Modeling Concepts - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 290

processes are usually driven by mechanical energy and are highly dissipative. In fact,
errors in hydrodynamic initial conditions result in an artificial initial energy supply,
which will be transformed into kinetic energy before being dissipated. Stability prop-
erties of the model are limited by the values of the velocity. Very unrealistic initial
conditions can generate very high velocities and create conditions for numerical insta-
bility. For these reasons the most efficient way to initialize a hydrodynamic model is
to start from null velocity and a horizontal free surface equal to the average level at
the open boundaries.
Properties associated with the ecology of the system have to be initialized with
realistic values. In fact, ecological systems are resilient and once they are submitted
to a string of perturbations, they do not necessarily recover toward the state they
assumed before that perturbation. Initializing ecological models with unrealistic
values is equivalent to stating that the system has been highly disturbed. In fine-
resolution models, a lot of information must be supplied by interpolating between
points where information is available. Specific initialization software tools can
simplify the initialization procedure.
Sediment transport models are also very dependent on the initial conditions. If
initial suspended matter concentration is overestimated, settling will remove it.
Conversely, if initial suspended matter is underestimated, erosion will supply the miss-
ing matter (if deposited matter is available). In this case the difficulty is not the
initialization of the water column concentration, but rather the initialization of the
erodable amount of sediment lying on the bottom. The user has to implement a
procedure compatible with the information available and with the objectives of the
simulation. If the information is scarce and the aim of the simulation is to obtain
realistic values of the concentration in the water column, then a convenient procedure
is to assume that there is no net erosion in any point of the lagoon. In this case the
model is initialized assuming that there is a thin erodable layer everywhere in the
lagoon, and it is run until an equilibrium solution is obtained. After this period,
erosion and deposition areas are identified and the model is initialized assuming that
there is no erodable sediment in the areas where erosion was identified. If the aim
of the model is to simulate erosion processes in the lagoon (e.g., due to anthropogenic
modifications of its morphology), a consolidation erosion module of the bottom has
to be considered. In fact, after deposition, sediment are submitted to a consolidation
process resulting in an increase in the critical shear stress for erosion. To simulate
the erosion process an initial vertical profile of consolidation has to be specified.

6.4.3 B OUNDARY C ONDITIONS

Boundary conditions can be specified in terms of a specified value, a specified flux,
or a specified law of property variation at the boundary. The radiation boundary
conditions used on ocean hydrodynamic models fit in the last group. A typical
coastal lagoon has one or several sea inlets and receives land discharges through
one or several rivers. In general, at sea inlets, the most convenient procedure is to
specify the values of the properties or to estimate those values as a function of the
values inside the modeling area. At the river boundary the flux is the river flow rate
times the specific value of the property (concentration in case of a mass). In this