Experimental study on self‐accelerating turbidity currents

Abstract

A self‐accelerating current is a particle‐driven gravity flow moving on a sloping bottom whose velocity increases in the downstream direction as a result of increasing suspended sediment concentration due to sediment entrainment from the bed. This implies that the net balance between deposition from the current onto the bed and erosion into the flow must be favorable to the latter; thus, a larger mass of particles is being picked up into suspension than is settling out. The self‐accelerative stage cannot continue indefinitely. Either the downstream bed slope drops off to the point where self‐acceleration cannot be maintained or an autosuspensive stage may be reached where the net balance between deposition and erosion is zero and the channel bed is partially or completely free of alluvium. Once such a state is reached on a constant bed slope, the current can persist indefinitely without any external supply of energy other than the potential energy offered by the slope itself. This paper documents experimental turbidity currents composed of lightweight plastic particles ranging from 20 to 200 μm with a specific density between 1.3 and 1.5. These particles were either noncohesive or slightly cohesive. The experiments were performed in a 15‐m long flume with a bottom slope of 0.05. Self‐acceleration of the head of the flow was achieved in some of the tests reported here. Measurements of velocity and suspended sediment taken at different stages of head evolution document this self‐acceleration. In addition, these measurements are in agreement with previous empirical studies relating to head thickness, concentration, velocity, and water depth. Stratigraphic analysis of the deposit shows the key role bed material plays in determining whether a given turbidity current will or will not accelerate. This factor ties the dynamics of a self‐accelerating current to the existence of deposits laid down by antecedent currents. The conditions of the present tests appear to fulfill previous autosuspension criteria relating to flow velocity, particle settling velocity, and bed slope. Densimetric Froude number similarity analysis is used to estimate equivalent parameters for field scale turbidity currents.

The Fluid Mechanics of Pyroclastic Density Currents

“…Finally, there are important physical processes that still need to be incorporated to simulate the full range of pyroclastic density current behaviors. An important example involves the erosion and bulking of currents during transport (Sparks et al 1997) that may have an analogous impact on increasing the runout distance as bulking in debris flows and turbidity currents (Mangeney et al 2010, Parker et al 1987, Sequeiros et al 2009). Advances in 14:27 understanding erosion will likely come from incorporating a range of experiments that examine processes as diverse as turbulent erosion (Sequeiros et al 2009), force chain dynamics (Estep & Dufek 2013), and erosion of the bed by low-pressure regions of the flow due to fluidized transport (Roche et al 2013).…”

Effects of topography on lofting gravity flows: Implications for the deposition of deep-water massive sands

“…It has been demonstrated that lofting gravity currents are stratified in terms of grain size and concentration (Sequeiros et al, 2009). This means that lofting will occur in the upper mixing zone first, where mixing with the ambient fluid is strongest and sediment concentration is lowest (Fig.…”

“…Hence densimetric Froude similarity can be used to scale up laboratory experimental observations to field scale. Here we use the method ofSequeiros et al (2009) (and references therein) to scale up the results of this experimental study. The densimetric Froude Number from the experimental tests can be related to natural flows by: Sediment distribution plots illustrating the differences in deposition between obstructed versus unobstructed lofting dynamics.…”

Field-scale numerical modeling of breaching as a mechanism for generating continuous turbidity currents

“…Sustained turbidity currents need not be produced by suffi cient sediment supply; neither need they be produced by continuous, sustained events. Under the right conditions, even a small turbidity current plume can accelerate and entrain sediment from its bed in a self-reinforcing cycle, so strengthening and elongating (e.g., Sequeiros et al, 2009). A notable case is Scripps Submarine Canyon (offshore California, USA), where the fi eld research of Inman et al (1976) suggested that nearshore sediment plumes suspended near the head of the canyon by storms are responsible for sustained turbidity currents in the canyon.…”