Erodibility of cohesive sediment: The importance of sediment properties

The goal of this paper is to discuss the spatial averaging concept in environmental hydraulics and develop it further by considering transport equations for fluid momentum, passive substances, and suspended sediments. The averaging theorems, the double-averaged (in time and in space) fluid momentum equation, and advection-diffusion equations for a passive substance and suspended sediments are introduced and their limitations and applications for modeling rough-bed flows, experimental design, and data interpretation are discussed. The suggested equations differ from those considered in terrestrial canopy aerodynamics and porous media hydrodynamics by accounting for roughness mobility, change in roughness density in space and time, and particle settling effects for the case of suspended sediments. We show that the form of the double-averaged equations may depend on the type of decomposition of flow variables and that this difference may have important implications for modeling. We also show that the suggested methodology offers better definitions for hydraulic characteristics, variables, and parameters such as flow uniformity, flow two dimensionality, and bed shear stress.

Double-Averaging Concept for Rough-Bed Open-Channel and Overland Flows: Theoretical Background

The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63 μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice-rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006GC001284

Size sorting in marine muds: Processes, pitfalls, and prospects for paleoflow-speed proxies

New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These diagrams can be used to distinguish between turbulent Newtonian, transitional, and laminar non-Newtonian flow behavior, on the basis of the balance between turbulent forces (approximated by the horizontal components of flow velocity and turbulence intensity) and cohesive forces (approximated by the suspended clay concentration and rheology). Stability regimes for five different flow types are defined using a comprehensive series of laboratory flume experiments at depth-averaged flow velocities ranging from 0.13 m s−1 to 1.47 m s−1, and at volumetric kaolinite clay concentrations ranging from 0.03% (= 0.8 g L−1) to 16.7% (= 434 g L−1). As clay concentration increases, five flow types can be distinguished: turbulent flow, turbulence-enhanced transitional flow, lower and upper transitional plug flow, and quasi-laminar plug flow. The turbulent properties of transitional flows are shown to be considerably more complex than the common notion of gradual turbulence damping. Turbulence-enhanced transitional flows display higher turbulence intensity than turbulent flows of similar velocity, with such enhancement originating from development of a highly turbulent basal internal shear layer within ~ 0.01 m of the bed. In lower transitional plug flows, the basal internal shear layer separates a lower region of high vertical gradient in horizontal velocity and strong turbulence from an upper region of plug flow with a much gentler velocity gradient and lower turbulence intensity. Kelvin-Helmholtz shear instabilities within the highly turbulent shear layer are expressed as distinct second-scale oscillations in the time series of downstream velocity. Turbulence damping dominates upper transitional plug flows, because strong cohesive forces, inferred to be caused by gelling of the high-concentration clay suspension, start to outbalance turbulent forces. In quasi-laminar plug flows, gelling is pervasive and turbulence is fully suppressed, apart from some minor residual turbulence near the base of these flows. With very few exceptions, all flows pass through the same development stages as clay concentration increases, regardless of their velocity, but the threshold concentrations for turbulence enhancement, gelling, and development of internal shear layers and plug flows are proportional to flow velocity. At flow velocities below ~ 0.5 m s−1, only low concentrations (< 0.75%) of kaolinite are required to induce transitional flow behavior, thus potentially affecting many slow-moving and decelerating clay flows in natural sedimentary environments. However, at flow velocities above 1 m s−1, clay concentrations of at least 6% are required in order for flows to enter the transitional flow phase, but even at these velocities the transitional flow phases make up a significant proportion of the flow phase space. By converting the experimental data to nondimensional Froude number (momentum term) and Reynolds number (cohesive term), it is shown that each boundary between the turbulent, transitional, and laminar flow phases can be described by a specific narrow range of Reynolds numbers. Within the duration of the experiments, settling of clay particles occurred only in plug flows of low flow velocity (and low Froude number), when the flows lacked the strength to support the entire clay suspension load.

https://eartharxiv.org/repository/object/1472/download/3272/

A Phase Diagram for Turbulent, Transitional, and Laminar Clay Suspension Flows

Modeling a dynamically varying mixed sediment bed with erosion, deposition, bioturbation, consolidation, and armoring

1 Introduction 2 Boundary layer flow 3 The nature of cohesive sediment 4 Flocculation processes 5 Settling and sedimentation 6 Sediment-fluid interaction 7 Self-weight consolidation 8 Mechanical behaviour 9 Erosion and entrainment 10 Biological effects 11 Gas in cohesive sediment Appendix A: Nomenclature Appendix B: Definitions and useful relations Appendix C: Measuring techniques Appendix D: Tensor analysis Appendix E: The 1DV Point Model

https://www.elsevier.com/__data/promis_misc/703315errata.pdf

Introduction to the Physics of Cohesive Sediment in the Marine Environment

3 - The Nature of Cohesive Sediment

2 - Boundary Layer Flow

A subscour soil displacement model is a key component to determine the safe burial depth of offshore pipelines in ice scoured environments. In order to calibrate numerical models and physical tests carried out in geotechnical centrifuges, 3D ice scour tests were carried out in a dredging flume at a scale approaching that of observed scour depths in the Caspian sea. Two soil failure mechanisms were observed. For steep keels, the soil is mainly pushed up into a mound in front of the keel. For shallow angle keels most of the scoured soil is forced under the keel and to the side of the indentor. Each mechanism results in a different subscour displacement pattern. Subscour displacement equations that include the effect of soil properties and keel angle are presented. While these equations are considered an improvement on existing sub-scour models, there remain limitations to their application in 3D sub-scour until further information relating scour depth and width to soil strength becomes available.Copyright © 2008 by ASME

Walther G.M. van Kesteren

Papers

Introduction to the Physics of Cohesive Sediment in the Marine Environment

3 - The Nature of Cohesive Sediment

2 - Boundary Layer Flow

Subscour Displacement in Clays From Physical Model Tests

11 - Gas in Cohesive Sediments