Sorry but I can't answer your question it's out opf my skill (I'm not expert in CFD) and I don't know any publication with Fluent and DDPM-DEM in concentrated suspension. But there are several works with Monte Carlo method. With such CFD the difficulty is to explore a large surface to obtain qualitative average results.

There are different tools apart from ANSYS Fluent for DDPM-DEM modelling like ANSYS CFX and ABAQUS, which might be easier for user definitions, a bit more precise, as well as couples solid and fluid. However, it all depends on the physics considered. We also carried out DDPM-DEM approach using COMSOL Multiphysics and ANSYS Fluent, and both tools were effective and published.

Based on your system, you may consider using virtual massless particles to see which areas of the bridge are most affected by scouring. It is actually pier scouring, which means scouring of the sediment around the pier.

Based on the physics, let me share this: you can use GKT (Granular Kinetic Theory) in ANSYS-Fluent in the previous papers that are suggested to review. Nevertheless, the authors also used GKT in ANSYS-CFX in the following paper of particles transport (slurry mix) (this was related to flow assurance for oil and gas application). ANSYS-Fluent permits to use the three components of the solid viscosity in GKT, i.e., friction, collision and kinetic, while ANSYS-CFX only permits to use collision and kinetic components.

However, as you will see in this paper, ANSYS-CFX does a very good job as well. The authors were able to capture particles segregation due to gravity and distribution with moderate discrepancies with experiments only on region of wall due to the lack of frictional term in viscosity (as per their conclusion in the paper)

Conference Paper CFD Modeling of Slurry Flows in Horizontal Pipes

They showed that ANSYS-Fluent is better suited than ANSYS-CFX to deal with particle transport using GKT for slurry flows (at least in ANSYS CFX v.13).

These attached files may help you with answering the question and you can check the references on these papers for more info. Different models are available in ANSYS Fluent and CFX to simulate multiphase flows. These are two models that can be used to simulate the dispersion of sand in gas flow:  

Eulerian – Lagrangian approach: The primary phase is the continuous phase (air for example) and the secondary phase is the discrete phase (particles – sand for example).  Conservation equations are solved for the continuous phase and the particle phase is tracked by solving the equations of motion for each particle. The trajectories of particles (sand) are computed in a Lagrangian frame. The particles can exchange heat, mass, and momentum with the continuous gas phase.

Euler-Euler approach:  In Euler-Euler model all phases are treated as continuous. This model can be used for separated flows where both phases can be described as a continuum. The Euler-Euler approach is used to model dispersed flows when the overall motion of particles is of interest rather than tracking individual sand particles (sand bed for example). The dispersed phase equations are averaged in each computational cell to achieve mean fields. This model is valid (describe a dispersed phase as a continuum), when the volume fraction is high. This model is suitable for dense flows (sand bed for example).

See some mathematical description in this one too on mutliphase flow

Article A two-fluid model with a tensor closure model approach for f...