mud and sand (Torfs (1994b); Torfs et al (1996); Ockenden & Delo (1988)). If all the sand and mud settle simultaneously over a short period, two well-sorted layers are formed: sand grains pass through the non-consolidated mud and a sandy layer is formed below the muddy one.…”

Section: Erodability and Sedimentation Of Sand/mud Mixturesmentioning

confidence: 99%

Modelling sand/mud transport and morphodynamics in the Seine river mouth (France): an attempt using a process-based approach

et al. 2007

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The mouth of the Seine River estuary (France) has undergone marked morphological evolution over several decades mainly due to engineering works aimed at improving access to Rouen and Le Havre harbours. The intertidal areas are decreasing in size and the lower estuary is accumulating sediment and prograding. In order to understand and better describe the major morphological behaviours of the estuary, a morphodynamic numerical model was developed within the Seine-Aval program. At the end of the 1st part of the research program, a validated fine sediment transport model (3D) was available (Le Hir et al., 2001b). As the present morphological study addresses medium-term issues (a few decades), and because of the need to investigate impacts of local structures or events, we chose to use the so-called ''process-based approach'' starting from the existing model. First, the existing model was upgraded to account for (suspended) sand transport, and to achieve coupling between morphological changes and sediment transport. Erodability of the sediment accounts for the respective proportions of mud and sand. Simulations starting from an arbitrary surficial sediment cover show that the model is able to reproduce realistic sediment patterns. For example, it is able to change the sediment nature on the intertidal flat near Le Havre from sand to mud. Observed structures of suspended sediment are also reproduced: fine particles mainly follow the turbidity maximum whereas significant concentrations of sand grains in suspension are found where the hydrodynamic stresses are intense. Concerning morphodynamics, simulations with real forcing over one year are discussed. The effect of waves on the bathymetric evolution of the mouth is shown and the sensitivity of morphodynamics to the coupling procedure is tested.

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“…When sand is added to a predominantly muddy matrix, Mitchener et al (1996) found that this increased the binding potential between the clay particles, for example as found in the subtidal mud patches off Sellafield in the Irish Sea (Feates and Mitchener, 1998). Thus the physical effect of adding cohesive mud to a sandy environment can create increased bed stability, which can potentially lead to mixed sediment flocs forming when the eroded bed is entrained (Kamphuis and Hall, 1983; Alvarez-Hernandez, 1990; Williamson and Ockenden, 1993;Torfs, 1994;Mitchener et al, 1996;and Panagiotopoulus et al, 1997). Even where sand and mud are considered to be fairly well segregated at the bed, sand and mud can co-exist in suspended sediment transport.…”

Section: Segregation and Flocculationmentioning

confidence: 99%