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

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen For calculation of shear stress exerted by flowing water, hydraulic parameters, roughness, flow velocity and fluid density have been taken into account. The actual version of the shear stress equation calculates the average shear stress by depth averaging of momentum equation for steady uniform flow per area and time, but this definition doesn’t fit to all definitions used in literature. Some factors have been developed from empirical studies [15-26]. In most cases, the theoretical basis of the equations is not clear. The formula used in Chisci et al. [27] is derived from Landau and Lifchitz [57]. Other versions of the Landau-Lifchitz equation are to find in literature [2-5]. The critical shear stress is the force needed to detach a soil particle. So it corresponds to a soil parameter and therefore, input for calculations should also be depending on soil characteristics. Only in the WEPP model [34], the critical shear stress is calculated using only soil parameters like texture, organic matter content and dry bulk density. But in some equations the so called “critical” shear stress consists of hydraulic parameters like water depth, water width, Manning friction factor or fluid density [16, 28, 33], hydraulic and soil parameters are used equally [34]. In the studies of Partheniades & Paaswell [30], Andrews [31] and Andrews & Erman [32] the empirical nature of the development is clearly noticeable. That means the equations are not deduced from physical laws but from empirical studies. In many studies [12, 35, 37-41], neither critical shear stress nor shear stress are used for the calculation of the transport capacity. In other studies shear stress is used to calculate transport capacity and detachment capacity [36] or transport rate [42] and critical shear stress to calculate the detachment capacity [36]. In both cases it is clear that shear stress and critical shear stress operate against each other, the important parameter is the difference between these two variables. A summary of these equations can be found in Reid and Dunne [66], on the EPA-homepage [67] and in Hessel and Jetten [68]. The second reason for the bad correlations between hydraulic parameter and soil detachment can be the lack of turbulence parameters in the equations. In the study of Knapen et al. [14] the Reynolds number shows very different correlations to the detachment rate, the same is to notice in the given results of this study. The reason could be that the turbulence, described by the Reynolds number, does not directly operate on substrate, it influences the acting shear stress, that means the calculated shear stress is much lower than the operating shear stress. This relation has been confirmed in several studies. Nearing et al. [69] found that turbulence can increase the active shear stress by a factor of several thousands. They measured flow shear stresses ranged from 0.5 to 2 Pa, while tensile 195
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen strengths ranged from 1 to 2 kPa, a difference in magnitude of 1000. Despite this conflict, detachment rates of nearly 300 g m -2 s -1 were measured. He explained this result with turbulent burst events which are much greater than the average flow shear stresses. Another study about the influence of turbulence on detachment rates was published by Nearing & Parker [70]. They showed that under turbulent flow conditions the same shear stress value caused a clearly higher detachment rate. In their flume experiments the difference between detachment rate caused by turbulent and laminar flow increased with increasing shear stress value. That means, if given hydraulic conditions lead to a high shear stress value, the influence of turbulence on soil erosion is higher than in low shear stress value ranges. The shear stress equation as well as those related to the other hydraulic parameters assumes that drag forces are dominant for controlling erosion. But rill erosion is the result of the combination of different processes including headcut erosion, sidewall sloughing, tunnelling, micro-piping, slaking piping and sapping [14, 45, 71-75]. This is the third reason for the problems of the model equations. The percentage of headcutting in the different studies ranges between “four times higher than the contribution of bed scours” [75] to “60 % of total rill erosion” [76]. <strong>Stefan</strong>ovic and Bryan [77] showed that concentrated flow causes sediment production primarily from knickpoints, chutes, meanders and bank failure. Govers [45] distinguished between hydraulic erosion, mass wasting processes on rill sidewalls, gullying and piping. Hydraulic rill erosion mostly occurred during three extreme runoff events. Mass wasting processes caused 37 % of total erosion in rills. Gullying, the retreat erosion at knickpoints and headcuts caused about 12% of rill erosion rates. In the presented experiments, mainly mass wasting and gullying processes occurred, so the correlations between hydraulic parameters and detachment rate 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. The percentage of material which is transported independent of shear stress is very high on the water front samples. Here the transport of loose material is probably more important than in the other samples. That means this process is mainly independent of shear stress. In these cases of transport rate vs. transport capacity < 1 the independence of shear stress cannot be excluded, in the other cases the processes controlled by shear stress can occur. It is to conclude that in the case of T R > T C , not only shear stress controlled processes provide the material; at least the difference between T R and T C is caused by processes independent of shear stress. The presented experiments show that the correlation between hydraulic parameters and detachment rate does neither change from one experiment to another nor from one run to 196
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Stefan Wirtz Vom Fachbereich VI (Ge
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Danksagung Danksagung Die vorliegen
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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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