97
The erosion ?ux for cohesive sediments is formulated using [IV-6]:
?? = ??(1 ? ?)?? ?
??
???,?
? 1? ?? ?? > ???,? (IV-15)
where ?cd is the critical shear stress for the deposition of cohesive sediments and ?cr,0 is the
critical shear stress for the erosion of cohesive sediments.
For the mixture of cohesive and non-cohesive sediments the assumptions made by [IV-7] were
followed. This is based on the parameter of critical cohesive sediment fraction in the seabed.
Erosion of mixtures of cohesive (mud) and non-cohesive sediments (sand) is independent if
clay content is below critical. Above critical clay content, the bed behaves cohesively. In the
non-cohesive regime, exchange of sand and mud with bottom is independent, whereas in a
cohesive regime an erosion of mud and sand occurs simultaneously as cohesive sediment. The
deposition is an independent process for cohesive and non-cohesive sediments.
Cohesive sediments are able to form ?ocs that may have a larger e?ective diameter than primary
particle size and, therefore, larger settling velocity. The equilibrium model is used to calculate
?oc size and settling velocity [IV-8]:
?? =
(?????)?
?????
? ? ??
?
???.?????.???
(IV-16)
where Re = WsD/? is the Reynolds number, ? is water kinematic viscosity, DF is the diameter
of original particles and:
? = ?? +
???
???
?
??
(IV-17)
with ? = ?? ?? , where ? is dissipation rate, kA and kB are parameters describing aggregation
and ?oc break-up respectively.
IV-4. 3-D RADIONUCLIDE DISPERSION MODEL
IV-4.1. Eulerian model
The equations for the temporal variations of dissolved radionuclide concentrations in the water
column (Bq/m3), for the concentration of particulate phase radionuclides for each sediment size
class i in the water column (Bq/m3), for the bottom layer averaged concentration of radionuclide
in the pore water (Bq/m3) and for concentration of particulate phase of radionuclide for each
sediment size class i in the sediment (Bq/kg of sediments) are written in Cartesian coordinates
as:
??
?
??
+ ??? ???? = ????(??? ? ??,???,????? ? ???) ? ???? + ????(???) (IV-18)
??,?
?
??
+ ??? ???,?? = ??,?
???,?
?
??
+ ?????????,?? ??,?? ? ??,?? ? ? ???,?? + ????(??,?? ) (IV-19)
?????
?
??
= ???????(??) ? ???? ? ???????(1 ? ?)(??? ? ??,?????? ?? ? ???)???? (IV-20)
