- Email: [email protected]
Holocene sediment budgets in two river catchments in the Southern Upper Rhine Valley, Germany
Available online at www.sciencedirect.com
Geomorphology 92 (2007) 198 – 207 www.elsevier.com/locate/geomorph
Holocene sediment budgets in two river catchments in the Southern Upper Rhine Valley, Germany Jochen Seidel ⁎, Rüdiger Mäckel Department of Physical Geography, University of Freiburg, Werderring 4, 79085 Freiburg, Germany Received 30 September 2005; received in revised form 31 March 2006; accepted 24 July 2006 Available online 24 May 2007
Abstract The Holocene sediments of two catchments in the southern Upper Rhine valley have been quantified as part of the German LUCIFS Programme (RheinLUCIFS), which aims to quantify sediment fluxes in the Rhine catchment since the onset of agriculture in the Neolithic about 7500 years ago. The spatial distribution of the alluvial and colluvial sediments was derived using geological maps, with information on the thickness of these sediments from various sources including auger profiles and data from excavations. The sediments were subdivided into characteristic sedimentary storage types according to the different types of landscapes. For each of the sedimentary storage types an average thickness was assessed so that an integral sediment balance for the Holocene could be derived. For the different types of landscapes in the study area, 32 Holocene sedimentary storage types were determined, 21 in the Elz catchment (1500 km2) and 11 in the Möhlin catchment (230 km2). By adding up the sediment volumes of all single sedimentary storage types the total Holocene sediment volumes for the two catchments were calculated. Erosion depths were determined by dividing the sediment volumes through the potential erosion areas (slope N 2%) and by assuming a sediment delivery ratio (SDR) between 0 and 0.4. The total erosion for the potential erosion areas during the Holocene was calculated as 31–61 cm in the Elz catchment and 44–79 cm in the Möhlin catchment. © 2007 Elsevier B.V. All rights reserved. Keywords: Holocene sediments; Sediment budgets; Upper Rhine Valley; Erosion rates
1. Introduction The research project presented in this paper was conducted as part of the German LUCIFS Programme (RheinLUCIFS), which is part of the PAGES Focus 5 “Past Ecosystem Processes and Human-Environment Interactions” science activities. The general aim of the LUCIFS (Land Use and Climate Impacts on Fluvial ⁎ Corresponding author. Tel.: +49 761 203 9117/3507; fax: +49 761 203 3596. E-mail address: [email protected] (J. Seidel). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.07.041
Systems during the period of agriculture) research project is to understand the particulate fluxes in fluvial systems since the period of agriculture (Lang et al., 2003a; Dikau et al., 2005). For the RheinLUCIFS project, the catchment of the River Rhine was selected as suitable study site, since the first settlements in this area date back to the Neolithic and there is a variety of studies and results already available. The first RheinLUCIFS project phase lasted from 2002 to 2004 and included five projects from different disciplines (geomorphology, archaeology and historical geography). The objective of the geomorphological projects within RheinLUCIFS was to develop sediment
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
199
Fig. 1. Location of study area.
budgets in selected regions and catchment sizes using different approaches. Within this framework, the aim of the research project at the University of Freiburg was to quantify the volume of Holocene sediments and to determine erosion amounts in two mesoscale river catchments (Elz and Möhlin) by evaluating data which were already available and supplemented by field work. The basic assumption was, that the first significant human impact on the natural landscape in the Upper Rhine Valley took place in the Neolithic about 7500 years ago. This time marks the onset of settlement and agricultural activities, which led to soil erosion and affected the sediment fluxes in river catchments. It is assumed that most parts of central Europe were covered by woodland and no recognizable soil erosion had taken place before the Neolithic (Bork et al., 1998). This
situation was also discovered by several investigations in the Upper Rhine area (Friedmann, 2000; Sudhaus, 2005; Lechner, 2005), which are based on the DFGPriority Programme “Changes of the Geo-Biosphere During the Last 15,000 Years” and the DFG Research Training Group “Formation and Development of Present-Day Landscapes” (Mäckel et al., 2004). 2. Study area The river catchments of the Elz and Möhlin are situated in the southern Upper Rhine Valley between the River Rhine and the Black Forest (Fig. 1). The source of the River Elz is located at an altitude of 1089 m a.s.l. in the Black Forest. In the Upper Rhine Valley, the course of the River Elz is strongly influenced by river engineering.
200
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
Fig. 2. Map with location of the evaluated data.
At the town of Riegel, where the important tributary rivers Dreisam and Glotter flow into the Elz, the river is split up. Most of the discharge follows the 12-km-long Leopold Canal directly into the River Rhine. This canal was put into operation in 1842 to reduce the flood risk in the Upper Rhine Valley north and east of the Kaiserstuhl. A small but constant amount of water is directed into the “Alte Elz”, the old riverbed of the Elz, which corresponds to the original course of the Elz before the Leopold Canal was built. The original mouth of the Elz into the Rhine was near Nonnenweier, but due to the massive engineering of the Rhine, the water is now directed parallel to the bank of the Rhine for several kilometres. This part of the Elz has not been considered in this study. The area of the Elz catchment examined is about 1500 km2 . The River Möhlin also originates in the Black Forest and flows through the Münster Valley (Münstertal) and across the Upper Rhine Valley towards its junction with the River Rhine south of the town of Breisach. In the 19th century the lower course of the River Möhlin was completely regulated and embanked. The size of the Möhlin catchment is 228 km2.
