Assuming a z^ of 0.1 cm and a flow speed of 1 cm/sec at 10 cm above the 

 bottom, Figure 6.15 shows that uR^ is about 100 at neutral stability. Thus 

 the hydrodynamically induced deposition velocity is on the order of 

 0.01 cm/sec. In the presence of a stable stratification, the deposition 

 velocity is decreased to a value of 0.0025 cm/sec when L is 10 cm. Unstable 

 stratification, on the other hand, increases the deposition velocity to about 

 0.02 cm/sec when L is -10 cm. Increasing the bottom roughness, while keeping 

 other parameters the same, the deposition velocity at 10 cm height can be 

 increased by almost an order of magnitude. 



Sublayer Resistance (R^) 



The deposition of sediment particles depends primarily on the particle 

 diameter and particle density. The effects of particle density can be 

 incorporated into the particle size by increasing the equivalent particle 

 size. Particles <30 ym in radius tend to follow all of the turbulent eddies 

 and can be diffused in the same manner as a gaseous species, except within the 

 viscous sublayer next to a surface. Transport of particles in the sublayer is 

 determined by three main contributions - Brownian motion, gravitational 

 settling and inertial impaction. 



For submicron size particles, Brownian diffusion dominates. 

 Gravitational settling and inertia impaction dominate for particles with 

 diameters of 10 pm or more. There is a transition range, for particles with 

 diameters between 0.1 and 1 ym, wherein none of the mechanisms is very 

 effective in bringing the particles across the sublayer. Lewellen and 

 Sheng (1981) derived the following formula for deposition velocity within the 

 subl ayer: 



c.(^)°-'^ 



2 2 



U^'^r -0.075q T_/V 



0.1 Cf q — ^ (1-e "^ ) + T^g (6.6) 



where c^ is a skin friction coefficient, Dg is the Brownian diffusion 

 coefficient, q is the turbulence velocity, u* is the friction velocity, and t^ 

 is the particle relaxation time. The three terms on the r.h.s. of (6.6) 

 represent the effects of Brownian diffusion, inertial impaction, and 

 gravitational settling, respectively. 



164 



