SEDIMENTARY PROCESSES/Deposition from Suspension 11 Figure Regions of the turbulent boundary layer for a flow 10 m deep In the centre, the linear representation of flow speed vs height cannot resolve the viscous sublayer, but the speed vs log height (z, expressed as a Reynolds number) shows it very well air, for the same stress, its thickness is similar, about 0.5 mm.) However, this is ten times the diameter of very fine sand The shear across this layer is very large; for U* ¼ 0.01 m s 1, the speed goes from to 0.1 m s in just mm Weak aggregates cannot survive this shear and break up Above this sublayer, there is a transition (‘buffer layer’) to a region in which the flow speed varies as the logarithm of height above the bed (Figure 3) As the flow speed decreases, U* decreases and dv increases, so that, in deposition, most particles that are going to become part of the geological record have to get through the viscousdominated layer Although viscous dominated, this layer is actually not laminar Spatially, it has a structure of high- and low-speed streaks, and temporally very high-speed ‘bursts’ of fluid out of the layer and ‘sweeps’ of fluid into it from outside These are associated with stresses typically up to 10 times the average (and extremes of 30 times), and so the mean shear example given above is a minimum, and even strongly bound particles may find themselves ripped apart just as they were getting within sight of the bed and posterity Above the viscous sublayer, the ‘buffer layer’ is overlain by a zone in which the flow speed varies as the logarithm of distance from the bed (the ‘log layer’) This zone is fully turbulent with eddies becoming longer with height above the bed and turbulence intensity becoming smaller The roughness of the bed positively influences the drag and turbulence, but also provides quiet regions in between large grains where fine particles can settle Fine sediment can thus be deposited in the interstices of gravel, affecting several processes, e.g., the spawning of salmon Critical Conditions for Suspension Two views of the critical suspension condition are as follows: (1) at critical movement conditions, the turbulent intensity can hold particles up, and so suspension depends on whether the particles are ejected from the viscous sublayer; and (2) sublayer ejections are fast, and so suspension depends on whether the vertical turbulent velocity can hold the particles up after injection into the flow The second view was held by many, but recent work suggests that the first view may be correct This view is based on high-speed video observations of particles close to the bed, which show that there is a threshold level of shear stress for the particles to respond to turbulent ejections of fluid from the viscous sublayer The second view would mean that fine to very fine sand would immediately go into suspension as soon as it moved For example, for 100 mm sand, the critical erosion shear velocity U* is 0.012 m s 1, and the settling velocity of this very fine sand is 0.008 m s 1, and so it is capable of being held up by the flow, but video data show that it is not suspended This means that there is a region of bedload transport for all particles of settling velocity, at least down to $30 mm silt This is shown on a conventional nondimensional erosion diagram in Figure The significance of this is that, in a decelerating flow, below the suspension threshold, material may continue to move, but not in suspension Experimentally it has usually been found easier to determine the critical suspension condition with increasing flow, rather than failure of suspension on decreasing flow It is generally assumed that the two views are equivalent Transport in Suspension Once material is moved out of the near-bed region, it is held in suspension by the action of fluid turbulence For this, because the vertical turbulent component of velocity is about the same as the shear velocity U*, the normal suspension criterion is that ws/U*