In 2 research projects we want to use 3D particle tracking in a partially obstructed river/reservoir section to simulate transport of large woods and their interaction with logbooms, and in the other project oil spill barriers on the surface. The implementation of 3D gates (chapter B.3.1 of Flow Manual) within the hydrodynamic model (Delft3D flow, structured grid, sigma layers) works great, where we were able to block the flow in specific positions, directions, and layers. Using drogues (which however only follow surface layer velocities) the structures inhibit the drogues passing them (even though getting stuck at the 3D gates). This is nice already, but we would like to allow particles to pass below the structure. Running Delft3D PART we obtain nice 3D particle tracks, but the particles pass the 3D gates. I assume that PART doesnt recognize the 3D gates, and uses the velocity field only, to track the particles, thus allowing them to pass the gates. We tried making gates around several grid cells, to block the flow in larger regions, but still, quite some particles pass the gates. Any suggestion on how to overcome this problem are highly appreciated. Thin dams are not an option, as we want to allow the flow to pass below the structure. Thanks in advance.