Individual Resistances which Determine v^ 



Deposition of sediment particles through the bottom boundary layer is 

 influenced by the resistance of the various layers through which the particles 

 must pass to reach the bottom. For ease of analysis, it is convenient to 

 break up the boundary into three regions. The total resistance to deposition, 

 defined as the inverse of the deposition velocity, is then the sum of 

 resistances presented in each region. 



-1 

 ^d " '^hydrodynamic "^ '^sublayer "^ '^bottom (^-^^ 



'^hvdrodvnamic represents the sum of the resistances to turbulent transport in 

 the water column. ^sublayer accounts for that resistance due to the thin, 

 viscous, relatively laminar sublayer next to the bottom or the leaf surface of 

 vegetation canopy. The third resistance is the chemical or biological 

 resistance the particles encounter after they reach the bottom. 



Hydrodynamic Resistance (R^ ) 



The hydrodynamic resistance to deposition can be estimated by considering 

 the constant flux region above the bottom. This should be a reasonable 

 approximation so long as the reference height for defining deposition velocity 

 does not exceed approximately 100 cm. 



The similarity solution which exists for the vertical gradients of 



velocity and particle concentration in the constant flux region can be 



integrated in the z direction to yield expressions for the hydrodynamic 



resistance which are a function of height z; bottom roughness height, z.; 



-1 

 stability, L , expressed as the inverse of Monin-Obukhov length; and flow 



speed u at that height z. Since the resistance is inversely proportional to 



the flow speed, the product of the flow speed and the hydrodynamic resistance 



is presented. The results are shown in Figure (6.15) for three heights (1 cm, 



10 cm, 100 cm) and three roughness heights (0.01 cm, 0.1 cm, 1 cm) covering 



the unstable range (L <0), the neutral case (L =0), and the stable range 



(l"So). 



162 



