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coastal and open-sea environments where wind forcing remaining suspended, or descending – depending on
significantly influences transport dynamics: 39,40 environmental conditions such as temperature, salinity,
V leeway = V wind × (a + b.r) (I) and biofouling.
where V mind represents the wind speed at a height The OpenDrift model specifically employs the
of 10 m; a is the intercept coefficient, accounting for Langevin equation to simulate vertical diffusion, as
the portion of drift that is not linearly dependent on follows: 42,43
wind speed; b is the linear coefficient, representing dz wdt . 2 K dt R. (III)
z
the portion of drift that is proportional to wind speed;
and r is the random noise coefficient, introduced to where dz is the change in depth; w is the settling/
model uncertainties in the drift. The coefficients a and rising velocity (buoyancy or sinking velocity); Kz is the
b are empirically determined and specific to the type of vertical diffusivity coefficient; dt is the time step; and
drifting object. R is a standard random number drawn from a normal
distribution, Normal (0,1).
3.1.2. Stokes drift module In this modeling framework, seawater temperature
This module simulates the effect of ocean waves on the and salinity are utilized as primary inputs to compute
transport of floating particles by calculating the net drift the buoyant velocity of plastic debris within the
resulting from wave-induced orbital motion. Stokes drift marine environment. These parameters are integrated
is a critical factor in modeling the movement of plastic into the Thermodynamic Equation of Seawater 2010
debris, especially in coastal and offshore environments (TEOS-10), which is implemented in the Vertical
where wave activity is significant. The module accounts Mixing module of the OpenDrift modeling system.
for the cumulative displacement of particles caused by The relative density contrast between plastic particles
wave motion, which contributes to long-range transport. and the ambient seawater – determined by TEOS-10
To quantify this effect, the Stokes Drift module – governs their vertical displacement. This vertical
applies a mathematical formula that estimates the drift motion significantly influences the transport pathways
velocity based on wave parameters such as amplitude and spatial accumulation patterns of plastic waste in the
and frequency, as described in previous studies. 40,41 ocean, particularly in stratified water columns where
This component enhances the accuracy of trajectory density gradients are pronounced.
simulations by incorporating wave-driven transport
mechanisms into the overall particle movement model: 3.1.4. Coastal module
awk This module was integrated to robustly simulate
2
vz 2 e 2 kz (II) the interaction between plastic debris and coastal
s
where a is wave amplitude; k is wave number, environments, encompassing shoreline dynamics and
nearshore processes. The module uses logical conditions
calculated as 2π/λ (where λ is the wavelength); ω is and geospatial checks to handle the behavior of particles
the angular frequency, calculated as 2π/T (where T is when they encounter land. It accounts for mechanisms
the wave period); z is depth (negative when moving such as particle washout, refloating due to wave
downward into the water column); and v (z) is the action, and entrapment within coastal features (e.g.,
s
Stokes drift velocity at depth z. estuaries, mangroves, and tidal zones). By resolving
The Stokes drift velocity is highest near the surface these processes, the module enables a high-resolution
(z = 0) and diminishes with increasing depth. representation of plastic waste transport, retention,
and redistribution in coastal regions, which are critical
3.1.3. Vertical mixing module zones for accumulation and ecological impact.
This module is designed to simulate the dynamic
vertical distribution of particulate matter within the 3.1.5. PlastDrift module
aquatic column. It incorporates key physical processes, The PlastiDrift module is not a standalone module; it is
including turbulent diffusion, buoyant forces, and a specialized version of OpenDrift designed for plastic
stratification driven by density gradients. This module waste simulation. In this research, PlastDrift serves as
plays a critical role in accurately representing the vertical the integrative core of the modeling system, synthesizing
transport and fate of plastic debris, which may exhibit input parameters from four specialized submodules
variable buoyancy characteristics – either ascending, to generate scenario-specific outputs based on seeded

Volume 22 Issue 4 (2025) 78 doi: 10.36922/AJWEP025170129