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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
much degraded, that the most erodible material is removed and the rill's bottoms still reached
the Flysch-plates . But it is a fact, that even in the slump material rills are developed. But for
developing or increasing much higher water quantities and runoff intensities are needed.
Considering the influence of initial soil moisture, one experiment consists of two runs. The
influence of initial soil moisture was accounted by GOVERS et al. (1990) and GOVERS
(1991). In their experiments under dry conditions the sediment concentrations almost reached
the runoff's transport capacity, under higher initial soil moistures, the sediment concentrations
were lower. In the rill experiments in the Arnás-catchment, the transport capacity was not
reached because of the lack of transportable material and the low water amount. The influence
of the initial soil moisture was clearly to notice, in the second run under saturated conditions
the flow velocities and the runoff values were higher than in the first run with a soil moisture
of about field capacity caused by snow melt . The starting moistures of the rainfall
simulations were between 22 and 24 grav. %. But some experiments also show higher
sediment concentrations under the higher initial soil moisture conditions in the second run
(almost saturated). Under wet conditions, the flow velocities are higher and the cohesion
forces are weaker, but there is less loose material in the rill. Under dry conditions, (in this
case: less wet) the sediment concentrations are higher if there is a high quantity of lose
material in the rill, if all this material is removed in the first run and if the higher flow
velocities in the second run cannot cause an incision into the rill’s bed.
5 Conclusion
Three rill experiments according the described method were executed in the Arnás-catchment
in the Pyrenees.
In these experiments, as well naturally developed rill, anthropogenically induced rills like rills
developed from motorcycle trails as real lanes were tested. The used 72 liters of water, that
were disposed with an intensity of 9 l min -1 into the rill, caused material loss in the rill, but the
dimensions were really different. There were samples without detectable sediment, the highest
sediment concentrations reached values of 6.31 g l -1 . These results indicate that in the Arnás
catchment, the rills have developed and are still active today under higher runoff intensities
than the ones used in the experiments.
From the induced 72 l of water, between about 8 and 49 l reached the end of the test length,
what means a rate between 11 and 68 %. Is the passed flow length accounted, r-l-factors of
between 2.1 and 17.1 were calculated. This factor is necessary, because in the third
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