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

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen shear stress, unit length shear force or stream power. The process of gullying, the retreat erosion at knickpoints and headcuts is not considered in rill erosion formulas. This process caused about 12% of rill erosion rates in the study of Govers (1987). In our experiments, we only cause mass wasting and gullying processes, so the relations between hydraulic parameters and sediment concentration are mostly low. But the hydraulic rill erosion only occurs in extreme runoff events, in most cases, the runoff values are too low to cause this process (Govers 1987). All these observations agree with our own observations and measurements. In these cases, we can also observe that the areas with high erosion rates exceed the calculated transport capacities and show a trend towards an increase with flow duration. This suggests that sediment production by knickpoints, chutes, etc. is important for understanding rill behaviour. Nevertheless, they are highly variable throughout a single rill, and the combination of the processes can change between run a and run b of one experiment. The different processes can be identified by comparing the sediment concentrations, the flow velocities and the absolute level of the sediment concentrations. The first process is the “bulldozer-effect” (Seeger et al., 2005, Regües et al., 2000), a mobilisation of the loose material available in the rill before an experiment starts. The highest sediment concentration at the MP is collected with the water front despite the lowest flow velocity. This effect can be detected in the first run of the Negratin experiment. The flow velocity in run a and run b are close but in the first run the average sediment concentration is clearly higher. The material transport within the second run has to be attributed only to the erosion effect of the flowing water (bulldozer-effect (marginal) + erosion (low)), since the loose material has removed in the first run, mostly by the water front (bulldozer effect (high) + erosion (low)). In run a, the bulldozer-effect is more powerful than the erosion process. If the erosion process is more powerful, the sediment concentrations increase at the MP from the first to the last sample. The waterfront only mobilizes the available loose material, but because of the low flow velocity, the erosion process starts later and the sediment concentration increases (bulldozer-effect (lower) + erosion (higher)). This can be observed in the first run of the Freila experiment. In the second run, the sediment concentrations are lower, because the bulldozer-effect is able to mobilize only the material left behind after run a (bulldozer-effect (marginal) + erosion (high)). If there is no available loose material and the erosion process is very active from the beginning, the sediment concentrations increase at each measuring point and the run with the higher flow velocity shows the higher average sediment concentration. This was given in the Salada experiment: run a (bulldozer-effect (marginal) + erosion (high)) had a lower average flow velocity and a lower average sediment 117
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen concentration than run b (bulldozer-effect (marginal) + erosion (even higher)) with a higher average flow velocity and also a higher average sediment concentration. In some rills the erosion process could not be activated with the inflow intensity and water quantity used. Even the higher flow velocity in the second run was not sufficient to start erosion processes. In this case the available loose material is removed by the slowly flowing waterfront of the first run. This situation occurred in the Belerda experiment, where the highest sediment concentrations were measured at the waterfront run a (bulldozer-effect (high) + erosion (marginal)), while in run b both the bulldozer-effect and erosion processes were very low (bulldozer-effect (marginal) + erosion (marginal)). The differences in runoff-length-coefficient between the two runs can be seen clearly. In Salada, the RC L factor in run b is about twice as high as in run a, in Salada even the factor 3 is observed. In Belerda, the differences between the two runs are not significant, but the absolute values show that this rill is an effective runoff collector for the gully. In Freila only the first run could be measured, but this first run had the highest RC L factor of all runs. This is likely due to the fact that the rills in this area show a low infiltration rate, which corresponds to the high bulk densities measured. It also shows that, in these areas, flow concentration and the runoff accelerating effect of the rills within the landscapes are considerable regardless of the erosion processes inside the rills. Differences in the flow velocities of the water fronts between the two runs are particularly noteworthy. The velocities of the waterfronts in the first runs are clearly lower than all other velocity curves. This may be attributed, in part, to the higher transporting activity the water has to perform during the first runs, combined with a higher infiltration rate in most cases, resulting in considerably higher loss of water mass during the experiment. But the other, and surely most important, factor is that of initial soil moisture as was reported also by Govers et al. (1990) and Govers (1991): higher soil moisture reduces the infiltration rate which results in more water flowing along the experimental length. Higher soil moisture at the surface layer of the soil reduces the water repellency of the soils. This can be important for the dynamics of the water front under dry conditions. This was observed, even under very moist conditions, in the Pyrenees (<strong>Wirtz</strong> et al., 2010). Here we have that the observed velocities during the first run may underestimate the runoff during a rainfall event, as the rain producing the runoff would moisten the rill at the same time, thus generating the conditions for fast flow. The influence of initial soil moisture on sediment transport was reported by Govers et al. (1990) and Govers (1991). In their experiments under dry conditions the sediment concentrations almost reached the runoff's transport capacity while, under higher initial soil 118
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Stefan Wirtz Vom Fachbereich VI (Ge
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Danksagung Danksagung Die vorliegen
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Inhaltsverzeichnis Inhaltsverzeichn
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Inhaltsverzeichnis Abbildungsverzei
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Experimentelle Rinnenerosionsforsch
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Dipl. Geogr. Stefan Wirtz; Physisch
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Stefan Wirtz Vom Fachbereich VI (Geographie/Geowissenschaften ...

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