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Quantification of mining subsidence in the Ruhr District (Germany)

Quantification de la subsidence d’origine minière dans la région de la Ruhr (Allemagne)
Stefan Harnischmacher
p. 261-274

Résumés

Cet article présente la variabilité spatiale des altitudes dans la région de la Ruhr suite à l’extraction du charbon à grande échelle au cours des 100 dernières années. A l’aide d’un système d’information géographique, les données altimétriques mentionnées sur les cartes historiques de 1892 ont été numérisées. Interpolée, cette surface de référence a été comparée au modèle numérique de terrain actuel afin de calculer les différences d’altitude. Les résultats montrent que les valeurs de subsidence les plus élevées, se chiffrant à plus de 25 m, sont observées dans les bassins miniers de l’ancienne mine de charbon de « Zollverein » qui se distingue par sa longue histoire minière et son statut de patrimoine mondial. L’analyse détaillée des villes d’Essen et de Dortmund révèle que les dépressions associées à d’autres formes de terrain sont touchées par les affaissements miniers. Ce type de transformations de surface n’est pas visible sur le terrain et nécessite une comparaison de modèles topographiques numériques afin de les mettre en évidence. La valeur moyenne de l’abaissement de la surface a été calculée à partir de l’ensemble des cartes numérisées et analysées : elle varie de 0,51 m sur la carte de Kamen, ville située à la frontière orientale de la région de la Ruhr, à 5,16 m sur la carte de Gelsenkirchen, dans la plaine alluviale centrale de la Emscher d’une surface totale de 128,5 km². L’abaissement moyen de la surface totale de la région étudiée (environ 2700 km²) est de 1,6 m.

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Notes de la rédaction

Article soumis le 29 octobre 2009, accepté le 10 juin 2010

Texte intégral

Texte intégral en libre accès disponible depuis le 01 octobre 2012.

The research project was supported by the land registry office of the Federal State “North Rhine-Westphalia” (Germany). The author thanks Dr. Annette Schiller (Schiller Translations, Dublin, Ireland) for proofreading the manuscript in English, and Dr. Denis Mercier (university of Nantes, France) for the editing of the text in French. The valuable comments of the reviewers are gratefully acknowledged. Last but not least, I am grateful to the editors of the journal and Dr. Dénes Lóczy (University of Pécs, Hungary), chair of the IAG/AIG working group “Human Impacts on the Landscape (HILS)”.

Introduction

1The Ruhr District in the western part of Germany, formerly a major industrial region, was dominated by coal mines and steel mills from the second half of the 19th to the end of the 20th century. Early primitive open pit coal mining was recorded for the first time in the year 1298 (Dege and Dege, 1983). Over decades the area has developed into a conurbation with many different service industries. Due to the process of de-industrialisation, the cities in the Ruhr District succeeded in compensating for job losses in the secondary sector by attracting new jobs in the tertiary sector (Bronny et al., 2004). Remnants of coal mining activity are still visible in the Ruhr District particularly in the form of numerous waste heaps (Harnischmacher, 2007). Some of them such as one of the biggest mining waste heaps at the border of the cities Recklinghausen and Herten, the mining waste heap “Hoheward”, have already been developed as a landscape structure, well integrated into the surrounding area, meeting ecological as well as recreational demands. Negative effects on nature functions and biotopes caused by the heaping operation were compensated for by adequate measures (Hofmann and Winter, 1991). Furthermore, landscape-related leisure and recreation aspects were considered by planning pathways, viewpoints or attractive artworks. Besides positive forms of landscape disturbances such as heaps, there are numerous negative forms resulting from ground surface subsidence frequently filled with water. One example is the lake “Lanstrop” in the city of Dortmund, localised in depression following mining activities in a coal mine nearby which was shutdown in 1985 (Bell et al., 2000). Today, this subsidence lake has been turned into a nature reserve and a popular recreation area. Mining subsidence in the Ruhr District causes severe problems not only for buildings, streets and service pipes, but especially for drainage capability, because the flow direction of rivers has been disturbed and in part reversed. In order to protect the Ruhr District from severe flooding problems much of the area has to be drained by a large number of pumping stations, although mining has ceased in the southern and central part of the Ruhr District (Peters, 1999). In some locations not only troughs, wetland areas or lakes occur in the most subsiding areas but also larger hills like the “Mechtenberg” in Essen, which is located in the middle of a subsidence area with a surface lowering of approximately 20 m. The study presented here has focused on the quantification of the magnitude and the distribution of mining subsidence in the Ruhr District and has also allowed for the identification of subsidence areas, which are less striking due to different surface features. Since area-wide data on mining subsidence is not available or was not investigated to date, our project has led to the first large-scale and systematic collation of data on mining subsidence in the entire Ruhr District. As a contribution to the quantitative approach of anthropogeomorphology, a comparison of mining induced surface lowering rates with denudation rates of rivers will finally be discussed.

Study area, mining history, longwall mining and mining subsidence

2The Ruhr District is a conurbation with an overall population of 5.3 million living in eleven municipal authorities and four districts (Bronny et al., 2004; Regionalverband Ruhr, 2006; fig. 1). It extends from the river Rhine in the west, to the river Ruhr in the south and borders rural areas in the north and east. Its west to east extension is 116 km, from north to south it measures 67 km. The total area of the Ruhr District amounts to 4435 km2. The Ruhr District covers at least three significant landscapes with different surface characteristics (Liedtke, 1993; Boldt and Gelhar, 2008): to the west, there is the Lower Rhine Plain with the lowest point within the Ruhr District (13 m a.s.l.). The Westphalian Embayment, consisting of Upper Cretaceous strata in the underground, partly covered by ground moraine, loess and fluvial deposits, is located in the central and the northern part (Hahne, 1965). It is divided by two major rivers crossing the Ruhr District from east to west opening into the river “Rhine”: the river “Lippe” in the northern part of the Ruhr District and the river “Emscher” in the central part. In the south of the Ruhr District follows the low mountain range of the so called “Süderbergland” with a maximum elevation of 444 m a.s.l., composed of faulted and fractured Carboniferous siltstones, sandstones and schist, with coal seams in the Upper Carboniferous (Richter, 1996). The Upper Carboniferous strata dips down to the north, where it is covered by the younger Upper Cretaceous of the Westphalian Embayment with an increasing thickness of up to 1000 m north of the river “Lippe” (Drozdzewski, 1993). The river “Ruhr” crosses the low mountain range from east to west and flows into the river “Rhine”.

