The above formula has been compared with some data of particles deposited 

 on the underside of a flat plate in the range where inertial impaction 

 dominates (Lane and Stukel , 1978) in Figure 6.16(a). In Figure 6.16(b), this 

 formula is compared with Sehmel's (1973) data for deposition on a smooth 

 surface for particles in the transition regime. Both comparisons appear 

 satisfactory. The deposition velocities shown in these figures are for 

 particles settling in the atmosphere and hence are much bigger than those for 

 sediment particles in water. 



For cohesive sediments in marine water with a particle size distribution 

 shown in Figure 6.2, Brownian diffusion is negligible for most of the 

 particles. Inertial impaction and gravitational settling are the primary 

 mechanisms for deposition through the sublayer. Gravitational settling 

 velocity is a lower bound for the deposition velocity of these particles. In 

 fresh water environment, however, smaller particles with diameters of 1 um or 

 less constitute a bigger fraction of the particle size distribution. A 

 comprehensive formula like Equation (6.5) should yield an accurate estimate on 

 the deposition velocity. 



In the presence of a vegetation canopy, the analysis is more complicated. 

 Simple constant flux relation (6.4) no longer holds within the vegetation 

 canopy. The presence of the canopy introduces source and sink terms into the 

 basic equations for momentum, heat, and species concentration. Lewellen and 

 Sheng (1980) developed a model of turbulent flow within a canopy using a 

 second-order closure model of turbulent transport (see Appendix D). Within 

 the canopy, the total drag force has to be partitioned into a profile drag and 

 a skin friction drag. Resistance to deposition within the sublayer next to 

 the leaf surfaces can be incorporated into the canopy model to estimate the 

 total canopy resistance to deposition. The deposition velocity within the 

 canopy thus depends on the total leaf area per unit volume; the ratio of 

 total wetted area to the projected frontal area; the Schmidt number of the 

 species; the leaf surface resistance; flow speed above the canopy; and the 

 stability above the canopy. 



165 



