Geoscience Reference
In-Depth Information
and evolution is a complex process that may or may not
indicate climatic control (Cooke and Reeves, 1976).
On much shorter timescales, badlands typically re-
spond rapidly to the extreme events that characterise dry-
lands (see Chapter 11). Wainwright (1996a) used rainfall-
simulation experiments to demonstrate that the high rates
of runoff production on Terres Noires badlands in south-
ern France during high-intensity rainfall correspond to
rapid decreases in the surface friction, leading to high ve-
locity flows. These conditions lead to concentrated flows
for much of the slope length, and thus high erosion rates.
In rainfall-simulation experiments on similar badlands,
Oostwoud Wijdenes and Ergenzinger (2003) generated
miniature debris flows. A subsequent modelling study
(Wainwright, 1996b) suggested that the small area of bad-
lands (when combined with relatively small agricultural
areas) were responsible for disproportionate amounts of
runoff and erosion entering the channel system in the
catastrophic floods at Vaison-la-Romaine in 1992. The
low friction and steep slopes also meant that the badlands
typically produced runoff much earlier in the storm than
surrounding areas.
Likewise, the monitoring results of Canton et al . (2001)
suggested that the 5 % of storm events in which more
than 80 mm of rain fell produced around 35 % of all the
sediment exported in a small catchment in the Tabernas
badlands over a two-year period. Francke et al . (2008)
found that badlands in northern Spain similarly produced
very high sediment yields in intense late summer storms,
but had a much more moderate response in lower inten-
sity storms in the autumn, although Garcıa-Ruiz et al.
(2008) found less variability in subhumid badlands in the
same region. Torri et al . (1999) explained the effects of
high rainfall intensities as being due to rapid mechanical
crusting, producing a rapid decrease in infiltration rates
(Chapter 11), together with the same sort of decrease in
surface roughness observed by Wainwright (1996a). For
the soils of Torri et al ., the dominant effect on infiltration
and thus runoff occurred once a cumulative rainfall energy
of about 0.5 kJ/m 2 had been reached. These studies all
point to important variations relative to when a particular
storm occurred. Kasanin-Grubin and Bryan (2007) took
this idea further in demonstrating that the seasonal dynam-
ics of rills in Alberta, as discussed above, relate to changes
in surface conditions that are a function of the duration
and intensity of rainfall input. Short wetting cycles lead
to 'popcorn'-type surfaces on smectite-rich mudstones,
while longer wetting cycles will produce a breakdown
of aggregates and thus much smoother surfaces (Figure
10.9). Different patterns of storms in different years could
Cycle 1
Cycle 3
Cycle 5
Cycle 7
Cycle 10
10 min
30 min
60 min
Search WWH ::




Custom Search