Geoscience Reference
In-Depth Information
Including the following effects in the model defines its structure and complexity:
• Variations of variables in time
• Variations of variables in space
• Advection of momentum
• Advection of admixture
• Diffusion of momentum
• Dispersion of admixture
• Wind and bottom friction effect
• Wind waves and their interaction with general water movements
• Coriolis force
• Buoyancy effect due to stratification (i.e., vertical variations of water
density)
• Gravity force influence due to spatial variation of water density or/and
water level
Eigen or induced lagoon behavior
In any case, the water movement in real lagoons is time dependent and three
dimensional, and it develops under all forcing factors found in nature. We can use
fewer spatial dimensions (geometry facilitation) or physically simplified models (some
forcing factors or processes are reduced) only for estimation of the real situations with
some uncertainty. In the most favorable cases, the residual error between the model
solution and reality may be less than the accuracy required for the task, but that success
depends on many circumstances.
6.3.5.2
Possible Simplifications in Spatial Dimensions
The real current structure in lagoons should be simulated only by the 3D model, but
some simplification is possible because in many tasks the current solution itself is
not needed. For instance, the fluxes between subbasins of a lagoon are the important
variables needed for the ecological modules, so it is possible to reduce the spatial
dimensions in the ecological modeling.
In an elongated lagoon, the solution given by horizontal 2D model (vertically
integrated motion) for fluxes through a transversal slice is nearly the same as a 3D
model would provide. Only in cases of the narrowing of the lagoon basin (e.g., by
an island or a peninsula), when the cross section is reduced by, say, 80%, the
difference between fluxes computed from 2D and 3D solutions becomes considerable
when the winds are longitudinal. In the case of transversal winds, the 2D approach
gives large errors in the flux solution. 38
It could be possible to use a 1D model for advection-dispersion problems if the
basin is essentially elongated: or (see the
parameters in Table 6.1) . In other situations it is more reasonable to use a 2D or 3D
shallow water approach.
The 2D approach is useful for modeling of advection-dispersion transport and
wind-wave resuspension if spatial mean water depth is less than wave penetration
p
max
≥⋅
10
p
min
max( ,
ab
)
≥⋅
10
min( ,
ab
)
shell
shell
 
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