Turbulent Suspension of Sediments in the Deep Sea

The high-energy benthic boundary layer experiment demonstrated the existence of high energy events capable of suspending large amounts of sediment at the base of the Nova Scotian Rise. The currents that cause these storms are episodic pulses of 25-35 cm s$^{-1}$ flows lasting four to seven days. The build up and decay of the currents is too rapid for local equilibrium of the suspended sediment distribution to be achieved. Therefore, a fully time-dependent model of the turbulent boundary layer and the suspended sediments was developed to describe the events in detail. The period of high flow is erosive for only a few hours. The surface erodible bed sediments are quickly removed. The dominant processes resulting in the development of the suspended sediment profile are then restricted to turbulent diffusion and entrainment. The depth of penetration of the suspended sediments into the water column is limited by stratification induced by suspended sediments. After the shear generated turbulence collapses most of the eroded sediment remained in suspension far above the expected `equilibrium' height for a `non-storm' turbulent boundary layer. Scaling arguments, and the model, show that fine clay particles kept in suspension by turbulent diffusion dominate settling during the low level turbulence present during `non-storm' conditions. Level 2 and 2${\textstyle\frac{1}{2}}$ energy closure models with stratification predict quite different structures of the nepheloid layer.