Page 310 - Soil and water contamination, 2nd edition

Patterns in the soil and in the vadose zone 297

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Figure 16.8 Interpolated 137 Cs deposition density in the Mochovce catchment, Slovakia (Bq m ) (Van der Perk
et al., 2002).
attributed to floodplain sedimentation (which was not included in the erosion–deposition
137
model), to the initial pattern of Cs deposition, and to short-range spatial variation.
As was demonstrated by the above 137 Cs study in Slovakia, floodplain deposition
can be a substantial source of variation of contaminants in catchments. The amounts and
spatial variability of deposition of sediment-associated contaminants on floodplains during
overbank flooding depend on several factors, including frequency and duration of the
flooding, suspended sediment concentration in the main river channel, and the flow patterns
and stream velocities in the floodplain. The spatial variability of floodplain deposition is
related to floodplain topography. In general, high deposition rates are found on low-lying
floodplains that are frequently inundated for relatively long periods. The fine sediments that
carry the greatest contaminant load tend to accumulate in the lower parts of the floodplain
(Leenaers, 1989; Leenaers et al., 1990). At the same time, the deposition rates generally
decrease with distance from the source, i.e. the river channel (Burrough et al., 1996). Figure
16.9 illustrates these general trends for a floodplain along the river Meuse, the Netherlands.
However, the relationships between floodplain contamination on the one hand and
floodplain elevation and distance to the main channel on the other are essentially empirical
and often only apply to small stretches of floodplain. They may become inapplicable if the
floodplain area is larger and has an irregular topography with depressions, backwaters, minor
dikes, and other objects that influence sedimentary patterns. In these areas, a physically-based
approach is required to explain the patterns of floodplain sedimentation and contamination.

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