Fig. 1 – Map of the Ruhr District and location of digitised historical maps.
Fig. 1 Carte de la région de la Ruhr et localisation des cartes historiques numérisées.

Fig. 1 – Map of the Ruhr District and location of digitised historical maps. Fig. 1 – Carte de la région de la Ruhr et localisation des cartes historiques numérisées.

Insert: Location of the Ruhr District in Germany. 1: Ruhr District; 2: digitised map; 3: river; 4: district or city.
Carton de localisation : la région de la Ruhr en Allemagne. 1 : région de la Ruhr ; 2 : carte historique numérisée ; 3 : cours d’eau ; 4 : arrondissement rural ou cité.

3Mining began in the south of the Ruhr District in the Ruhr valley and its tributaries, where the coal is exposed at the surface within the outcrops of the Upper Carboniferous strata. At the beginning, in the 13th century, the coal was mined in shallow open pits, later on in primitive tunnels, and used for forging (Wiggering and Zimmermeyer, 1993; Drozdzewski and Koetter, 2008). The mining in shafts began in the 18th century, before the invention of the steam engine allowed for the dewatering of coal mines and deep-seam mining underneath the Cretaceous strata north of the Ruhr valley, within the so called “Hellwegzone” in 1840 (Wegener, 1998; Huske, 2007). From that date on, the Ruhr District developed from a rural area to the famous industrial region, where coal mines, the development of railways and steam boats as well as the establishment of steel works with its coke blast furnaces contracted a driving coalition in the second half of the 19th century (Steinberg, 1988). At the same time, coal mining gradually moved northwards into the central part of the river “Emscher” floodplain, already reaching the river “Lippe” in the north of the Ruhr District during a time of prosperity in the first decade of the 20th century (Steinberg, 1995). After 1920, the mechanisation of mining and especially the demand for energy and steel just before the Second World War led to the highest coal production in the history of the Ruhr District in 1939 (130 Mt) and a total number of 151 coal mines (Wiggering and Zimmermeyer, 1993 ; Huske, 2006). Up to the 1950s, hard coal remained the dominant source of energy and it also became an important raw material for the chemical industry. After the Second World War, the coal output was rapidly increased and reached a final climax in 1956 (124 Mt ; Bronny et al., 2004 ; Huske, 2006). At that time, coal was important for war reparation payments and reconstruction measures, especially the smelting of iron ore, the generation of electricity, the chemical industry, for transport and domestic fuel. Due to the competition of oil and gas in the heat market, coal mining in the Ruhr District decreased from the early 1960s on. Furthermore, coal mining suffered from its diminishing importance for energy supply and decreasing steel production (Wiggering and Zimmermeyer, 1993). Today, only four big coal mines remain, which are located at the western, northern and eastern edge of the Ruhr District, with a total coal production of 14.2 Mt (Gesamtverband Steinkohle, 2008). Presumably, the last coal mine in the Ruhr District will be closed in 2018. Due to its position along the edge, of the Ruhr District, subsidence due to active mining no longer affects the central part along the Emscher River lowlands. However, flooding was characteristic of this area even before coal mining began and this has been exacerbated by the irreversible disturbance of drainage capability due to mining subsidence, giving rise to a situation where approximately 38 % of the Emscher catchment and 15 % of the Lippe catchment are an artificially drained polder region (Emschergenossenschaft-und-Lippeverband, 2008). If the pumping stations in the polder regions were shutdown, most parts of the Ruhr District would be inundated (Peters, 1999).

4One reason for the extensive mining subsidence in the Ruhr District is the method of coal mining. Since the middle of the 19th century longwall mining has been introduced into the Ruhr District. This method is particularly suited to the extraction of seams of a type of relatively large lateral extent and a fairly consistent thickness. It is not limited by depth of mining and represents one of the safest methods for mining at great depths, where special rock stress problems pose a hazard to safe operation. One of its advantages is to permit controlled subsidence which can be accurately pre-assessed in terms of magnitude, effects and duration (Whittaker and Reddish, 1989). When longwall extraction is applied, a panel of coal is removed by working a face of up to 300 m in width between two parallel roadways, more than 1000 m underneath the surface. The roof is supported only in and near the roadways, and at the working face. After the coal has been won and loaded, the face supports are advanced, leaving the strata in the areas where the coal has been removed to collapse into the caved area (Bell et al., 2000). In the Ruhr District, subsidence at the surface follows the advance of the working face with a maximum lag of approximately six months, depending on the intensity of the mining. Approximately three years after the removal of coal, up to 90 % of the total subsidence can be observed, and after about three more years the subsidence completely dies out (Pollmann and Wilke, 1994). In general, the magnitude of the surface subsidence cannot exceed the extracted seam height (M). Field observations indicate a maximum surface subsidence of 0.9 M, depending on the nature of the overburden and type of goaf treatment such as total caving or some form of stowing (Whittaker and Reddish, 1989). Generally, the subsidence factor is regarded as being independent of depth (Bell and Donnelly, 2006). Observed values for the subsidence factor of coalfields in the Ruhr District range between 0.45 (pneumatic stowing), 0.5 (solid stowing) and 0.9 (caving ; Brauner, 1973). The surface area affected by ground movement is greater than the area worked in the seam, defined by the limit angle of draw or angle of influence which extends upwards and outwards from the working face. It varies from 8° to 45° depending on depth, seam thickness, dip of the seam and the local geology (Bell et al., 2000).