Both river catchments encompass different natural units of the Central and Southern Black Forest and the adjacent Upper Rhine Lowlands. The basins selected each represent a specific environmental milieu and show different responses to natural changes and human impact. Accordingly, the study sites in the climatically favoured loess areas of the Kaiserstuhl Mountain and the lower foothills as well as the plains and terraces of the River Rhine and its eastern tributaries show different histories and intensities of land use than the climatically cooler and moister upland areas of the Black Forest, which have been continuously populated since the Middle Ages. Qualitative studies on environmental changes and sediments in the study areas show that human impact varied during the Holocene (Schneider, 2000; Mäckel et al., 2001, 2002, 2003) with the most severe impacts occurring during Roman Times and from the Middle Ages onwards. 3. Quantification of Holocene sediments In order to obtain sediment volume for the Holocene sediments in the study area, the area as well as the
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
201
Fig. 3. Overview of the landscape types in the study area.
average depth of these archives had to be determined. The time from the Neolithic onwards is considered for the scientific framework of RheinLUCIFS, but since the border between pre- and post-neolithic sediments can usually not be determined in profiles, an integral calculation of the sediment volumes for the Holocene was conducted. The basic assumption to distinguish Holocene and Pleistocene sediments is, that the transition from the late Glacial to the Holocene is marked by a change in grain size of the sediments. Thus, coarse gravel and sand were allocated to the late Glacial and fine sediments were considered to be of Holocene age. Since the focus of this study was put on the evaluation of data that were already available, there was a spatially very heterogeneous database at hand (Fig. 2), which had to be evaluated regarding information about
the depth and age. Therefore in order to calculate the Holocene sediment volume the alluvial and colluvial sediments were spatially disaggregated into smaller units. 3.1. Determination of the Holocene sediment area The spatial distribution of Holocene sediments in the study area was derived from 1:25,000 scale geological maps. Due to their scale, these maps show some imprecision regarding the distribution of alluvial and colluvial sediments, but they are the only data sources that cover the whole of the study area. All mapped Holocene units were summarized in the two categories “Holocene Alluvial Sediments” and “Holocene Colluvial Sediments”.
202
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
Table 1 Number of Holocene sedimentary storage types in the study area Number of Holocene sedimentary storage types (HSST) Elz catchment 1. Offenburger Rhine Plain 2. Foothill Zone of LahrEmmendingen 3. Kaiserstuhl 4. Freiburger Bucht 5. Black Forest 6. Markgräfler Hill Country 7. Markgräfler Rhine Plain Möhlin Catchment 8. Freiburger Bucht 9. Markgräfler Rhine Plain 10. Markgräfler Hill Country 11. Black Forest Total:
4 3 2 5 5 2 -
3 2 4 2 32
In the next step, both river catchments were subdivided according to the different landscape types in the Water and Soil Atlas of Baden-Württemberg (WaBoA, Ministerium
für Umwelt und Verkehr Baden-Württemberg and Landesanstalt für Umweltschutz Baden-Württemberg, 2001). In this way 11 different landscape types were determined for the two river catchments (Fig. 3). Then characteristic Holocene sedimentary storage types were determined for each of the 11 landscape types in the two river catchments. Areas with similar features regarding relief, soil, geology and land use history were merged, resulting in a total of 32 Holocene sedimentary storage types for the whole study area (Table 1). Hillwash sediments displaced from their area of origin to another landscape type (e.g. the fringes of the colluvial loess in the foothill zone of Lahr-Emmendingen) were still considered as part of their area of origin. Thus the borders of the Holocene sedimentary storage types vary to a certain degree from the landscape types according to the WaBoA. In the case of the landscape type of the “Markgräfler Rhine Plain” (Elz catchment, 7 in Fig. 2), all mapped sediments originate from the adjacent landscape types, and therefore there were no sediments which could be allocated to this landscape type.
Table 2 Overview of the age and thickness of the Holocene sedimentary storage types in the Elz catchment Sedimentary storage type (landscape type)
Average thickness
Age of the sediments
Data appraisal (quality/quantity)
Floodplain sediments with Black Floodplain Soil (OGP) Floodplain sediments without Black Floodplain Soil (OGP) Filled river courses (OGP) Other colluvial sediments (OGP) Hillwash colluvial sediments (FZL) Valleys with colluvial fillings (FZL) Floodplains in the foothill zone (FZL) Valleys with colluvial fillings (KAI) Hillwash colluvial sediments (KAI) Floodplain sediments of the Rivers Elz, Glotter and Dreisam (FBE) Floodplain sediments at the fringe of the foothill zone of Lahr-Emmendingen (FBE) Valleys with colluvial fillings in the Tuniberg (FBE) Hillwash colluvial loess sediments at the Tuniberg and Mengen (FBE) Colluvial sediments in the northeastern part of the Freiburger Bucht (FBE) Floodplains in the Black Forest except Brettenbach Valley (BLF) Floodplains in the Brettenbach Valley(BLF) Floodplains in the Basin of Zarten (BLF) Colluvial sediments in the Elz Valley (BLF) Colluvial sediments west of the Main Black Forest Fault (BLF) Floodplain sediments east of the Schönberg (MHE) Other floodplain and colluvial sediments (MHE)
150 cm 130 cm 80 cm 70 cm 320 cm 600 cm 300 cm 500 cm 300 cm 100 cm 50 cm
≤Neolithic ≤Neolithic (?) ≤Modern times ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic
+/+ o/− ο/ο −/− −/− ο/− o/ο +/o o/o o/− −/o
450 cm 100 cm 100 cm
≤Neolithic ≤Neolithic ≤Neolithic
−/− o/− −/−
90 cm 200 cm 90 cm 110 cm 400 cm 70 cm 200 cm
≤Middle ages ≤Roman times (?) ≤Middle Bronze Age ≤Middle Ages ≤Neolithic ? ≤Neolithic
+/+ +/+ +/+ −/ο o/− −/− −/−
Abbreviations for landscape types: ORP = Offenburger Rhine Plain, FZL = Foothill Zone of Lahr-Emmendingen, KAI = Kaiserstuhl, FBE = Freiburger Bucht (Elz Catchment), BLF = Black Forest (Elz Catchment), MHC = Markgräfler Hill Country (Elz Catchment). Age of sediments. ≤ = younger or same age as. ? = uncertain. Data appraisal: +, good; o, sufficient; −, minimal.
