HOW TO USE FLOODPLAINS FOR FLOOD RISK ... - SGGW

48
PART III – Guidelines
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High nutrient, heavy metal and sediment concentrations
in water are associated with poor water quality
and environmental degradation. On the other hand
minimum concentrations of some chemicals are required
to maintain surface water bodies at optimum
environmental quality, and sediment deposition on
soils can help maintain fertility and soil development.
Floodplains can have the ability to regulate these
properties in a variety of ways, and consequently can
perform soil and water quality improvement functions.
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There is a range of functions and processes that occur
in naturally functioning floodplains that can affect
soil and water quality. Generally these involve the import,
transformation, export and/or storage of chemicals
or particulate matter. Processes that involve the
transformation of chemicals from one form to another
are known as biogeochemical processes, and these
can play a significant role in the regulation of nutrients
and heavy metals. Processes that involve the regula-
Figure 29. A generalised view of the wetland biogeochemical cycle
Source: After Kadlec and Knight, 1996
tion of sediment are generally physical processes
such as erosion, transportation (usually by water but
sometimes by wind) and deposition or sedimentation.
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The type and rate of biogeochemical processes that
occur in floodplain soils depends largely upon their
hydrology. Well drained soils generally are aerobic,
that is they contain considerable amounts of oxygen
and water passes rapidly through them, often providing
little opportunity for biogeochemical transformations
to take place. On the other hand poorly drained
soils typically are anaerobic, containing little or no
oxygen, have high organic matter content and the
residence time of water often is long, providing plenty
of opportunity for biogeochemical transformations to
occur. For this reason most of the water and soil quality
functions and benefits that occur in floodplains take
place in the wetter soils found within them, and these
soils will form the focus of the information provided
here.
Few biogeochemical transformation processes are
unique to wetland soils, but the combinations and particular
dynamics of biogeochemical
cycles and processes operating
within them generally are not
found in many other ecosystems.
Wetland or hydric soils often
have unique distributions of oxygen
rich and oxygen depleted
zones resulting in sequences of
transformations of nutrients and
metals that cannot occur in other
ecosystems. The combination of
biological, chemical and physical
processes that occur in wetlands
result in biogeochemical interactions
that can mobilise, immobilise,
transform and even remove
completely from the wetland/aquatic
system a wide range
of compounds and elements. A
generalised diagrammatic representation
of the wetland biogeochemical
cycle is shown in Figure
29.
Many of the transformation processes
within wetlands are con-