Flow Modeling - Environmental Modeling - page 210

Chapter 11
Flow Modeling
Flow, of course, is a crucial topic in many environmental sciences. Flow is the
carrier of advective transport (see Chap. 3). Biogeochemical species are transported
by flow through environmental compartments and through systems of compart-
ments. Often transport with the flow is the fastest process by which a species of
potentially hazardous impact, starting from a source, reaches a sensitive region.
Let's take a repository for radioactive waste as an example. In several countries
of the world final storage facilities are envisaged located in some nearly imperme-
able geological environments in the deep sub-surface. The main safety problem
with these waste disposal sites is concerned with the identification of flow. Contain-
ments and barriers of any type are not able to shut off heat producing, acid,
radioactive and/or toxic material for long time periods. There is the danger that in
the long run hazardous substances find a subsurface flow path, which takes them up
to the surface. Even if that are long distances and long time periods, the potential
thread still remains, as those nuclides with long half-lives remain active for 1,000s
of years. Though the transport along the flow path may take several 1,000s or
10,000s of years, this is a much faster process than any other one. Thus, it is
important to understand and model flow paths and fields (Fig. 11.1 ).
Speaking of flow in environmental sciences not always means the same thing.
There are various types of fluids and fluxes. In the hydrosphere, containing as
different compartments as creeks, rivers, lakes and the sea with coastal waters,
continental margins and the deep sea sub-compartments, water is the flow medium.
Aquifers and the pore space of aquatic sediments also belong to the hydrosphere.
Air is the flow medium of the atmosphere. In the unsaturated zone, between the
earth surface and the groundwater table, water and air are both fluids, although with
quite different characteristics. There is also flow in the biosphere, for example,
when water is taken up by the roots and transferred to the green parts of plants
above the earth surface.
According to the differences concerning the medium and the compartments and
sub-compartments respectively, there is not the one and only approach to modeling
flow phenomena. In the sequel, the term free fluids is used if the flow medium
occupies the entire volume. A contrasting term is porous media flow, where the