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207 Table 3 Overview of the age and thickness of the Holocene sedimentary storage types in the Möhlin catchment Sedimentary storage type (landscape type)
Average Age of the thickness sediments
Data appraisal (quality/ quantity)
Colluvial sediments of the Mengener Brücke (FBM) Valleys with colluvial fillings in the Tuniberg (FBM) Hillwash colluvial sediments of the Tuniberg (FBM) Alluvial sediments with Black Floodplain Soil (MGR) Alluvial sediments without Black Floodplain Soil (MGR) Narrow floodplains (MHM) Floodplains of the Möhlin from Ehrenstetten onwards (MHM) Floodplains of the Neumagen (MHM) Colluvial sediments of the Markgräfler Hill Country (MHM) Floodplains of the Möhlin (BFM) Floodplains of the Neumagen (BFM)
350 cm
≤ Neolithic
ο/−
450 cm
≤ Neolithic
−/−
100 cm
≤ Neolithic
−/−
140 cm
≤ Neolithic
+/+
100 cm
≤ Neolithic
−/−
160 cm
o/−
80 cm
≤ Middle Ages ≤ Latène
o/o
80 cm
?
−/−
250 cm
≤ Neolithic ? − / −
105 cm
≤ Middle Ages ≤ Middle Ages
90 cm
o/o −/−
Abbreviations for landscape types: FBM = Freiburger Bucht (Möhlin Catchment), MGR = Markgräfler Rhine Plain, MHM = Markgräfler Hill Country (Möhlin Catchment), BFM = Schwarzwald. Age of sediments. ≤ = younger or same age as. ? = uncertain. Data appraisal: +, good; o, sufficient, −, minimal.
3.2. Assessment of the average thickness and volume of the Holocene sedimentary storage types After determining the spatial distribution of the Holocene sediments, the average thickness of each of the Holocene sedimentary storage types was assessed. This was carried out by evaluating all available data from previous research projects, which were mainly scientific studies by the Department of Physical Geography (University Freiburg) conducted within the DFG-Priority Programmes “Fluvial Geomorphodynamics of the Younger Quaternary” (Mäckel and Zollinger, 1989, 1995) and “Changes of the Geo-Biosphere During the Last 15,000 Years” (Mäckel and Friedmann, 1998; Schneider, 2000, Schneider et al., 2000; Mäckel et al., 2001, 2002,
203
2003). Further data on the thickness of the Holocene sediments were gained by means of our own fieldwork and by evaluating auger profiles from the mapping of the geological maps in the study area. In addition, information from archaeological sites within the DFG Research Training Group “Formation and Development of Present-Day Landscapes” was also considered for this study (Seidel, 2004; Mäckel et al., 2004). The available data for determining the thickness of the Holocene sedimentary storage types vary from 0 to 17 auger profiles and/or cross section through valley floors. For Holocene sedimentary storage types based on sufficient data, the thickness was obtained by determining the average base of the Holocene sediments from the auger profiles and excavations. In those cases, where there were only few or no data available, assessments about the thickness of the Holocene sediments were made by comparing data from Holocene sedimentary storage types from other landscape units with similar features. An overview of the gained results and an assessment of the age and an appraisal of the data base for both river catchments in the study area are shown in Tables 2 and 3. These tables also include a specification of the age of the Holocene sediments. This information was derived from more than one hundred 14C-datings, associated with field work in the course of this study and the research projects mentioned above. 3.3. Results of the sediment quantification The volume of the Holocene sediments was calculated by multiplying the average thickness with the area of the Holocene sedimentary storage types and adding these results. Table 4 shows an example of this procedure of the landscape unit “Black Forest” in the Elz catchment with five different Holocene sedimentary
Table 4 Results of the sediment quantification in the Black Forest (Elz catchment) Holocene sedimentary storage type
Ø Area Volume thickness (km2) (km3) (m)
Alluvial sediments in the Black Forest (without Brettenbach Valley) Alluvial sediments in the Brettenbach Valley Alluvial sediments in the Basin of Zarten Colluvial sediments in the Elz Valley Colluvial sediments west of the Black Forest main fault
0.9 2
54.42 0.048978 4.09 0.00818
0.9
10.74 0.009666
1.1 4
4.79 0.005269 0.86 0.00344
204
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
Table 5 Results of the sediment quantification
Area Total sediment volume Alluvial sediments Colluvial sediments Total area of Holocene sediments Area of alluvial sediments Area of colluvial sediments Ø thickness of alluvial sediments Ø Thickness of colluvial sediments
Elz catchment
Möhlin catchment
1502.60 km2 0.447 km3 0.161 km3 0.286 km3 270.49 km2 188.15 km2 82.34 km2 0.86 m 3.47 m
228.10 km2 0.073 km3 0.046 km3 0.027 km3 51.71 km2 42.09 km2 9.62 km2 1.09 m 2.80 m
storage types. An overview of the complete results is shown in Table 5. The sediment quantification in the Elz catchment added up to a sediment volume of 0.447 km3 with 0.161 km3 of alluvial sediments and 0.286 km3 of colluvial sediments, respectively. Thus, the greater parts of the Holocene sediments (63%) are accumulated in form of colluvial sediments. This can be explained by the fact, that wide areas of the Elz catchment are covered by loess (Kaiserstuhl Mountain, foothill zone of Lahr-Emmendingen). These loesscovered areas are characterized by thick colluvial sediments (up to 7 m) in the valley floors. In the Möhlin catchment, the total sediment volumes amount to 0.073 km3. 0.046 km3 are alluvial sediments and 0.027 km3 are colluvial sediments. In contrast to the
Elz catchment, the greater part of the Holocene sediment in the Möhlin catchment was deposited as alluvial sediment. The colluvial sediments make up only about 36% of the Holocene sediment volume, which is due to the relatively little loess-covered potential erosion surface (8 km2). 4. Calculation of average Holocene erosion in the study area The results of the sediment quantification were used to calculate average erosion for the Holocene. Generally, an erosion depth (E) is obtained by dividing the volume of the eroded material (V) through the potential erosion surface (A) according to Eq. (1). This equation also considers the sediment delivery ratio (SDR), since river catchments are “open systems”, in which eroded material is carried to the receiving stream to a certain degree. E ¼ V =Að1 SDRÞ
ð1Þ
The areas of accumulation are identical with the areas containing the alluvial and colluvial sediments shown on the 1:25,000 geological maps. The areas of erosion were derived from the digital elevation model (SRTM, 90 m resolution) of the study area. For the determination of the potential erosion surface, all areas outside the mapped Holocene sediments with slope over 2% were
Fig. 4. Relationships between the average erosion and the sediment delivery ratio (SDR) in the two river catchments.