Methods

5In order to detect mining subsidence areas in the Ruhr District and to calculate the magnitude of mining subsidence, we used historical maps from 1892 with a scale of 1 :25,000 (“Preußische Neuaufnahme”) to get the earliest possible area-wide data on surface elevation. Subsequently we compare it with current Digital Elevation Models and finally calculate the magnitude of differences in elevation. After georeferencing 21 historical maps in total, every contour line and each geodetic point were digitised on a scale of 1 :3000 with the help of a Geographic Information System (ArcGIS). The number of points digitised in each map varies between 13,000 and more than 100,000, their average distance ranges from 34 m to 99 m, both depending on the density of contour lines and the surface characteristics. Tab. 1 and fig. 1 give an overview of all maps digitised, covering those parts of the Ruhr District, which are mostly affected by an intensive and long-lasting extraction of coal.

Tab. 1 – Data on digitised maps.
Tab. 1 – Données extraites des cartes digitalisées.

Nr.

Map

Area (in km2)

Number

of digitised points

Number

of digitised points (in m)

1

Bochum

128.98

92,454

37.4

2

Bottrop

128.67

79,524

40.2

3

Datteln

128.34

57,793

47.1

4

Dinslaken

128.60

34,325

61.2

5

Dorsten

128.29

65,531

44.2

6

Dortmund

128.56

51,866

49.8

7

Duisburg

128.87

33,905

61.7

8

Essen

128.90

112,314

33.9

9

Gelsenkirchen

128.58

43,493

54.4

10

Haltern

128.02

99,564

35.9

11

Hamm

128.42

32,423

62.9

12

Herne

128.58

34,607

61.0

13

Huenxe

128.39

37,237

58.7

14

Kamen

128.62

55,304

48.2

15

Lünen

128.30

45,548

53.1

16

Marl

128.35

73,297

41.8

17

Moers

128.88

25,419

71.2

18

Mülheim

128.88

65,018

44.5

19

Recklinghausen

128.34

57,84

47.1

20

Rheinberg

128.60

38,877

57.5

21

Wesel

128.34

13,033

99.2

Minimum

13,033

33.9

Mean

54,732

52.9

Maximum

112,314

99.2

6Finally, an area-wide capture of historical elevation data could be achieved (2700 km2 in total). The digitised data of contour lines and geodetic points was used to interpolate an elevation model with the help of ArcGIS by applying the Kriging algorithm and fitting an appropriate variogram model for every single map. The goodness of fit was evaluated by calculating a crossvalidation. For the same map a current Digital Elevation Model with a resolution of 10 m was used to interpolate the present surface by executing the Inverse Distance Weighted (IDW) function (De Smith et al., 2008). Finally, we intersect the two interpolated surfaces and calculated the differences in elevation, represented here for the map of Gelsenkirchen (fig. 2). Negative values indicate a lowering of the surface, positive values a rising. Differences between -2 m and +2 m are printed in white, due to errors emerging from the accuracy of historical elevation information and the interpolated data. In order to estimate the error of the Kriging interpolation the standard error of the predicted value was calculated and accounted for the interpretation of differences in elevation. Moreover, to evaluate the accuracy of the calculated differences in elevation and the interpolated elevation data from historical maps we used survey data which was allocated by the land registry office of the Federal State “North Rhine Westphalia”. The land registry office conducts a levelling along several profiles crossing the Ruhr District which is repeated every two years (Fröhlich and Müller, 1986 ; Haupt, 1999). Some of the levelling points were surveyed from the late 19th century on, most of the data reaches back to the 60s and 70s of the 20th century. Finally, the location of calculated differences in elevation was compared with the position of coal mines and coalfields in order to identify human impacts on the ground surface other than those from coal mining. In addition, the results were overlaid by data on the geology of the Carboniferous for further interpretation purposes.

Fig. 2 – Calculated differences in elevation for the map of Gelsenkirchen. Fig. 2 Dénivellations calculées sur la carte de Gelsenkirchen.

Fig. 2 – Calculated differences in elevation for the map of Gelsenkirchen. Fig. 2 – Dénivellations calculées sur la carte de Gelsenkirchen.

1 : less than -20 m ; 2 : -20 m to -14 m ; 3 : -14 m to -10 m ; 4 : -10 m to -6 m ; 5 : -6 m to -2 m ; 6 : -2 m to +2 m ; 7 : more than +2 m.
1 : moins de -20 m ; 2 : de -20 m à -14 m ; 3 : de -14 m à -10 m ; 4 : de -10 m à -6 m ; 5 : de -6 m à -2 m ; 6 : de -2 m à +2 m ; 7 : plus de + 2 m.

Results

7The presentation of selected results will focus on three maps in the Ruhr District covering the cities of Essen, Gelsenkirchen and Dortmund and located within or near the central river Emscher floodplain (fig. 1).

8Fig. 3 shows the calculated differences in elevation for a section of the map of Essen, which were derived from the elevation data of the historical map from 1892 and the current Digital Elevation Model. The map section is located northeast of the city of Essen and covers the coalfields of one of the most famous coal mines in the Ruhr District, the coal mine Zollverein. In 2002, UNESCO declared the Zollverein coalmine industrial complex a World Heritage Site. The World Heritage Committee praised the Zollverein coalmine as a representative example of the development of traditional heavy industries in Europe. The coal mine Zollverein was established in 1847 and shutdown in 1986 (Huske, 2006 ; Hermann and Hermann, 2008).

Fig. 3 – Calculated differences in elevation for a section of the map of Essen.
Fig. 3 Dénivellations calculées sur une partie de la carte de Essen.

Fig. 3 – Calculated differences in elevation for a section of the map of Essen. Fig. 3 – Dénivellations calculées sur une partie de la carte de Essen.

