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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
b - 1 - 0:30 0 5.8 0 91.5 0 92.9
b - 1 - 1:30 9.7 0 0 90.4 0 58.9
b - 1 - 2:30 0 0 0 87.5 0 22.9
b - 2 - 0:00 94.9 62.1 0 97.3 0 92.6
b - 2 - 0:30 0 0 0 88.7 0 59.4
b - 2 - 1:30 0 0 0 84.5 0 96.4
b - 2 - 2:30 0 0 0 83.9 0 82.3
b - 3 - 0:00 0.5 42.2 59.0 99.7 0 0
b - 3 - 0:30 32.5 0 44.1 99.4 0 72.4
b - 3 - 1:30 0 0 0 99.4 0 80.9
b - 3 - 2:30 0 0 0 99.3 0 84.1
Mean 41.4 16.0 15.7 94.0 6.0 76.1
Mean 24.3 58.7
Mean 41.5
4 Discussion
A comparison with results of other research groups shows that the measured values are in a
realistic range. Ghebreiyessus [3] measured shear stress values up to 40 Pa, in the experiments
of Nearing et al. [4] Reynolds numbers up to 100000 and unit stream power values up to 10 m
s -1 were reached. Giménez & Govers [5] found unit stream power values up to 0.4 m s -1 and
unit length shear force values up to 6 N m -1 , in the paper of Zhang et al. [9], shear stress
values up to 30 Pa and unit stream power values up to 0.5 m s -1 are published. Govers [13]
measured shear stress values up to 100 Pa and effective stream power values up to 10000 W
m -1 . The presented measurements are in the same order of magnitude and therefore realistic.
But in contradiction with the cited papers, there are no clear linear correlations between
hydraulic parameters and erosion parameters in the results of the field experiments. Therefore,
these outcomes indicate that linear models are not applicable for description of experimental
processes in natural rills. The question to be answered is: why doesn’t this concept that has
been used for over 30 years work?
In literature, four possible reasons can be found. (I) Unclear calculation and definition of the
used parameters, (II) disregarding the influence of turbulence, (III) responsibility of processes
which are not controlled by shear stress for material transport and (IV) a high spatial and
temporal variability of those processes.
The first reason could be the unclear calculation of shear stress. It is to differentiate between
flow shear stress τ, a hydraulic parameter, and critical shear stress τ c or τ cr , a soil parameter
which is similar to soil strength. Shear stress exerted by flow must exceed the critical shear
stress to cause erosion.
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