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207 Table 6 Erosion amounts during the Holocene in the study area
Total volume of the Holocene sediments (km3) Erosion amount on potential erosion surface during the Holocene (cm) Erosion rates on potential erosion surface during the Holocene (m3ha− 1a− 1)
Elz catchment
Möhlin catchment
0.447
0.073
46–77
53–88
0.61–1.03
0.71–1.17
considered. According to the slope classifications of AG Boden (1994), these are areas with the slope classes N1 (slope of 2–3,5%) to N6 (slope N 36%). Beside the accumulation areas and the potential erosion surfaces, there are also sites in the study area, which were not or scarcely affected by erosion or accumulation processes, which is indicated for example in loess areas with uneroded luvisol profiles. Areas with a slope less than 2% (slope class N0 according to AG Boden, 1994) were classified as neutral areas regarding erosion processes. These are mainly areas covered with late glacial fluvial loams and mires, as well as relatively flat areas along the watersheds of sub-catchments, especially in the foothill zone of LahrEmmendingen. An indication of erosion and displacement processes within the accumulation areas could not be established, and therefore these processes cannot be regarded in this study. According to the classification above, the total potential erosion surface is 1108.5 km2 for the whole study area, divided into 970 km2 in the Elz catchment and 139 km2 in the Möhlin catchment. Eq. (1) for the calculation of the erosion amount includes two unknown variables (E and SDR) which can be set into the relationship. The result of this is shown in Fig. 4, in which the values determined for the sediment volumes (V) and the area of the potential erosion surface (A) in the study site are considered. The curve shows the erosion amount for both river catchments on the x-axis for different SDR values on the y-axis. The result resembles an exponential function. For low SDR values, the erosion remains around 1 m, but it rises quickly at SDR rates over 0.5 and draw asymptotically nearer to the y-axis. Unfortunately, no local studies on the SDR in the Upper Rhine Valley are available, and consequently an estimation of the SDR had to be derived from other studies. Leser et al. (1998) determined SDRs of 8–30% for different catchments in Germany and Switzerland. A study by Walling (1983) focusing on sites in the USA
205
and former USSR shows, that the SDR decreases with increasing catchment sizes. This is also observed by Van Rompaey et al. (2001) in the Dijle catchment in Belgium. Here the SDR decreases gradually to values of 20% when the upslope area exceeds 500 km2. Following these papers, the SDR in the Elz and Möhlin basins should be expected to be approximately 10%. In order to obtain an erosion amount on the basis of Eq. (1), the range of the SDR in the study area was considered to be between 0 and 40%. On the basis of the studies mentioned above, this seems a plausible range for the Elz and Möhlin catchment, since higher SDR values would also implement far higher erosion amounts, for which no evidence can be found in the study area. Assuming that the SDR ranges from 0% to 40%, the average erosion on the potential erosion surface during the Holocene could be deduced to be 46–77 cm in the Elz catchment and 53– 88 cm in the Möhlin catchment (Table 6). The average erosion rates for the Holocene range between 51± 13 mm ka− 1 in the Elz basin and 58.5 ± 14.5 mm ka− 1 in the Möhlin basin. 5. Chronological differentiation of the Holocene sediments On the basis of the available data, an area-wide chronological differentiation of the Holocene sediments has not been possible in the context of this study. The general problem is the fact, that datings in redeposited material such as alluvial and colluvial sediments only give information about the age of the material but not necessarily about the age of the sedimentation process. This is especially problematical in colluvial sediments, where cascaded transport and reworking of the material can occur (Lang et al., 2003b). Therefore the time of sediment deposition and the age of the dated material may not be coherent when using only 14C-datings. The evidence from fieldwork shows that the first erosion and accumulation processes took place in the loesscovered areas during the Neolithic. In less favourable landscapes, like the uplands of the Black Forest, the onset of erosion processes is marked by the colonization of these areas from the Middle Ages onwards. At some sites, it has been possible to establish a chronological differentiation of the alluvial sediments. On the fringes of the foothill zone of Lahr-Emmendingen in the northern parts of the Elz catchment for example, alluvial sediments with a thickness of 90 cm were deposited during the time of Neolithic to the late Bronze Age. These are overlain by another 60 cm of younger alluvial deposits, which date from the late Bronze Age to Modern Times (Schneider, 2000; Seidel, 2004). In the
206
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