1 : less than -20 m ; 2 : -20 m to -14 m ; 3 : -14 m to -10 m ; 4 : -10 m to -6 m ; 5 : -6 m to -2 m ; 6 : -2 m to +2 m ; 7 : more than +2 m ; 8 : location of a coal mine shaft ; 9 : cross profile (fig. 4).
1 : moins de -20 m ; 2 : de -20 m à -14 m ; 3 : de -14 m à -10 m ; 4 : de -10 m à -6 m ; 5 : de -6 m à -2 m ; 6 : de -2 m à +2 m ; 7 : plus de + 2 m ; 8 : position d’une cheminée d’aération ; 9 : profil en travers (fig. 4).

9Next to shafts 3, 7 and 10 of the coal mine Zollverein a surface lowering of more than 25 m was calculated. The Mechtenberg hill, which was mentioned in the introduction, is located in the southeast of the map section and experienced a calculated surface lowering of round about 20 m. A profile across the Mechtenberg hill visualises the surface changes (fig. 4) : regardless of the former topography, mining subsidence not only captures surface depressions but also elevation features. This kind of subsidence was not visible in the field, unless historical and current elevation data were compared as demonstrated here. The magnitude of subsidence of the Mechtenberg hill shows a tower on top of the hill with a height of 16.75 m completely dropped underneath the former ground surface of 1892. One reason for these high values of mining subsidence is the characteristic of the Carboniferous strata : Overlaying the outcrop of coal seams on top of the Carboniferous strata illustrates the position of a syncline, the so called “Essener Mulde” (Hesemann, 1975). Areas with high values of a surface lowering are located along the axes of that syncline with slightly inclined limbs. These are the structures preferentially mined for efficient and modern coal extraction, because nearly-horizontal layering facilitated the use of machines and conveyer belts.

Fig. 4 – Cross-profile (A-B) in a section of the map of Essen.
Fig. 4 Profil en travers (A-B) d’une partie de la carte de Essen.

Fig. 4 – Cross-profile (A-B) in a section of the map of Essen. Fig. 4 – Profil en travers (A-B) d’une partie de la carte de Essen.

1 : former ground surface ; 2 : today’s ground surface ; 3 : difference in elevation.
1 : surface topographique historique ; 2 : surface topographique actuelle ; 3 : dénivellations calculées.

10Another example of a mining subsidence affecting an elevation feature is located north of the city of Dortmund (fig. 5). A profile across the area demonstrates the surface lowering of a hill on a distance of round about 3000 m (fig. 6). The maximum difference in elevation amounts to more than 17 m. This subsidence area is located next to the former coal mine “Minister Stein”, which was established in 1856 and shut down in 1987 (Huske, 2006). Regarding the calculated differences in elevation on the map, it is quite striking that the position of shafts 1, 2, 4 and 7 was purposely not impacted by mining subsidence in order to avoid any damage.

Fig. 5 – Calculated differences in elevation for a section of the map of Dortmund.
Fig. 5 Dénivellations calculées sur une partie de la carte de Dortmund.

Fig. 5 – Calculated differences in elevation for a section of the map of Dortmund.Fig. 5 – Dénivellations calculées sur une partie de la carte de Dortmund.

1 : less than -20 m ; 2 : -20 m to -14 m ; 3 : -14 m to -10 m ; 4 : -10 m to -6 m ; 5 : -6 m to -2 m ; 6 : -2 m to +2 m ; 7 : more than +2 m ; 8 : location of a coal mine shaft ; 9 : levelling point ; 10 : cross profile (fig. 6).
1 : moins de -20 m ; 2 : de -20 m à -14 m ; 3 : de -14 m à -10 m ; 4 : de -10 m à -6 m ; 5 : de -6 m à -2 m ; 6 : de -2 m à +2 m ; 7 : plus de + 2 m ; 8 : localisation d’une cheminée d’aération ; 9 : repère de nivellement ; 10 : profil en travers (fig. 6).

Fig. 6 – Cross-profile (A-B) in a section of the map of Dortmund.
Fig. 6 – Profil en travers (A-B) d’une partie de la carte de Dortmund.

Fig. 6 – Cross-profile (A-B) in a section of the map of Dortmund. Fig. 6 – Profil en travers (A-B) d’une partie de la carte de Dortmund.

1 : former ground surface ; 2 : today’s ground surface ; 3 : difference in elevation.
1 : surface topographique historique ; 2 : surface topographique actuelle ; 3 : dénivellations calculées.

11A levelling point regularly measured by the land registry office of the Federal State of North Rhine-Westphalia, located at the northern border of the subsidence area, allows a comparison of the interpolated elevation data from 1892 with the precise levelling data, starting in 1968 (fig. 7). Furthermore, the elevation data can be related to the mining history of the coal mine “Minister Stein” and can allow for detection of a possible correlation between periods of increased subsidence and the level of coal extracted. Although the interpolated elevation data from 1892 are much older than the earliest levelling data, the historical value seems to be plausible, because the interpolation of the two data points as well as the amplified subsidence following the year 1968 match the history of the coal mine. The highest level of coal production after the Second World War correlates to successive high subsidence rates, documented by the levelling data. Interpolating the historical value from 1892 and the levelling data of 1968 reveals an earlier beginning of a period with high subsidence rates, verified by the highest coal production in 1941. Taking a look at the end of the data series proves the 6-year period of declining subsidence generally observed after coal extraction has ended.

Fig. 7 – Levelling point data located in the map of Dortmund in comparison with interpolated surface elevation data from 1892.
Fig. 7 Valeurs de nivellement extraites de la carte de Dortmund et comparées avec les valeurs altimétriques interpolées de 1892.

Fig. 7 – Levelling point data located in the map of Dortmund in comparison with interpolated surface elevation data from 1892.Fig. 7 – Valeurs de nivellement extraites de la carte de Dortmund et comparées avec les valeurs altimétriques interpolées de 1892.

12Overlaying the tectonic features of the Carboniferous strata reveals the influence of another tectonic feature on coal mining. One of the biggest faults detected in the Carboniferous of the Ruhr District, the so called “Quintus Sprung” (Grabert, 1998), touches the subsidence area in the southwest, avoiding any progress of coal extraction along the working face in the southwest direction due to a sudden vertical displacement. As a consequence, the calculated differences in elevation observed at the ground surface are abruptly reduced across the fault line, thus representing an image of the tectonic situation in the underground. From this result we may conclude that tectonics of the Carboniferous strata become apparent at the surface by limiting the extraction of coal and thereby reducing the surface lowering.

