DHIJWASv Software FEFLOW 6.1

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or in a steady-state solution to take care of nonlinearities
in the basic equations.
• The determination of an appropriate time-step
length in automatic time-stepping procedures
based on the deviation of calculated from predicted
solutions.
The absolute error (e.g., head difference) is normalized
by the maximum value of the corresponding primary
variable in initial or boundary conditions
(maximum hydraulic head, maximum concentration,
etc.).
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By default, a newly created FEFLOW model
reflects a confined aquifer. Saturated simulations of
unconfined conditions require specific treatment of the
phreatic groundwater table.
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In unconfined two-dimensional models with a horizontal
projection the saturated thickness is iteratively
adapted to the resulting hydraulic head. For this purpose,
material-property input includes the aquifer top
and bottom elevations.
When the hydraulic head exceeds the aquifer top
elevation, the model calculations presume confined
conditions in the respective area, i.e., the saturated
thickness is limited to the difference between top and
bottom elevation. Aquifers with partly confined conditions
are thus easily simulated.
Two-dimensional vertical cross-sectional and axisymmetric
models are always assumed to be completely
confined. Modeling of unconfined conditions in
these cases strictly requires a simulation in unsaturated/
variably saturated mode, hereby applying Richards’
equation.
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In three-dimensional models with gravity in the
direction of the negative z axis (default ’top view’
models) two different strategies can be applied in
FEFLOW for handling the phreatic surface besides
simulating in unsaturated mode. It is important to note
that these methods were originally designed for
regional water-management models. They are clearly
limited (with very few exceptions) to cases with a single
phreatic surface. Simulations where partial desaturation
of the model is expected below saturated parts,
e.g., due to drainage in a lower aquifer, have to be simulated
in unsaturated/variably saturated mode. In contrast
to an often-heard opinion, despite the nonlinearity
of Richards’ equation unsaturated models can be as
computationally efficient or even more efficient than
saturated models with phreatic-surface handling if
appropriate simplifications are applied to the unsaturated
material properties.
As both options for phreatic-surface handling have
their specific advantages and disadvantages, the methods
applied in FEFLOW will be explained in detail:
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This mode takes care of the phreatic surface by vertically
moving the calculation mesh in a way that the
top of the model is exactly at the water-table elevation
at any time. For this purpose, layer elevations are
changed at each time step (transient simulation) or for
each iteration (steady-state simulation). The movement