basin of Zarten, the first alluvial sediments were dated to the middle Bronze Age. Data gained by field work show, that these sediments have an average thickness of about 50 cm (Seidel, 2004). An area-wide chronological differentiation of colluvial sediments is rather difficult, since colluvial deposits only provide information about local processes in the surrounding areas. The evaluation of the data on colluvial deposits has also shown that the chronological information derived from these sediments is partially inconsistent. Generally, the evaluated data indicate that the first thin colluvial sediments were deposited from the Neolithic onwards. During excavation works for a pipeline in 2002, relatively thin colluvial sediments (e.g. on Blankenberg Hill) could reliably be dated between the Neolithic and the Iron Age (Seidel et al., 2004). Qualitative statements can be made regarding the intensity of erosion and accumulation. The evaluated data show, that the greater part of these Holocene sediments were deposited during and after the Subboreal. Before this time, erosion and accumulation are at a rather low intensity. This changes from the Subatlantic onwards when the human impact on the environments increases noticeably (Mäckel et al., 2003). The evidence from colluvial sediments show, that most of the these sediments in the study area were deposited from the Middle Ages onwards (Seidel, 2004). 6. Discussion of results The approach used to determine the volumes of Holocene sediments in the study area by disaggregating them into different storage types does not allow us to specify an overall error range, since the errors determining the volume for each Holocene sedimentary storage type can add up or be compensated. Generally, the volumes of the alluvial sediments are more reliable than those of the colluvial deposits, since these sediments do not have a distinct relief, and the subsurface on the flood plains is usually relatively even. In contrast, colluvial sediments normally have a relief of their own, and information on the subsurface on which they were deposited is scarce and difficult to determine. This circumstance therefore implies a less precise determination of the sediment volumes regarding these storage types. A general problem is the fact, that there are several sites which were erosion sites first and subsequently covered with sediments, especially the lower slopes of the loess-covered areas. This is indicated by an eroded luvisol buried by colluvial sediments. Due to the lack of information on the dimension of these areas this case could not be considered in this study.
The obtained results from this research project can be compared with two recent studies within the RheinLUCIFS framework. Hoffmann et al. (2007) calculated a simplified sediment budget for the whole Rhine catchment and averaged an erosion rate of 38.5 ± 10.7 mm ka− 1 for the Holocene. This result does not take colluvial sediments on the hill slopes into account and it is based on a SDR of 10% and a mean floodplain thickness of 1.6 m. Therefore the results for the erosion rates are rather low (Hoffmann et al., 2007), but they correspond quite well to the results obtained for the Elz and Möhlin catchment in this study. Houben et al. (2007) conducted a detailed field-based empirical sediment budget in a small river catchment (10 km2) in the loess area of the Wetterau basin near Frankfurt/Main. These results show an averaged Holocene erosion amount of 50–70 cm and a catchment SDR of approximately 30%. The major part (∼ 90%) of the Holocene sediments remaining in the catchment are stored as colluvial slope deposits. That empirical study shows, that the SDR in a relatively small catchment is at the upper limit of what was considered to a plausible range for the Elz and Möhlin catchments. Another study by Rommens et al. (2005), who established a sediment budget in a small catchment in the Belgian loess belt, derived SDR between 20% and 42%. The results from these two studies and the fact, that the SDR decreases with increasing catchment size (see above, Section 4) therefore support the thesis, that the SDR in the two examined river basins in the Upper Rhine Valley are most likely to be noticeably less than 40% and within the range of what Hoffmann et al. (2007) assumed as SDR for the Rhine catchment (∼ 10%). 7. Conclusion In this study, the volumes of Holocene sediments in two river catchments (Elz and Möhlin) tributary to the River Rhine were deduced from the thickness of the sediments in combination with data on the spatial distribution of these sediments. This proves to be a promising approach to quantify Holocene sediments in mesoscale river catchments. The sediment volumes obtained were used to calculate integral denudation amounts and erosion rates for the Holocene. Though there were more than 100 14Cdatings and several archaeological findings available, only a limited amount of Holocene sediments in some storage types could be referred to certain cultural phases or time spans. The problem is, that the time of deposition of the Holocene sediments cannot be determined accurately by using information from 14C-dates and archaeological findings. These datings only indicate a terminus post
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
