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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
L of water were induced with an intensity of 9 L min -1 into the rill. Flow velocities, sediment
concentrations, rill cross sections, water levels, runoff values and slope were measured and
different hydraulic parameters were calculated. It was noticeable that, despite the low inflow
intensity, extremely high sediment concentrations were reached in some cases, with the
transport of loose material having in some cases a higher, and in other cases a lower,
influence on the sediment budget. In one experiment, where only low erosion activity took
place, the measured sediment concentrations were mainly caused by the removal of the loose
material. The runoff effectiveness showed very different values, and the higher effectiveness
was reached in each case under conditions of higher initial soil moisture.
During the experiment, the main sediment delivering processes were observed to be retreat
erosion at knickpoints or steps and bank failure processes. Depending on the tested rill, a
simple cutting in the rill's bottom also played an important role. In some cases, the high
sediment concentrations during the first run indicate that the mobilisation of loose material
may be important too. This may lead to loose material dominating the amount of transported
material unless the other processes described above occur.
In most cases, the erosion rate was near the transport capacity. In each experiment, the
transport capacities and the sediment loads showed high variability even at a single measuring
point. In some cases, samples exceeded the traditionally calculated transport capacity. This
behaviour is a clear indicator of variable processes and process intensities in the natural
environment. The limitation of detachment or transport equations is that important sediment
preparing processes, like headcut retreat, step-pool effects or bank failure, are not considered.
Measured total rill erosion rates are the sum of erosion rates caused by a combination of
different soil erosion processes with different spatial and temporal distribution. There are two
different ways. The first way is to modify existing physical based model concepts or, the
second way, to start a new direction, the concept of stochastic soil erosion modelling. In both
cases, much more field experiments are needed to provide the required data and the question
is, if the actual used experimental setups can deliver the requested data or if a totally new
experimental setup must be created.
The novel method for monitoring rill processes presented here allows a lot of data which are
useful for understanding the behaviour of rills in a catchment area to be directly measured or
calculated from the measured data.
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