Depositional processes, bedform development and hybrid bed formation in rapidly decelerated cohesive (mud–sand) sediment flows

Abstract

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 known to be a function of both clay concentration and type, as well as the applied fluid stress, but so far the interaction of these transitional flows with a loose sediment bed has received little attention. Information on this type of interaction is essential for the recognition and prediction of sedimentary structures formed by cohesive transitional flows in, for example, fluvial, estuarine and deep‐marine deposits. 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, turbulence‐enhanced transitional flow, lower transitional plug flow, upper transitional plug flow and a quasi‐laminar plug flow. 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 turbulent flow, turbulence‐enhanced transitional flow and lower transitional plug flow. 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 turbulence‐enhanced transitional flow and lower transitional plug‐flow regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater lee side erosion. As clay concentration increases further from a lower transitional plug flow, ripples cease to exist under the upper transitional plug flow and quasi‐laminar plug flow 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 ref...

Short length-scale variability of hybrid event beds and its applied significance

“…In the Gottero case, the thicker event beds (>2 m) also have thick associated mudstone caps suggesting these flows carried significant clay loads, but these were deposited after the emplacement of the H3 divisions, possibly from ponded suspensions given that they are of similar thickness to the sandy lower part of the bed. The decelerating experimental flows examined by Baas et al (2011) were unable to separate clay and sand once the concentration of clay exceeded 12%, emplacing thick fluid mud layers (see also Sumner et al, 2009); the sandy mudstones that might be expected were clay-rich flows to have arrested in this manner are not seen in this study.…”

Disconnected submarine lobes as a record of stepped slope evolution over multiple sea-level cycles

“…Hybrid event beds (Haughton et al, 2003(Haughton et al, , 2009) are most commonly identified toward the bases and fringes of lobe deposits (e.g., Hodgson, 2009;Talling, 2013). However, they can form in any environment where mud and mudstone clasts are entrained into the turbulent flow, damping turbulence, and developing high-concentration to pseudo-laminar flow conditions (e.g., Ito, 2008;Haughton et al, 2003;Talling et al, 2004;Baas et al, 2011;Pierce et al, 2018).…”

Bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions

“…Therefore, van Rijn (2006) replaced the factor 0.5 in Eq. (3) with a bedform shape factor, f, which has been shown to be approximately 0.6 for current ripples and dunes (van den Berg, 1987;Hoekstra et al, 2004;Baas et al, 2011). Eq.…”