quem of the sedimentation process. In order to establish a chronological differentiation of the alluvial and colluvial sediments, dating techniques like OSL would have to be used to determine more reliable information about the course of sedimentation. The evidence from field work points out, that the greater part of the erosion and accumulation processes took place from the Subboreal onwards, so the determined erosion rates of 51 ±13 mm ka− 1 in the Elz basin and 58.5 ± 14.5 mm ka− 1 in the Möhlin basin are not linear. It can be assumed, that the erosion rates were much lower at the beginning of the Holocene and rose to higher rates than stated above. Acknowledgements The authors wish to thank the Deutsche Forschungsgemeinschaft (DFG) for the support of this research project (Ma 557/15). We would also like to thank Jens Vogel for comments proof-reading. References AG Boden, 1994. Bodenkundliche Kartieranleitung, 4th edition. Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover. Bork, H.-R., Bork, H., Dalchow, C., Faust, B., Piorr, H.-P., Schatz, T., 1998. Landschaftsentwicklung in Mitteleuropa. Wirkungen des Menschen auf Landschaften. Klett, Stuttgart. Dikau, R., Herget, J., Hennrich, K., 2005. Land use and climate impacts on fluvial systems during the period of agriculture in the River Rhine catchment (RhineLUCIFS) — an introduction. Erdkunde 59, 177–183. Friedmann, A., 2000. Die spät-und postglaziale Landschafts-und Vegetationsgeschichte des südlichen Oberrheintieflands und Schwarzwalds. Freiburger Geographische Hefte 62 Freiburg. Hoffmann, T., Erkens, G., Dikau, R., Houben, P., Seidel, J., Cohen, K.-M., 2007. Holocene flood plain sediment storage and hillslope erosion within the Rhine catchment. The Holocene 17 (1), 105–118. Houben, P., Burggraaff, P., Hoffmann, T., Kleefeld, K., Zimmermann, A., Dikau, R., 2007. Reconstructing Holocene land-use change and sediments budgets in the Rhine system. PAGES Newsletter 15 (1), 17–18. Lang, A., Bork, H.-R., Mäckel, R., Preston, N., Wunderlich, J., Dikau, R., 2003a. Changes in sediment flux and storage within a fluvial system: some examples from the Rhine catchment. Hydrological Processes 17, 3321–3334. Lang, A., Hennrich, K., Dikau, R., 2003b. Concepts and approaches to long term ans large scale modelling of fluvial systems. In: Lang, A., Hennrich, K., Dikau, R. (Eds.), Long Term Hillslope and Fluvial Aystem Modelling. Concepts and Case Studies from the Rhine River Catchment. Springer Verlag, Berlin, pp. 9–15. Lechner, A., 2005. Paläoökologische Beiträge zur Rekonstruktion der holozänen Vegetations-, Moor-und Flussauenentwicklung im Oberrheintiefland. Dissertation at the Faculty of Forest and Environmental Sciences, University of Freiburg, Freiburg. Leser, H., Prasuhn, V., Schaub, D., 1998. Bodenerosion und Landschaftshaushalt. In: Richter, G. (Ed.), Bodenerosion. Analyse
207
und Bilanz eines Umweltproblems. Wissenschaftliche Buchgesellschaft, Darmstadt, pp. 97–109. Ministerium für Umwelt und Verkehr Baden-Württemberg and Landesanstalt für Umweltschutz Baden-Württemberg. Wasser und Bodenatlas Baden-Württemberg. Mäckel, R., Friedmann, A., 1998. Wandel der Geo-Biosphäre in den letzten 15000 Jahren im südlichen und mittleren Oberrheintiefland und Schwarzwald. Freiburger Geographische Hefe 54 Freiburg. Mäckel, R., Zollinger, G., 1989. Fluvial action and valley development in the central and southern Black Forest during the Late Quaternary. Catena Supplement 15, 243–252. Mäckel, R., Zollinger, G., 1995. Holocene river and slope dynamics in the Black Forest and Upper Rhine Lowlands under the impact of man. Zeitschrift fur Geomorphologie N.F. Supplementband 100, 89–100. Mäckel, R., Friedmann, A., Seidel, J., Schneider, R., 2001. Natural and anthropogenic changes in the palaeoecosystem of the Black Forest and Upper Rhine Lowlands since the Bronze Age. Regensburger Beiträge zur Prähistorischen Archäologie 7, 143–160. Mäckel, R., Schneider, R., Friedmann, A., Seidel, J., 2002. Environmental changes and human impact on the relief development in the Upper Rhine Valley and Black Forest (South-West-Germany) during the Holocene. Zeitschrift für Geomorphologie, N.F. Supplementband 128, 31–45. Mäckel, R., Schneider, R., Seidel, J., 2003. Anthropogenic impact on the landscape of Southern Badenia (Germany) during the Holocene — documented by colluvial and alluvial sediments. Archaeometry 45, 487–501. Mäckel, R., Steuer, H., Uhlendahl, T., 2004. Gegenwartsbezogene Landschaftsgenese am Oberrhein. Berichte der Naturforschenden Gesellschaft zu Freiburg i. Br. 94, 175–194. Rommens, T., Verstraeten, G., Poesen, J., Govers, G., Van Rompaey, A., Peeters, I., Lang, A., 2005. Soil erosion and sediment deposition in the Belgian loess belt during the Holocene: establishing a sediment budget for a small agricultural catchment. The Holocene 15, 1032–1043. Schneider, R., 2000. Landschafts-und Umweltgeschichte im Einzugsgebiet der Elz. Dissertation an der Geowissenschaftlichen Fakultät der Universität Freiburg, http://freidok.uni-freiburg.de/volltexte/125. Schneider, R., Friedmann, A., Mäckel, R., 2000. Hangsedimente und Kolluvien in den Lößgebieten Südbadens. Berichte der Naturforschenden Gesellschaft zu Freiburg i. Br. 88/89, 1–16. Seidel, J., 2004. Massenbilanzen holozäner Sedimente am südlichen und mittleren Oberrhein. Dissertation an der Fakultät für Forst-und Umweltwissenschaften der Universität Freiburg, http://freidok.ub. uni-freiburg.de/volltexte/1565. Seidel, J., Faustmann, A., Rauschkolb, M., Sudhaus, D., 2004. Untersuchungen zur Landschaftsgenese entlang der TENP-Trasse im Raum Freiburg von 2001 bis 2003. Berichte der Naturforschenden Gesellschaft Freiburg i. Br. 94, 151–173. Sudhaus, D., 2005. Paläoökologische Untersuchungen zur spätglazialen und holozänen Landschaftsgenese des Ostschwarzwaldes im Vergleich mit den Buntsandsteinvogesen. Freiburger Geographische Hefte 64 Freiburg. Van Rompaey, A., Verstraten, G., Van Oost, K., Govers, G., Poesen, J., 2001. Modelling mean annual sediment yield using a distributed approach. Earth Surface Processes and Landforms 26, 1221–1236. Walling, D., 1983. The sediment delivery problem. Journal of Hydrology 65, 209–237.
