Monitoring and characterisation of sand-mud sedimentation processes (original) (raw)
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Sand Texture Sedimentology — 60 Years of Research
Abstract Only sand-sized sediments reflect their origin because they form by water flow in the transitional hydrodynamic regime — between the viscous flow (Stokes' law) and turbulent flow (Newton's law). That type of flow modifies strongest the sediment’s granularity (dispersity). Within the transitional range, the laminar layers, getting more energy, gradually separate from the sedimenting particle developing progressively growing turbulence. Particle size is not the genetically effective texture characteristic of sediments and sedimentary rocks. It is the settling rate — (length [distance] in time) that is directly related to the particle transportation and its location in space. Therefore, settling rate, not particle size is to be used as the independent distribution variable — dispersity unit (Rumpf, 1974). To measure the settling rate of sand populations correctly, the particle concentration must not exceed a particle size dependent limit, which is the most demanding measuring requirement. I developed a Sand Sedimentation Analyzer™ fulfilling this requirement. In addition, I developed a Universal Sedimentation Equation that includes Stokes’ law (laminar flow), Newton’s law (turbulent flow) and transitional range between them, which is valid not only for spheres but also for non-spherical particles whose shape is defined by Corey’s Shape Factor SF. Furthermore, I developed a Sand Sedimentation Separator™ enabling to take up to 25 samples from a flow of sedimenting particles. On narrow sieve (monosized) fractions, it operates as a Sedimentation Density Separator, capable of isolating heavy minerals and porous microfossils, e.g. Foraminifera, which are not separable by heavy liquids. A few (maximum of five) Gaussian components of perfectly measured settling rate distributions provide the best definition, classification and interpretation of the sand-sized sediment instead of using the current moment characteristics based on higher distribution derivatives.
Sedimentology, 2011
Flows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi-laminar plug flows. The characteristics of these transitional flows are a function of both clay concentration and type as well as the applied fluid stress. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds, which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow (TF), turbulenceenhanced transitional flow (TETF), lower transitional plug flow (LTPF), upper transitional plug flow (UTPF) and a quasi-laminar plug flow (QLPF). Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in TF, TETF and LTPF. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under TETF and LTPF regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater leeside erosion. As clay concentration increases further from a LTPF, ripples cease to exist under the UTPF and QLPF conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross-stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not to form in cohesive flows with a Reynolds number smaller than ~12,000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain-size segregation and longitudinal flow transformation.
Hindered settling of sand–mud flocs mixtures: From model formulation to numerical validation
Advances in Water Resources, 2013
Mixtures of non-cohesive and cohesive sediment are frequently encountered in natural environments such as estuaries. Depending on the concentration of both species, mixed sediment can record segregation effect or not, behaves like a non-cohesive sediment or like a cohesive sediment. The present study deals with the segregation effect between mud flocs and sand grains during hindered settling. Simulations of this process under various conditions of mixture are proposed by using two coupled mass conservation equations which are solved by a high order numerical model. Specific closure equations are proposed herein for the hindered settling of sand-mud mixed sediment. Comparisons between simulations and experiments are presented on vertical concentration profiles during the segregation process. Obviously, the model enables the description of hindered settling for pure non-cohesive (or pure cohesive) case and the segregation for bi-disperse suspensions.
Detailed simulation of morphodynamics: 2. Sediment pickup, transport, and deposition
Water Resources Research, 2013
1] The paper describes a numerical model for simulating sediment transport with eddyresolving 3-D models. This sediment model consists of four submodels: pickup, transport over the bed, transport in the water column and deposition, all based on a turbulent flow model using large-eddy simulation. The sediment is considered as uniform rigid spherical particles. This is usually a valid assumption for sand-bed rivers where underwater dune formation is most prominent. Under certain shear stress conditions, these particles are picked up from the bed due to an imbalance of gravity and flow forces. They either roll and slide on the bed in a sheet of sediment or separate from the bed and get suspended in the flow. Sooner or later, the suspended particles settle on the bed again. Each of these steps is modeled separately, yielding a physics-based process model for sediment transport, suitable for the simulation of bed morphodynamics. The sediment model is validated with theoretical findings such as the Rouse profile as well as with empirical relations of sediment bed load and suspended load transport. The current model shows good agreement with these theoretical and empirical relations. Moreover, the saltation mechanism is simulated, and the average saltation length, height, and velocity are found to be in good agreement with experimental results.
Observed and predicted bed forms and their effect on suspended sand concentrations
Coastal Engineering, 2004
In this paper, we study the effect of bed forms on suspended sand concentrations and we compare three existing bed form predictors to field and laboratory measurements over a wide range of conditions and propose a new predictor that better collapses the measured bed form dimensions. We apply the different bed form predictors to estimate the suspended sediment concentration distributions based on the 1D advection diffusion equation and the bed-form-enhanced shear stress, which includes the flow contraction effect according to Nielsen [Coast. Eng. 10 (1986) 23]. Predictions of suspended sand concentration improve when including a bed form predictor. Interestingly, it does not make much difference which bed form predictor is implemented but more whether a bed form predictor is implemented. However, the choice of a particular bed form predictor is more important when the bed form effect in the sand transport model is based a roughness length scale k s and thus on g 2 /k instead of g/k, where k s is Nikuradse's equivalent roughness length scale, g is the bed form height and k is the bed form length. This is the case in the Van Rijn [Van Rijn, L.C., 2000. General view on sand transport by currents and waves: data analysis and engineering modelling for uniform and graded sand (TRANSPOR 2000 and CROSMOR 2000 models). Z2899.20 Z2099.30 Z 2824.30, WLjDelft Hydraulics, Delft, The Netherlands] sand transport model, for example. We discuss the effect of so-called long wave ripples on the suspended sand concentrations.