Stefan Wirtz Vom Fachbereich VI (Geographie/Geowissenschaften ...

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
We measured distance and direction of the rock fragment translocation in experiment 3. The
mean translocation distance of the 3 experiments only reaches 8.8 cm, thus being much lower
than the mean displacement distance of 38 cm on Lesvos (Greece) that was determined by
OOSTWOUD WIJDENES et al. (2001). However, this rather short distance is a consequence
of the considerably flatter slopes of 4° - 11° in the present study vs. 2° - 24° in OOSTWOUD
WIJDENES et al. (2001). It is important to notice that OOSTWOUD WIJDENES et al.
(2001) measured the 38 cm after one year, the grazing peak is between April and June. It is
not reconstructible how far the rock fragments were displaced after a certain number of sheep.
Additionally, they corrected the negative distances (upslope movement) by adding the largest
negative distance plus one centimeter to all negative distances. This transformation resulted in
a slightly overestimated mean. Our smaller rock fragments are probably not the reason for
these different results, as OOSTWOUD WIJDENES et al. (2001) showed that the rock
fragment displacement distance is not depending on mass. The study started in May and in the
first 7 months the displacement distances are lower than in the second 5 months
(OOSTWOUD WIJDENES et al. (2000). In the 5-month period, only one of the grazing peak
months is included; in the 7-month period two of them. That means despite longer measuring
times and despite a higher trampling intensity, the values are higher in the 5-month period. A
reason could be the winter months December and January, in which other erosive processes
influence the displacement distances. This fact emphasizes the need for short-term
experiments to isolate the influence of animal trampling.
GOVERS & POESEN (1998) measured an average horizontal displacement distance of 38 cm
for rock fragments on slopes between 27° and 31°. But the test sites had different widths
(between 1.4 m and 4.1 m) and the number of sheep and goats ranged between 22 and 100. To
make the measured values comparable, they calculated a unit displacement distance in cm per
animal and meter. They presented values between 0.47 cm and 5.6 cm. The highest value was
calculated for the smallest plot, the lowest value for the largest plot. Again, we used a plot
width of 0.6 m and 45 goats and measured an average distance of 8.8 cm. Our test plot was
thus smaller than the smallest one used by GOVERS & POESEN (1998). Calculating the
same unit displacement distance (displacement distance / (number of animals / plot width))
we receive a value of 0.1 cm. Once more, slope is probably responsible for this much lower
value; its effect covers the fact that the plot width of the smallest Govers-Poesen-plot was
twice as large as our plot width. In our experiments, the marked rocks were positioned on top
of given overlying or embedded rock fragments. This means our rocks were probably easier to
move than naturally occurring rock fragments. Even after considering this, our values are still
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