Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.

Subaqueous sediment density flows: Depositional processes and deposit types

http://www.vliz.be/imisdocs/publications/ocrd/267179.pdf

Erosion of mud/sand mixtures

Erodibility of cohesive sediment: The importance of sediment properties

Sediment erodability in sediment transport modelling: Can we account for biota effects?

The erosion behaviour of sand–mud mixtures is not well understood. As a first step, a conceptual framework for the erosion behaviour of sand–mud mixtures is proposed and validated with data from field and laboratory experiments based on appropriate parameters for cohesion and network structure. Six bed types with different characteristics are distinguished in relational diagrams. Validation with experimental data shows that observed transitions in erosion behaviour are clearly delimited in these diagrams. Mud content (%

A conceptual framework for the erosion behaviour of sand-mud mixtures

Settling and consolidation of mud/sand mixtures

Modelling (partly) cohesive sediment transport in sewer systems

In a laboratory flume the formation and erosion of stratified sand-mud beds in uniform flow was studied. The experiments showed that depending on the initial conditions of suspended sediment concentration and mixture composition, consecutive inputs of a mud/sand suspension will lead to the formation of a layered sediment bed. Peaks in the measured density profiles show the different layers and indicate a possible segregation of the sand due to differential settling. During the erosion of the stratified bed, the sediments are eroded layer by layer. This results in a sequence of suspended load (fines) and bed load (sand) transport phases.

/pdf/erosion-of-layered-sand-mud-beds-in-uniform-flow-2l14vk2co8.pdf

Erosion of layered sand-mud beds in uniform flow

The formation of a cohesive sediment bed is a combination of settling and consolidation processes These processes strongly influence the structure, properties and erodibility of the sediment bed At low values of the bed shear stress, suspended sediments, individual particles or flocs, deposit onto the bed Subsequently, during the consolidation process, the flocs and aggregates rearrange themselves to form a denser structure The pore water, initially supporting the particles, is being expelled In an extensive set of laboratory experiments the influence of sand on the settling and the consolidation of mud has been studied The deposition of mud/sand mixtures has been closely followed in settling column experiments for different types of mud at different sand contents During the experiments settling rates, density profiles and pore water pressures have been measured The presence of a sand fraction in the initial suspension has a large impact on the bed formation processes The results show that the heavier sand particles settle faster and form a separate layer at the bottom of the column as long as the mud does not form a continuous network structure that prevents this segregation A continuous mud matrix is formed at low sediment supply rates or at high initial suspension concentrations, when the concentration of the mud fraction in the mixture exceeds the gel point density The settling rates of the mud/sand suspension increase with increasing sand content The local bed densities within the consolidating bed increase with increasing sand content of the initial suspension, even for the sand-free top layer Sand addition also speeds up the consolidation process and permeability increases However, these effects seem to be limited to a maximum sand content above which no additional effect is found Using the experimental results, guidelines for modelling the settling and consolidation of mud/sand mixtures have been formulated

Hilde Torfs

Papers

Erosion of mud/sand mixtures

Settling and consolidation of mud/sand mixtures

Modelling (partly) cohesive sediment transport in sewer systems

Erosion of layered sand-mud beds in uniform flow

Settling and consolidation of mud/sand mixtures