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

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen Applicability of different hydraulic parameters to describe soil detachment in eroding rills <strong>Wirtz</strong> S. a , Seeger M. a,b , Zell A. c , Wagner C. c , Wagner J.-F. d , Ries J.B. a a: Dep. of Physical Geography, Trier University, Germany; b: Dep. of Land Degradation and Development, Wageningen University, The Netherlands; c: Dep. 7.3- Technical Physics, Saarland University, Germany; d: Dep. of Geology, Trier University, Germany Abstract This paper presents the comparison of experimental results with assumptions used in numerical models. The aim of the field experiments is to test the linear relationship between different hydraulic parameters and soil detachment. The correlations between shear stress, unit length shear force, stream power, unit stream power and effective stream power and the detachment rate show that there is not a single parameter that continuously displays the best correlation. The best match not only changes from one experiment to another, but also from one measuring point to another. Different processes in rill erosion are responsible for the changing correlations. However, not all these procedures are considered in soil erosion models. Hence, hydraulic parameters alone are not sufficient to predict detachment rates. They predict the fluvial incising in the rill's bottom, but the main sediment sources are not considered sufficiently in its equations. The results of this study show that there is still a lack in understanding of the physical processes underlying soil erosion. Exerted forces, soil stability and its expression, the abstraction of the detachment and transport processes in shallow flowing water are still subject of unclear description and dependence. Keywords Soil erosion, rill hydraulics, shear stress, rill erosion, dynamic viscosity, field experiments 1 Introduction Soil erosion models use different composite factors to describe and predict soil detachment and transport capacity. The factors used most frequently are average shear stress [1-4], unit length shear force [5], stream power [4, 6-9], unit stream power [10, 11] and effective stream power [12, 13]. Giménez and Govers [5] define the different hydraulic parameters as follows: 179
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen (1) shear stress is the drag force exerted by the flow on the bed, (2) stream power is defined as the energy of the flow dissipated to the bed based on total discharge, (3) unit stream power is a product of slope and flow velocity, (4) the effective stream power can be seen as the power over a given rill cross section and (5) the unit length shear force is a hydraulic parameter calculated on a unit length basis [5]. In most cases, a linear equation describes the relation between the hydraulic parameters mentioned above and the detachment rate. By exceeding a certain threshold, erosion by concentrated flow begins and detachment rate increases. This threshold has a positive x-axis intercept, which means, there is no detachment below this point. Another option is to consider concentrated flow erosion as a nonlinear threshold phenomenon or as a two-part linear threshold phenomenon: below the threshold soil detachment takes place (first linear relationship) but after exceeding the threshold, detachment rate increases much faster (second linear relationship) [14]. But is this linear relationship really suitable? Knapen et al. [14] calculated the correlation between shear stress, unit length shear force, stream power and Reynolds number and the detachment rate from several WEPP datasets. The best average correlation was determined for stream power with R² = 0.59. The WEPPused shear stress is a variable that reaches only low R² values for all of the tested data sets. Knapen et al. [14, p. 80f.] describes the shear stress as follows: “Although the use of flow shear stress as soil detachment predictor can be contested, critical shear stress (τ cr ) and concentrated flow erodibility KC (...) have been selected as the most universal parameters to describe soil erosion resistance to concentrated flow.” The correlations between these factors and the soil detachment rate show very different results. There is not a single parameter that always shows the best correlation. These considerations lead to 2 main questions: - Are soil erosion, detachment and transport, directly dependent on water flow characteristics? - Are these concepts, as implemented in soil erosion models, suitable to describe rill erosion? These questions have been tackled by many research groups that have been searching for the equation that suits their observations best [1-13, 15-42]. However, taking into consideration the numerous and variable results, a deeper insight into the rill erosion processes on hillslopes is needed. To get this insight, different strategies can be applied [43]: (I) Modelling, (II) laboratory experiments (III) field observations and (IV) field experiments. Each of these methods shows different advantages and disadvantages. 180
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Stefan Wirtz Vom Fachbereich VI (Ge
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Danksagung Danksagung Die vorliegen
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Dipl. Geogr. Stefan Wirtz; Physisch
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Stefan Wirtz Vom Fachbereich VI (Geographie/Geowissenschaften ...

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