A Eulerian-Lagrangian approach to study the effect of river hydro-morphology on (small) floating particle transport and deposition

The transport and deposition of floating particles in flowing water is a key mechanism that drives the fate of contaminant, organic materials, and debris along river networks. The ability of human-made and naturally buyout particles to sit on the water surface makes them able to travel long distances. The mechanisms that allow these particles to deposit are closely linked to the hydraulics of the channel and the morphology of the river. Research has shown that particles such as plastics and debris tend to accumulate behind obstacles and recirculation areas, creating accumulation hotspots. The location of such hotspot also depends on the interplay between particle shape and size and flow conditions.  Although the influence of river morphology and flow regime is well acknowledged, the precise interaction between these components remains unclear.

To investigate this relationship, we used a new Eulerian-Lagrangian method based on a 2D depth-averaged flow solver simulating transport and dispersion of floating particles in a typical alpine river floodplain. This method offers a computationally efficient way to track the trajectory of single particles moving onto a flow field. Our approach integrates model simulations with data derived from outdoor and laboratory experiments, where we recorded deposition location of particles of different size, shape and material under different discharge conditions.

The results show that the number of floating particles decreases exponentially with the distance from the release point, with decay rates primarily correlated with the water discharge. We find that the deposition of particles depends on the hydraulics of the channel and the roughness elements in the channel, with particle sizes playing a secondary role. Unsteady flow conditions, namely receding water levels, promote particle deposition on shallow areas and channel shorelines. The use of the particle tracking model allowed us to extend the parameter space investigated experimentally, allowing for an in-depth analysis of the spatial and temporal dynamics of particles transport and deposition during floods.

These results deepen our understanding of transport processes of floating material at the reach scale, providing quantitative evidence on the central role played by channel hydromorphology. Although the effect of particle shape and size is not fully understood, the study can offer valuable insights into the dispersion mechanisms of different floating particles, from plastics to organic materials.