13The last result presented refers to the calculated differences in elevation for the total map of Gelsenkirchen, which is located in the middle of the central Emscher plain (fig. 1 and fig. 2). A look at fig. 8 demonstrates the high proportion of areas with a surface lowering : almost 93 % of the total map area (128.5 km2) are lower than in the year 1892. Only the embankments of highways and especially the mining waste heaps stand out as features with an increased height. The coal mining waste heap “Hoheward” is located next to one of the biggest former coal mines in the Ruhr District, the coal mine “Ewald”. The use of Geographic Information Systems not only allows for the calculation of differences in elevation but also of volume budgets by comparing the former and the current surface elevations. For the map of Gelsenkirchen approximately 0.777 km3 account for the cut volume, that is the volume loss first of all as a result of subsidence. Together with the fill volume of 0.113 km3, which is basically ascribed to mining waste heaps a net volume loss of 0.664 km3 can be calculated. After dividing the net volume loss by the total area of the map under investigation (128.5 km2), a net surface lowering of 5.16 m can be estimated. Taking into account the time span between the date of the survey in 1892 and today, an average annual surface lowering of approximately 44 mm can be calculated. The average subsidence in areas only affected by a surface lowering even amounts to 6.5 m after dividing the cut volume by the corresponding area (93 % of 128.5 km2 = 119 km2).

Fig. 8 – Areas within the map of Gelsenkirchen revealing a surface lowering or a surface rising.
Fig. 8 Les zones situées sur la carte de Gelsenkirchen montrant un exhaussement ou un abaissement de la surface topographique.

Fig. 8 – Areas within the map of Gelsenkirchen revealing a surface lowering or a surface rising. Fig. 8 – Les zones situées sur la carte de Gelsenkirchen montrant un exhaussement ou un abaissement de la surface topographique.

1 : area affected by a surface rising ; 2 : area affected by a surface lowering ; 3 : shaft of coal mine “Ewald“.
1 : zone affectée par un exhaussement de la surface topographique ; 2 : zone affectée un abaissement de la surface topographique ; 3 : cheminée d’aération de la mine « Ewald ».

14After combining all maps digitised and analysed, fig. 9 gives an overview of the Ruhr District with the calculated differences in elevation between the interpolated elevation data from 1892 and the current Digital Elevation Model. All blue coloured areas indicate the position of mining waste heaps while most parts of the Ruhr District investigated so far exhibit a surface lowering of at least -2 m. The highest values of a surface lowering can be observed along lines crossing the Ruhr District from west-southwest to east-northeast. This direction is in accordance with the strike of syncline axis of the Carboniferous strata, indicated in fig. 9 by the black lines. Furthermore, most of the predominant faults in the Carboniferous (green lines) seem to confine the extraction of coal and therefore the amount of mining subsidence : in some cases, the fault lines border on high values of calculated differences in elevation or take course in areas with low values of surface lowering. However, most of the areas with a surface lowering seem to correlate with tectonic features of the Carboniferous strata and, therefore, with the extraction of coal.

Fig. 9 – Calculated differences in elevation for all maps under investigation.
Fig. 9 Dénivellations calculées sur toutes les cartes utilisées.

Fig. 9 – Calculated differences in elevation for all maps under investigation. Fig. 9 – Dénivellations calculées sur toutes les cartes utilisées.

1 : less than -10 m ; 2 : -10 m to -5 m ; 3 : -5 m to -2 m ; 4 : fault ; 5 : syncline ; 6 : digitised map.
1 : moins de -10 m ; 2 : -10 m à -5 m ; 3 : -5 m à -2 m ; 4 : faille ; 5 : synclinal ; 6 : carte historique numérisée.

15Plotting the calculated average differences in elevation for all maps digitised and analysed so far, the map of Gelsenkirchen shows the highest value, followed by Essen, Bottrop, Dinslaken and Herne (fig. 10). All these areas are located within the central Emscher floodplain along two major synclines of the Carboniferous strata with slightly inclined limbs and a gentle dip of the coal seams, favoured for an intensive mechanised extraction of coal. Furthermore, those areas reveal a mining history reaching back to the early beginning of deep-seam mining in the middle of the 19th century. For the total area of more than 2700 km2 under investigation an average net surface lowering of 1.6 m can be calculated. This value includes not only areas affected by subsidence and open pit mining, but also anthropogenic elevation features such as mining waste heaps and embankments of highways or railways.

Fig. 10 – Comparison of average calculated differences in elevation.
Fig. 10 Comparaison de la moyenne des dénivellations.

Fig. 10 – Comparison of average calculated differences in elevation.Fig. 10 – Comparaison de la moyenne des dénivellations.

Discussion

16The presented results of differences in elevation between the surface of 1892 and the current ground surface are primarily based on the method of digitising contour lines and elevation points in historical maps of the Ruhr District. Hence, an estimation of accuracy needs investigations on errors emerging from the use of historical maps for the interpolation of elevation data. At the current stage of our investigations not all the errors can be quantified due to missing data or information, but at least an estimation or description of possible errors will be presented here.

17One of the first sources of errors emerges from georeferencing the historical maps. All maps were georeferenced allowing for a root mean square error of all data points used of not more than 10 m. Taking into account the scale of the historical maps (1 :25,000) this error corresponds to a distance of only 0.4 mm on the map. Secondly, the accuracy of contour lines on the historical maps has to be questioned. Even if the levelling of individual survey points were accurate at that time, the drawing of contour lines might be faulty, especially after considering, that contour lines resulted from drawings on a plane survey sheet in the field with the scale of the original map (1 :25,000 ; Grothenn, 1994). On the other hand, there is much evidence of a high level of accuracy in drawing the contour lines, because even on flat areas such as floodplains elevation differences of 1.25 m were accounted for by the use of corresponding contour lines.

