Turbulent Suspension and Sediment Grains Transport in Natural Flows

Sand grain entrainment and suspension in low-concentration flows is generally assumed to be controlled by the magnitude of the basal shear stress and the resulting flow velocity fluctuations. Basal shear stress and velocity fluctuations are key fundamentals of the classic “mixing-length” theories of turbulence developed by Reynolds, Prandtl, von Karman and others. However, concerns have been raised about discrepancies between estimates of basal shear stress obtained from the traditional methods of the Law-of-the-Wall and Reynolds stress decomposition which challenges the continued reliance on the turbulence-suspension theory as it applies to sediment grains transport.

This research uses state-of-the-art, high resolution (in space and time) velocity fluctuations data obtained from a flume tank experiment to re-examine and validate key assumptions of the turbulence-suspension theory and investigate how well the turbulence model support thin long run-out turbidity-current flows transporting sand into Agadir basin, offshore Morocco, Northwest Africa.
In validating the theory, six flow cases characterised by varying flow conditions including flow discharge rate, thicknesses of flow and bed roughness were measured in a clear water flume tank experiment instrumented with a Nortek II ADV flow velocity sampling instrument. A total of 45 flow experimental runs that generated over 30,000 instantaneous flow velocities with depth measurements for each run were undertaken.

Results and data analysis show a reasonable agreement between the measured flume-tank data with that predicted by the turbulence model with respect to the time-averaged velocity profile with depth as well as the estimates of basal shear stress from the Reynolds and Law- of-the-Wall methods. Thus, this widely used mathematical approximation of turbulent suspension remains supported by experimental evidence and can continue to be confidently utilized.

However, reported field observations and inferences of long run-out thin turbidity current flows transporting sand-sized sediments into the Agadir basin could not be replicated by a numerical flow model that is based solely on the turbulence-suspension theory. Instead, the flow model demonstrated that thicker mud-rich turbidity current flows can achieve long distance transport of sand in Agadir basin. Therefore, it is suggested that turbulence and other sediment grain support mechanisms such as hindered settling and grain-grain collisions not incorporated in the flow model may have significantly contributed to the long-distance transport of sand grains by thin turbidity current flows across the Agadir basin.