Geomorphology 92 (2007) 198 – 207 www.elsevier.com/locate/geomorph
Holocene sediment budgets in two river catchments in the Southern Upper Rhine Valley, Germany Jochen Seidel ⁎, Rüdiger Mäckel Department of Physical Geography, University of Freiburg, Werderring 4, 79085 Freiburg, Germany Received 30 September 2005; received in revised form 31 March 2006; accepted 24 July 2006 Available online 24 May 2007
Abstract The Holocene sediments of two catchments in the southern Upper Rhine valley have been quantified as part of the German LUCIFS Programme (RheinLUCIFS), which aims to quantify sediment fluxes in the Rhine catchment since the onset of agriculture in the Neolithic about 7500 years ago. The spatial distribution of the alluvial and colluvial sediments was derived using geological maps, with information on the thickness of these sediments from various sources including auger profiles and data from excavations. The sediments were subdivided into characteristic sedimentary storage types according to the different types of landscapes. For each of the sedimentary storage types an average thickness was assessed so that an integral sediment balance for the Holocene could be derived. For the different types of landscapes in the study area, 32 Holocene sedimentary storage types were determined, 21 in the Elz catchment (1500 km2) and 11 in the Möhlin catchment (230 km2). By adding up the sediment volumes of all single sedimentary storage types the total Holocene sediment volumes for the two catchments were calculated. Erosion depths were determined by dividing the sediment volumes through the potential erosion areas (slope N 2%) and by assuming a sediment delivery ratio (SDR) between 0 and 0.4. The total erosion for the potential erosion areas during the Holocene was calculated as 31–61 cm in the Elz catchment and 44–79 cm in the Möhlin catchment. © 2007 Elsevier B.V. All rights reserved. Keywords: Holocene sediments; Sediment budgets; Upper Rhine Valley; Erosion rates
1. Introduction The research project presented in this paper was conducted as part of the German LUCIFS Programme (RheinLUCIFS), which is part of the PAGES Focus 5 “Past Ecosystem Processes and Human-Environment Interactions” science activities. The general aim of the LUCIFS (Land Use and Climate Impacts on Fluvial ⁎ Corresponding author. Tel.: +49 761 203 9117/3507; fax: +49 761 203 3596. E-mail address: [email protected] (J. Seidel). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.07.041
Systems during the period of agriculture) research project is to understand the particulate fluxes in fluvial systems since the period of agriculture (Lang et al., 2003a; Dikau et al., 2005). For the RheinLUCIFS project, the catchment of the River Rhine was selected as suitable study site, since the first settlements in this area date back to the Neolithic and there is a variety of studies and results already available. The first RheinLUCIFS project phase lasted from 2002 to 2004 and included five projects from different disciplines (geomorphology, archaeology and historical geography). The objective of the geomorphological projects within RheinLUCIFS was to develop sediment
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
199
Fig. 1. Location of study area.
budgets in selected regions and catchment sizes using different approaches. Within this framework, the aim of the research project at the University of Freiburg was to quantify the volume of Holocene sediments and to determine erosion amounts in two mesoscale river catchments (Elz and Möhlin) by evaluating data which were already available and supplemented by field work. The basic assumption was, that the first significant human impact on the natural landscape in the Upper Rhine Valley took place in the Neolithic about 7500 years ago. This time marks the onset of settlement and agricultural activities, which led to soil erosion and affected the sediment fluxes in river catchments. It is assumed that most parts of central Europe were covered by woodland and no recognizable soil erosion had taken place before the Neolithic (Bork et al., 1998). This
situation was also discovered by several investigations in the Upper Rhine area (Friedmann, 2000; Sudhaus, 2005; Lechner, 2005), which are based on the DFGPriority Programme “Changes of the Geo-Biosphere During the Last 15,000 Years” and the DFG Research Training Group “Formation and Development of Present-Day Landscapes” (Mäckel et al., 2004). 2. Study area The river catchments of the Elz and Möhlin are situated in the southern Upper Rhine Valley between the River Rhine and the Black Forest (Fig. 1). The source of the River Elz is located at an altitude of 1089 m a.s.l. in the Black Forest. In the Upper Rhine Valley, the course of the River Elz is strongly influenced by river engineering.
200
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
Fig. 2. Map with location of the evaluated data.
At the town of Riegel, where the important tributary rivers Dreisam and Glotter flow into the Elz, the river is split up. Most of the discharge follows the 12-km-long Leopold Canal directly into the River Rhine. This canal was put into operation in 1842 to reduce the flood risk in the Upper Rhine Valley north and east of the Kaiserstuhl. A small but constant amount of water is directed into the “Alte Elz”, the old riverbed of the Elz, which corresponds to the original course of the Elz before the Leopold Canal was built. The original mouth of the Elz into the Rhine was near Nonnenweier, but due to the massive engineering of the Rhine, the water is now directed parallel to the bank of the Rhine for several kilometres. This part of the Elz has not been considered in this study. The area of the Elz catchment examined is about 1500 km2 . The River Möhlin also originates in the Black Forest and flows through the Münster Valley (Münstertal) and across the Upper Rhine Valley towards its junction with the River Rhine south of the town of Breisach. In the 19th century the lower course of the River Möhlin was completely regulated and embanked. The size of the Möhlin catchment is 228 km2.
