iii. Near-bottom velocity, shear, and suspended particle concentrations 



Near-bottom flow and shear is the physical cause of transport and erosion of sediment 

 In boundary layer flow, the velocity decreases from the free-stream value far above the 

 boundary to zero at the surface. The shear in velocity generates turbulence which in turn 

 transports momentum to the boundary; viscosity is not a significant effect in rough boundary 

 layers. The stress which the flow exerts on the sediment can be estimated from velocity 

 profiles, from correlation of velocity fluctuations (Reynolds stress), or from turbulent kinetic 

 energy. In wave-dominated flows, flows where the velocity reverses or approaches zero each 

 half wave period, the velocity profile is the most reliable estimate of bottom stress and the 

 spectrum of turbulent kinetic energy is a measure of wave energy and stress. 



When the stress exceeds the shear strength of the sediment, material is eroded. 

 Conversely, if turbulence levels are high, material in suspension will not be redeposited. 

 When material is suspended, the mean flow transports it. The flux of sediment is the vertical 

 integral of the vector velocity at each height times the sediment concentration. Since both the 

 velocity and sediment concentration vary with height, it is necessary to measure both at 

 enough heights to resolve the variability and to bracket the boundary layer. BASS tripods 

 measure vector velocities at 2 Hz and optical backscatter at 6 heights from 30 cm to 5 meters 

 above bottom which provides the flow and shear measurements plus the particle concentration 

 (with POC calibration of the sediments responsible for the optical signals) necessary for the 

 POC flux. 



To understand the relationship between the forces that influence shelf-water dynamics and 

 sediment transport events, the fundamental principles goveming flow in the BBL must be 

 understood. Recent studies indicate that previously accepted concepts about the dynamics and 

 structure of the BBL, such as Ekman transport and bottom friction, may be invalid on the 

 continental margin. Velocity and density profiles in the BBL may be markedly different from 

 those predicted by classical concepts. Modelling efforts are currently underway to assess how 

 previously neglected processes or properties (e.g. across-shelf variations in currents, bottom 

 slope, hydrography) may affect these conclusions. Direct assessment of velocity and density 

 profiles in the continental shelf BBL will be needed to evaluate the accuracy of the model 

 calculations. 



Hydrography studies in the OMP must extend close enough to the bottom to resolve the 

 BBL. The overlying water column structure sttongly influences conditions within the BBL, 

 and is in turn influenced by it. For example, the frictional stress exerted on the slope/margin 

 bottom depends not only on condition above the BBL (the current, the surface wave field, and 

 the internal wave field), it also strongly depends on the lateral density variation in the BBL 

 which in turn result from diabatic motions unique to the BBL. A simple diagnostic of the 

 effects of lateral density variation in the BBL on the bottom shear stress can be inferred by 

 examining the variation of BBL thickness along sections. It is important to recognize that the 

 most likely region in the world's ocean where the bottom shear stress is greatly reduced due to 

 lateral BBL density variations is on the upper continental margin. Thus it is critical that 

 hydrographic surveys extending seaward of the shelf break resolve the BBL. 



21 



