A three-dimensional mathematical model for the advection, diffusion and 
sedimentation of the fine-grained suspended particulate matter has been 
developed. The model is based on the Random Walk simulation (Lagrange-Monte 
Carlo Method). According to this method, a large number of particles 
representing a particular amount of mass, is introduced to the flow domain 
through a source. Their transport and fate is traced with time. Horizontal 
advection of the particulate matter is controlled by the local fluid velocity. 
Vertical advection is controlled by the local fluid velocity and the particle 
settling velocity. Turbulent diffusion is simulated by the random Brownian 
motion of the particles due to turbulence. The model includes processes like 
flocculation, settling, deposition and erosion-resuspension of the cohesive fine 
grained sediments.

An application is presented for Thermaikos Gulf, North Aegean Sea, where the 
rivers are the sources of the sediment particles. The model is coupled off-line 
with the hydrodynamic Princeton Ocean Model. The three-dimensional velocity 
field is provided to the transport model by a typical one-year simulation of the 
hydrodynamic model. The seasonal variability of the particle transport is 
compared to observational data of Suspended Particulate Matter. The importance 
of the sedimentary processes is assessed by comparing results to the transport 
of pure water particles.