103 ??? = ????1 ? |??? | ??? ? (V-8) where ? is the bottom friction stress (whose components are given by Eq. V-4) and ?cd is a critical deposition stress above which no deposition occurs since particles are kept in suspension by turbulence. The settling velocity of particles is obtained from Stokes’ law as mentioned above: ?? = ???? ?? ??? ??? (V-9) where ? and D are suspended particle density and diameter respectively and ? is the kinematic viscosity of water. The erosion rate is written in term of the erodability constant E: ?? = ??? ? |??? | ??? ? 1? (V-10) where fp gives the fraction of fine particles in the bed sediment and ?ce is a critical erosion stress below which no erosion occurs. The model can also calculate sedimentation rates (SR) as the balance between the deposition and erosion terms. V-3. RADIONUCLIDE TRANSPORT The dispersion model includes three phases, i.e. water, suspended matter in the water column and bed sediments. An advection–di?usion equation is solved in order to simulate the transport of radionuclides in the water column. Interactions between the dissolved phase and solid phases (suspended matter and bed sediments) are described through a dynamic approach, in terms of kinetic transfer coe?cients. Thus, assuming that adsorption–release reactions are described by a single reversible reaction, a coe?cient k1 characterizes the transfer from the liquid to the solid phase and a coe?cient k2 characterizes the inverse process. Dimensions of these coe?cients are [T]?1. The adsorption process is a surface phenomenon that depends on the surface of particles per water volume unit. This quantity has been denoted as the exchange surface [V-6–9]. Thus in general: ?? = ?(?? + ??) = ?? ??? + ????? (V-11) where Sm and Ss are the exchange surfaces for suspended matter and bottom sediments respectively (dimensions [L]?1). ? is a parameter with the dimensions of a velocity. It is denoted as the exchange velocity [V-6–9]. Assuming spherical particles, the exchange surfaces are written as [V-6–9]: ?? = ?? ?? (V-12) and: ?? = ????(???)? ?? (V-13)