18The error resulting from an inaccurate position of a digitised point on a contour line was minimised by digitising on a scale of 1 :3000. Nevertheless, the significance of this error for an estimation of elevation will increase with high surface slopes. Finally, the accuracy of the Kriging algorithm can be estimated by calculating the standard error of the predicted value (De Smith et al., 2008). The values of the standard error for the map of Gelsenkirchen vary between 0.06 and 6.02 m, depending on the distance to a digitised elevation point and the spatial characteristics of the elevation data. The highest values can be observed within the former river “Emscher” floodplain, designated by large distances of contour lines on the historical map. In general, relatively low values can be observed on maps with a low-relief energy and a high density of digitised contour lines. It seems that the standard error of the predicted interpolated values should be considered in future works, in order to estimate a total error of calculated elevation differences including all sources of error described above.

19As indicated above, the accuracy of interpolated elevation data from the historical maps can be estimated with the help of single levelling points measured by the land registry office of the Federal State of North Rhine-Westphalia. The data series of a levelling point located on the map of Gelsenkirchen and the corresponding value of the interpolated elevation data from 1892 are presented in fig. 11. A total subsidence of more than 4 m can be assessed after taking into account the measured elevation data of the levelling point starting in 1948. The backward trend of the graph indicates a surface elevation before 1948 which might be only slightly higher. Hence, the value of interpolated elevation data from 1892 is slightly too low, about 1 m. To date, at least 148 data series of levelling points going back to 1951 have been analysed and compared with the interpolated elevation data from 1892. The most helpful are 11 data series of levelling points starting in 1895. Only one series of those data shows a difference of more than 1 m between the measured surface elevation from 1895 and the interpolated surface elevation from 1892. All the others reveal a difference of less than 1 m. On a cautionary note, the surface characteristics of the levelling point location has to be borne in mind : All levelling points with data series reaching back to 1895 are located in lowland areas. In the low mountain range of the southern Ruhr District, the error of the interpolated surface elevation will be higher due to the slope-dependent accuracy of digitised points on a contour line. Otherwise, the standard error of the predicted interpolated value will be higher in lowland areas with a large distance of digitised points. However, the low level of difference between the interpolated and measured elevation data of levelling point series going back to 1895 is quite astonishing and allows one to assume a high level of accuracy of digitised and interpolated elevation data.

Fig. 11 – Levelling point data located in the map of Gelsenkirchen in comparison with interpolated surface elevation data from 1892.
Fig. 11 Valeurs de nivellement extraites de la carte de Gelsenkirchen et comparées avec les données altimétriques de surface interpolées de 1892.

Fig. 11 – Levelling point data located in the map of Gelsenkirchen in comparison with interpolated surface elevation data from 1892.Fig. 11 – Valeurs de nivellement extraites de la carte de Gelsenkirchen et comparées avec les données altimétriques de surface interpolées de 1892.

20The calculated differences in elevation of more than 20 m for wide areas of the Ruhr District seem to represent extraordinary high values in consideration of case studies from other countries. F.G. Bell and D.D. Genske (2001) for example describe the longwall extraction of coal from beneath Warrington New Town in Lancashire (England). The subsidence was up to 1 m and maximum subsidence from future workings was estimated to be up to 2 m. Another example from England (Hickleton, South Yorkshire) reveals a maximum subsidence of 1.7 m (Bell and Donelly, 2006). Investigations on mining subsidence in the Upper Silesian Coal Basin indicate an average subsidence of 4.54 m for a mining area (164 km2) at the Rybnik Plateau (Rahmonov et al., 2006).

Conclusions

21For the first time, an area-wide and large-scale calculation of differences in elevation between the situation in 1892 and today was conducted for the Ruhr District, a metropolitan region influenced by mining subsidence due to deep-seam mining starting in the middle of the 19th century. The elevation information on historical maps from 1892 was digitised with the help of a Geographic Information System, leading to a density of elevation points between 34 and 99 m. After applying a Kriging algorithm, the interpolated historical surface was intersected with a current Digital Elevation Model in order to calculate the differences in elevation. As a result, the highest values of elevation differences, amounting to more than 25 m, were observed within the coalfields of the former coal mine Zollverein which is distinguished by its long mining history and its World Heritage status. Regarding the surface characteristics, not only depressions but also elevation features suffer from mining subsidence, as two examples from the cities of Essen and Dortmund reveal. These kinds of surface transformations are not visible in the field without the help of an area-wide and systematic investigation as presented here. The examples of a surface lowering allow for a correlation with mining activities, because most of the mining subsidence areas are located next to a former coal mine. Furthermore, tectonic features of the Carboniferous strata in the underground are reflected by the location of subsidence areas, since they are located along synclines with a gentle dip of coal seams or confined by the location of predominant faults. The average level of surface lowering was calculated for all maps investigated, resulting in values of between 0.51 m for the map of Kamen, located at eastern border of the Ruhr District, and 5.16 m for the map of Gelsenkirchen within the central Emscher floodplain with a total area of 128.5 km2. Taking into consideration the time span investigated, the annual surface lowering rates were calculated and extrapolated for comparison with the highest denudation rates of river catchments in the world described by D.E. Walling (1987). Even the average surface-lowering rate of the total area under investigation exceeds nearly all of the highest denudation rates of river catchments in the world. However, this attempt of a comparison between the human impact on the ground surface induced by mining activities and landscape dynamics on different time- and spatial-scales can only offer an impression of the intensity of human impacts, since they are generally characterised by short time-scales as P.K. Haff (2003) emphasises. The analysis of errors in our investigation reveals several sources, originating from the first elevation data acquisition in the historical maps, the process of georeferencing maps, digitising contour lines and, last but not least, the interpolation process. Especially the standard error of the predicted interpolated elevation data after applying a Kriging algorithm is easy to quantify with the help of a Geographic Information System, but the quantification of some other errors discussed require further investigations. However, a comparison of single reliable elevation data, derived from surface levelling data of the land registry office in North Rhine-Westphalia, with the interpolated values from 1892 reveals an astonishing correlation with differences of not more than 1 m.

