Tracking individual particles rather than grid-cell concentrations lets you follow each object’s path in detail — where it came from, where it goes, and how long it stays. Historically the cost of doing this for large particle counts was prohibitive; that constraint has fallen away.

You can define particles as constant at a fixed depth or in fully 3D mode with optional settling or rise. Particle movement can be tied to EFDC+’s own horizontal and vertical diffusivity coefficients or to a user-specified coefficient.

Advection alone doesn’t reproduce real dispersion. In DSI’s testing, the randomness and turbulent diffusion acting on a particle cluster proved to matter as much as the velocity field in determining how the cluster spreads and where individual trajectories go. [EFDC+] EFDC+ therefore offers a 3D random walk option following Dunsbergen and Stelling (1993), which displaces each particle either side of its advected position at a magnitude set by the local diffusivity. Advection and the random component are solved separately, so you can enable random walk horizontally, vertically, both, or neither.

[EFDC+] Three numerical schemes are available for integrating particle motion including explicit Euler, predictor-corrector Euler, and fourth-order Runge-Kutta letting you trade computational cost against trajectory accuracy according to what the study needs.

DSI calibrated the LPT module against an analytical solution for quasi-steady, uniform flow in an open channel, and tested it further across a range of hydrodynamic regimes and geometries. The module’s performance has since been demonstrated across a variety of applications.

[EFDC+] The oil spill option uses the same transport approach as drifters, with each particle treated as a packet carrying a share of the total spilled mass. Packets can be released all at once or over a specified interval, and are held just below the water surface when rise and settling rates are set to zero.

Three processes distinguish oil from a passive particle:

• Wind drag: Surface oil is pushed by wind directly, over and above the wind-driven currents already in the flow field. Following Kim et al. (2014), EFDC+ applies a drag coefficient either as a single value from wind magnitude or resolved into separate x and y components.
• Evaporation: using the surface evaporation theory of Stiver and Mackay (1984). Where water temperature is simulated, each packet takes the temperature of the water around it, so evaporation responds to the thermal field rather than a fixed assumption.
• Biodegradation: as first-order decay following Stewart et al. (1993), a rate of 0.011 per day corresponds to roughly a two-month half-life. As with evaporation, a simulated temperature field supplies the reference temperature for optimal biodegradation.

Either loss term can be switched off by setting vapor pressure or degradation rate to zero, and packets whose remaining mass falls below a negligible threshold are retired from the computation.

This sub-model supports emergency response simulation as well as fate and transport analysis of historical spills.