Thresholds in small rivers? Hypotheses developed from fluvial morphological research in western Germany
Harnischmacher, Stefan
Department of Geography, Ruhr-University Bochum, 44780 Bochum, Germany
Geomorphology, Volume 92, Issue 3-4, pp. 119-133.
Abstract
The objective of this study was the formulation of fluvial morphological regularities for small rivers with a wide range of morphological and geological characteristics inNorth-Rhine-Westphalia (Germany) based on a statistical research methodology. Such empirical quantitative information on reference conditions is required for the restoration of small rivers in the former highly industrialised Ruhr-Area. Following the approach of some classic empirical works in fluvial morphology of the last century, several natural reference rivers in the entire research area have been observed in order to provide a statistical correlation between independent and dependent morphological variables. Regressions between valley-floor slope, bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and river planform on the other hand have shown some significant results. The regularities found are a quantitative contribution to the establishment of reference conditions as well as a useful tool for the restoration of small rivers, if the specific properties and values of the underlying random sampling are taken into account. In addition, the relation between stream power and sinuosity shows the likely existence of a threshold: Exceeding a stream power of 100 W/m, the sinuosity decreases after an increase for lower stream power values. Comparable thresholds were found for the relation between stream power and pool depth as well as stream power and step steepness. The thresholds could be explained by a change in the type of energy dissipation, due to different physio-geographical settings in highland rivers within forested v-shaped valleys. Here, large-woody debris seems to increase the channel roughness and possibly replaces the significance of coarse-grained bed material, pool depth and step steepness as contributors to energy dissipation.
Keywords: River restoration; Fluvial morphology; Correlation; Threshold; Energy expenditure
1. Introduction and objectives
The Ruhr-Area, a former highly industrialized region in the western part of Germany, extending over parts of the northern lowlands (Westphalian Bight) and the southern highlands (“Suederbergland”) (Fig. 1), has been dominated by coal mines and steel mills from the second half of the 19th to the end of the 20th century. In the early periods of industrialization the waste water of households and industries was dumped into the surface waters, leading to epidemics in the cities with high population densities, when floods occurred in areas affected by coal mining subsidences (see Peters, 1999). As a consequence, in the year 1899 an association was founded, the so called “Emschergenossenschaft”, entrusted with the disposal of waste water in concrete channels on the earth surface in order to avoid possible damage of sewer tunnels and subsurface contamination. Nowadays many coal mines are closed and the mining subsidences are weakening, especially in the southern and central parts of the Ruhr-Area.
Fig. 1. Simplified geological map of North-Rhine-Westphalia and its position in Germany. The numbering of the study reaches corresponds to the numbers in Table 1.
It is now possible to dispose of the waste water in sewer tunnels and to restore the concrete channels to natural conditions (see Londong, 1993, 1995). Fig. 2 shows an example of a restored river in the city of Castrop-Rauxel, the Deininghauser Bach. Within the former concrete channels sewer tunnels were laid, covered with earth material and used as footpaths. The restored river channel was constructed left of the footpath, flowing on a wide floodplain, the plants of it are partly protected by fences. When small rivers in the Ruhr-Area are restored, questions concerning reference conditions of the fluvial morphology arise. Especially quantitative information about channel depth, width, riffle-pool-sequences or meander belt width is required by the engineers of the commissioned environmental offices.
The objective of the present study was to develop quantitative, empirically derived morphological regularities by investigating independent and dependent morphological variables and their possible correlations in undisturbed small rivers (Harnischmacher, 2002; Harnischmacher and Zepp, 2005). According to the classical works of fluvial morphology, especially in Great Britain and the USA (for an overview see Hickin, 1983), an empirical approach was used: 32 study reaches at 29 small reference rivers, which are assumed to be in a dynamic morphological equilibrium, were selected. The study reaches are located in lowland (Westphalian Bight, see Fig. 1) and mountain regions (“Suederbergland”) adjacent to the river Ruhr. In a next step the fluvial morphology of the study reaches was surveyed (Fig. 3). From the results of the survey, values of the independent and dependent variables were determined, before the calculation of correlations and regressions helped to derive quantitative regularities. Finally, the applicability of the regularities as a support of river restoration projects was discussed.
4. Conclusions
Summarizing the observations and hypotheses described above, the following conclusions can be drawn: Significant regressions between valley slope, bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and the river planform on the other hand were found. These regularities offer a quantitative contribution for establishing reference conditions as well as a useful tool for the restoration of small rivers. Nevertheless, it is important to pay attention to the specific properties and values of the underlying random sampling when the regularities shall be transferred to different regions. Moreover, in many cases a scattering of variable values appeared, reflecting the influence of geological conditions, man-induced disturbances or large-woody debris. Thresholds were possibly found for relations between the independent variable stream power and the dependent variables sinuosity, pool depth and step steepness. These thresholds could be explained by a change in the type of energy dissipation, due to different physiogeographical settings, like bed material qualities or the occurrence of woody debris. Large-woody debris increases the channel roughness in highland rivers and possibly replaces the significance of coarse-grained bed material as a contributor to energy dissipation, because the step steepness decreases in the headwater areas of the rivers under investigation. As a conclusion, in addition to the observations cited by Chin and Wohl (2005), the large-woody debris possibly does not only accentuate the step height, but seems to represent an important independent roughness element in highland rivers contributing more to spill resistance than clast steps.
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