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Annexe

Version abrégée en français

Située dans la partie occidentale de l’Allemagne, la région de la Ruhr, autrefois région industrielle majeure entre la deuxième moitié du XIXe siècle et la fin du XXe siècle, a été marquée par les mines de charbon et les aciéries. Les vestiges de l’activité de charbonnage sont toujours visibles dans la Ruhr, à travers les nombreux terrils et les affaissements superficiels. L’affaissement lié aux extractions dans la Ruhr entraîne de sérieux problèmes en raison de son influence sur les modalités de drainage, la direction des écoulements des rivières étant perturbée, voire localement inversée. Pour protéger la Ruhr de graves problèmes d’inondation, de nombreux secteurs ont dû être drainés au moyen d’un grand nombre de stations de pompage, bien que l’extraction ait cessé dans la partie du sud et centrale de cette région. L’étude présentée ici quantifie l’ampleur et la géographie des affaissements. Les données sur les secteurs importants de subsidence n’étant pas disponibles ou jusqu’alors non examinées, l’article propose une première synthèse à grande échelle et systématique des données concernant l’affaissement d’origine minier à l’échelle de la Ruhr. Cette dernière constitue une conurbation de 5,3 millions d’habitants, répartis sur onze municipalités et quatre quartiers (figure 1). Elle s’étend du Rhin à l’ouest, à la rivière Ruhr au sud et borde des zones rurales au nord et à l’est. La Ruhr s’étend au total sur 4435 km2. L’extraction a commencé au sud, dans la vallée de la Ruhr et ses affluents, là où le charbon est à l’affleurement dans les strates du Carbonifère supérieur. Aujourd’hui, il ne reste que quatre grandes mines de charbon, situées dans la partie occidentale, septentrionale et orientale de la Ruhr. En raison de sa position bordière, l’affaissement lié à l’extraction active n’affecte plus la partie centrale le long des plaines fluviales d’Emscher. Cependant, ce secteur a été affecté par des inondations bien avant que le charbonnage ne commence et cela a été renforcé par la modification irréversible des conditions de drainage, en raison des affaissements. Une des raisons de l’affaissement de la Ruhr tient à la façon dont on a extrait le charbon. Depuis le milieu du XXe siècle, l’extraction « longwall » a été privilégiée. Cette méthode convient plus particulièrement à l’extraction de minerais sur de grandes surfaces. Les cartes historiques de 1892 au 1/25.000 ("Preußische Neuaufnahme") ont été utilisées pour détecter les secteurs d’affaissement et calculer leur ampleur. Les données extraites de ces cartes ont été comparées à celles des Modèles Numériques de Terrain (MNT) afin de mesurer les différences d’altitude. Après le géoréférencement de 21 cartes historiques, chaque courbe de niveau et chaque point géodésique ont été numérisés à l’échelle du 1/3000 à l’aide d’un logiciel de Système d’Information Géographique (SIG ; ArcGIS). La numérisation des courbes de niveau et des points géodésiques a servi à l’interpolation d’un MNT en appliquant l’algorithme Kriging et en adaptant un modèle Variogram approprié pour chaque carte. Sur chaque carte, un MNT d’une résolution de 10 m, a été utilisé pour interpoler la surface actuelle. Finalement, les deux surfaces interpolées ont été croisées afin de calculer les variations d’altitude. Par exemple, la figure 3 montre les variations d’altitude calculées sur une partie de la carte d’Essen, situé au nord-est de la Ruhr et couvrant les bassins houillers d’une des mines de charbon les plus célèbres de la région, la mine Zollverein. A proximité des puits 3, 7 et 10 de la mine de charbon Zollverein, un affaissement surfacique de plus de 25 m a été mesuré. La figure 8 apporte les preuves de la grande proportion de secteurs affectés par un affaissement superficiel : presque 93 % de l’ensemble de la carte (128.5 km2) ont des altitudes inférieurs à celles de 1892.
L’utilisation d’un SIG a permis le calcul de bilans d’érosion, en comparant les topographies anciennes et actuelles. A partir de la carte de Gelsenkirchen, la perte de volume suite à l’affaissement a été approximativement estimé à 0,777 km3. Avec un volume de 0,113 km3 (essentiellement attribué aux terrils), une perte de volume nette de 0,664 km3 a pu être proposée. En divisant la perte de volume nette par la surface totale étudiée (128,5 km2), l’abaissement net de la surface est de 5,16 m. La figure 9 montre les variations d’altitude entre les données topographiques interpolées de 1892 et le MNT actuel et ce, à l’échelle de la Rhur. La géographie de la plupart des secteurs affaissés semblent être corrélée avec les caractéristiques tectoniques des strates du Carbonifère (synclinaux et failles) dont on a tenu pour implanter les sites d’extraction du charbon.

