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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
literature, we did not homogenize the labelling of the different parameters, but used the ones
given by the authors. That means different parameters can be labelled equally. As it can be
seen in Table 5, for the calculation of shear stress exerted by flowing water, many different
parameters have been taken into account. Even the physical definition of the parameter “shear
stress” seems to be unclear. Some factors have been developed from empirical studies
(equation 10, equation 11). In most cases, the theoretical basis of the equations is not clear.
Equation 12 is derived from Landau and Lifchitz (1971). The critical shear stress is the force
needed to detach a soil particle, so it corresponds to a soil parameter and therefore input for
calculation should also be depending on soil characteristics. In the WEPP model (Table 6,
equation 26-28), the critical shear stress is calculated using only soil parameters like texture,
organic matter content and dry bulk density. However in some equations the so called
“critical” shear stress consists of hydraulic parameters like water depth, water width or fluid
density (Table 6, equations 17-20). In equation 21 hydraulic and soil parameters are used
equally. In equation 22, 23 and 24 the empirical nature of the development is clear. According
to equation 23 and 24 the critical shear stress is only dependent on a constant value and the
relationship between given particle size and the subsurface d 50 . In equation 26, the typical
parameters for calculating shear stress of flowing water are used to define critical shear stress.
This method uses additionally the Manning friction factor, which is a descriptor of the soils
surface roughness. Thus, the use of the term “critical shear stress” seems to be very unclear.
The different equations for detachment and transport capacity were developed from data sets
created from controlled laboratory experiments, field observations or field experiments. In
equations 29-32 (Table 7), neither critical shear stress nor shear stress is used for calculation
of the transport capacity. In equation 33, shear stress is used to calculate transport capacity, in
equation 34 transport rate is calculated using shear stress and critical shear stress and in
equation 35 the detachment capacity is calculated using critical shear stress and shear stress
(see Table 7). Equation 33 is a modification of the Yalin (1963) equation from 1963 (Foster et
al., 1995). In equation 34 and 35 it is clear to see that shear stress and critical shear stress are
opponents, the important parameter is the difference between these two variables. If critical
shear stress is higher than shear stress, no erosion can occur. A summary of these equations is
given by Reid and Dunne (1996), on the EPA-homepage (2009) and in Hessel and Jetten
(2007).It has been shown that the physical definition of the parameters is not clear and their
mostly empirical foundation does not fit the high temporal and spatial variability of processes
within a rill. Different processes dominate at different intensities and this fact causes high
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