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Table des illustrations

Titre Fig. 1 – Map of the Ruhr District and location of digitised historical maps. Fig. 1 Carte de la région de la Ruhr et localisation des cartes historiques numérisées.
Légende Insert: Location of the Ruhr District in Germany. 1: Ruhr District; 2: digitised map; 3: river; 4: district or city.Carton de localisation : la région de la Ruhr en Allemagne. 1 : région de la Ruhr ; 2 : carte historique numérisée ; 3 : cours d’eau ; 4 : arrondissement rural ou cité.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-1.jpg
Fichier image/jpeg, 348k
Titre Fig. 2 – Calculated differences in elevation for the map of Gelsenkirchen. Fig. 2 Dénivellations calculées sur la carte de Gelsenkirchen.
Légende 1 : less than -20 m ; 2 : -20 m to -14 m ; 3 : -14 m to -10 m ; 4 : -10 m to -6 m ; 5 : -6 m to -2 m ; 6 : -2 m to +2 m ; 7 : more than +2 m.1 : moins de -20 m ; 2 : de -20 m à -14 m ; 3 : de -14 m à -10 m ; 4 : de -10 m à -6 m ; 5 : de -6 m à -2 m ; 6 : de -2 m à +2 m ; 7 : plus de + 2 m.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-2.jpg
Fichier image/jpeg, 484k
Titre Fig. 3 – Calculated differences in elevation for a section of the map of Essen. Fig. 3 Dénivellations calculées sur une partie de la carte de Essen.
Légende 1 : less than -20 m ; 2 : -20 m to -14 m ; 3 : -14 m to -10 m ; 4 : -10 m to -6 m ; 5 : -6 m to -2 m ; 6 : -2 m to +2 m ; 7 : more than +2 m ; 8 : location of a coal mine shaft ; 9 : cross profile (fig. 4).1 : moins de -20 m ; 2 : de -20 m à -14 m ; 3 : de -14 m à -10 m ; 4 : de -10 m à -6 m ; 5 : de -6 m à -2 m ; 6 : de -2 m à +2 m ; 7 : plus de + 2 m ; 8 : position d’une cheminée d’aération ; 9 : profil en travers (fig. 4).
URL http://geomorphologie.revues.org/docannexe/image/7965/img-3.jpg
Fichier image/jpeg, 300k
Titre Fig. 4 – Cross-profile (A-B) in a section of the map of Essen. Fig. 4 Profil en travers (A-B) d’une partie de la carte de Essen.
Légende 1 : former ground surface ; 2 : today’s ground surface ; 3 : difference in elevation.1 : surface topographique historique ; 2 : surface topographique actuelle ; 3 : dénivellations calculées.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-4.jpg
Fichier image/jpeg, 144k
Titre Fig. 5 – Calculated differences in elevation for a section of the map of Dortmund.Fig. 5 Dénivellations calculées sur une partie de la carte de Dortmund.
Légende 1 : less than -20 m ; 2 : -20 m to -14 m ; 3 : -14 m to -10 m ; 4 : -10 m to -6 m ; 5 : -6 m to -2 m ; 6 : -2 m to +2 m ; 7 : more than +2 m ; 8 : location of a coal mine shaft ; 9 : levelling point ; 10 : cross profile (fig. 6).1 : moins de -20 m ; 2 : de -20 m à -14 m ; 3 : de -14 m à -10 m ; 4 : de -10 m à -6 m ; 5 : de -6 m à -2 m ; 6 : de -2 m à +2 m ; 7 : plus de + 2 m ; 8 : localisation d’une cheminée d’aération ; 9 : repère de nivellement ; 10 : profil en travers (fig. 6).
URL http://geomorphologie.revues.org/docannexe/image/7965/img-5.jpg
Fichier image/jpeg, 368k
Titre Fig. 6 – Cross-profile (A-B) in a section of the map of Dortmund. Fig. 6 – Profil en travers (A-B) d’une partie de la carte de Dortmund.
Légende 1 : former ground surface ; 2 : today’s ground surface ; 3 : difference in elevation.1 : surface topographique historique ; 2 : surface topographique actuelle ; 3 : dénivellations calculées.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-6.jpg
Fichier image/jpeg, 152k
Titre Fig. 7 – Levelling point data located in the map of Dortmund in comparison with interpolated surface elevation data from 1892.Fig. 7 Valeurs de nivellement extraites de la carte de Dortmund et comparées avec les valeurs altimétriques interpolées de 1892.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-7.jpg
Fichier image/jpeg, 116k
Titre Fig. 8 – Areas within the map of Gelsenkirchen revealing a surface lowering or a surface rising. Fig. 8 Les zones situées sur la carte de Gelsenkirchen montrant un exhaussement ou un abaissement de la surface topographique.
Légende 1 : area affected by a surface rising ; 2 : area affected by a surface lowering ; 3 : shaft of coal mine “Ewald“.1 : zone affectée par un exhaussement de la surface topographique ; 2 : zone affectée un abaissement de la surface topographique ; 3 : cheminée d’aération de la mine « Ewald ».
URL http://geomorphologie.revues.org/docannexe/image/7965/img-8.jpg
Fichier image/jpeg, 716k
Titre Fig. 9 – Calculated differences in elevation for all maps under investigation. Fig. 9 Dénivellations calculées sur toutes les cartes utilisées.
Légende 1 : less than -10 m ; 2 : -10 m to -5 m ; 3 : -5 m to -2 m ; 4 : fault ; 5 : syncline ; 6 : digitised map.1 : moins de -10 m ; 2 : -10 m à -5 m ; 3 : -5 m à -2 m ; 4 : faille ; 5 : synclinal ; 6 : carte historique numérisée.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-9.jpg
Fichier image/jpeg, 268k
Titre Fig. 10 – Comparison of average calculated differences in elevation.Fig. 10 Comparaison de la moyenne des dénivellations.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-10.jpg
Fichier image/jpeg, 208k
Titre Fig. 11 – Levelling point data located in the map of Gelsenkirchen in comparison with interpolated surface elevation data from 1892.Fig. 11 Valeurs de nivellement extraites de la carte de Gelsenkirchen et comparées avec les données altimétriques de surface interpolées de 1892.
URL http://geomorphologie.revues.org/docannexe/image/7965/img-11.jpg
Fichier image/jpeg, 93k
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Stefan Harnischmacher, « Quantification of mining subsidence in the Ruhr District (Germany) », Géomorphologie : relief, processus, environnement, 3/2010 | 2010, 261-274.

Référence électronique

Stefan Harnischmacher, « Quantification of mining subsidence in the Ruhr District (Germany) », Géomorphologie : relief, processus, environnement [En ligne], 3/2010 | 2010, mis en ligne le 01 octobre 2012, consulté le 24 novembre 2014. URL : http://geomorphologie.revues.org/7965 ; DOI : 10.4000/geomorphologie.7965

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Auteur

Stefan Harnischmacher

University of Koblenz - Department of Geography - D-56070 Koblenz - Germany (harnisch@uni-koblenz.de)

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Droits d’auteur

© Groupe français de géomorphologie

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