Both river catchments encompass different natural units of the Central and Southern Black Forest and the adjacent Upper Rhine Lowlands. The basins selected each represent a specific environmental milieu and show different responses to natural changes and human impact. Accordingly, the study sites in the climatically favoured loess areas of the Kaiserstuhl Mountain and the lower foothills as well as the plains and terraces of the River Rhine and its eastern tributaries show different histories and intensities of land use than the climatically cooler and moister upland areas of the Black Forest, which have been continuously populated since the Middle Ages. Qualitative studies on environmental changes and sediments in the study areas show that human impact varied during the Holocene (Schneider, 2000; Mäckel et al., 2001, 2002, 2003) with the most severe impacts occurring during Roman Times and from the Middle Ages onwards. 3. Quantification of Holocene sediments In order to obtain sediment volume for the Holocene sediments in the study area, the area as well as the
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
201
Fig. 3. Overview of the landscape types in the study area.
average depth of these archives had to be determined. The time from the Neolithic onwards is considered for the scientific framework of RheinLUCIFS, but since the border between pre- and post-neolithic sediments can usually not be determined in profiles, an integral calculation of the sediment volumes for the Holocene was conducted. The basic assumption to distinguish Holocene and Pleistocene sediments is, that the transition from the late Glacial to the Holocene is marked by a change in grain size of the sediments. Thus, coarse gravel and sand were allocated to the late Glacial and fine sediments were considered to be of Holocene age. Since the focus of this study was put on the evaluation of data that were already available, there was a spatially very heterogeneous database at hand (Fig. 2), which had to be evaluated regarding information about
the depth and age. Therefore in order to calculate the Holocene sediment volume the alluvial and colluvial sediments were spatially disaggregated into smaller units. 3.1. Determination of the Holocene sediment area The spatial distribution of Holocene sediments in the study area was derived from 1:25,000 scale geological maps. Due to their scale, these maps show some imprecision regarding the distribution of alluvial and colluvial sediments, but they are the only data sources that cover the whole of the study area. All mapped Holocene units were summarized in the two categories “Holocene Alluvial Sediments” and “Holocene Colluvial Sediments”.
202
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
Table 1 Number of Holocene sedimentary storage types in the study area Number of Holocene sedimentary storage types (HSST) Elz catchment 1. Offenburger Rhine Plain 2. Foothill Zone of LahrEmmendingen 3. Kaiserstuhl 4. Freiburger Bucht 5. Black Forest 6. Markgräfler Hill Country 7. Markgräfler Rhine Plain Möhlin Catchment 8. Freiburger Bucht 9. Markgräfler Rhine Plain 10. Markgräfler Hill Country 11. Black Forest Total:
4 3 2 5 5 2 -
3 2 4 2 32
In the next step, both river catchments were subdivided according to the different landscape types in the Water and Soil Atlas of Baden-Württemberg (WaBoA, Ministerium
für Umwelt und Verkehr Baden-Württemberg and Landesanstalt für Umweltschutz Baden-Württemberg, 2001). In this way 11 different landscape types were determined for the two river catchments (Fig. 3). Then characteristic Holocene sedimentary storage types were determined for each of the 11 landscape types in the two river catchments. Areas with similar features regarding relief, soil, geology and land use history were merged, resulting in a total of 32 Holocene sedimentary storage types for the whole study area (Table 1). Hillwash sediments displaced from their area of origin to another landscape type (e.g. the fringes of the colluvial loess in the foothill zone of Lahr-Emmendingen) were still considered as part of their area of origin. Thus the borders of the Holocene sedimentary storage types vary to a certain degree from the landscape types according to the WaBoA. In the case of the landscape type of the “Markgräfler Rhine Plain” (Elz catchment, 7 in Fig. 2), all mapped sediments originate from the adjacent landscape types, and therefore there were no sediments which could be allocated to this landscape type.
Table 2 Overview of the age and thickness of the Holocene sedimentary storage types in the Elz catchment Sedimentary storage type (landscape type)
Average thickness
Age of the sediments
Data appraisal (quality/quantity)
Floodplain sediments with Black Floodplain Soil (OGP) Floodplain sediments without Black Floodplain Soil (OGP) Filled river courses (OGP) Other colluvial sediments (OGP) Hillwash colluvial sediments (FZL) Valleys with colluvial fillings (FZL) Floodplains in the foothill zone (FZL) Valleys with colluvial fillings (KAI) Hillwash colluvial sediments (KAI) Floodplain sediments of the Rivers Elz, Glotter and Dreisam (FBE) Floodplain sediments at the fringe of the foothill zone of Lahr-Emmendingen (FBE) Valleys with colluvial fillings in the Tuniberg (FBE) Hillwash colluvial loess sediments at the Tuniberg and Mengen (FBE) Colluvial sediments in the northeastern part of the Freiburger Bucht (FBE) Floodplains in the Black Forest except Brettenbach Valley (BLF) Floodplains in the Brettenbach Valley(BLF) Floodplains in the Basin of Zarten (BLF) Colluvial sediments in the Elz Valley (BLF) Colluvial sediments west of the Main Black Forest Fault (BLF) Floodplain sediments east of the Schönberg (MHE) Other floodplain and colluvial sediments (MHE)
150 cm 130 cm 80 cm 70 cm 320 cm 600 cm 300 cm 500 cm 300 cm 100 cm 50 cm
≤Neolithic ≤Neolithic (?) ≤Modern times ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic ≤Neolithic
+/+ o/− ο/ο −/− −/− ο/− o/ο +/o o/o o/− −/o
450 cm 100 cm 100 cm
≤Neolithic ≤Neolithic ≤Neolithic
−/− o/− −/−
90 cm 200 cm 90 cm 110 cm 400 cm 70 cm 200 cm
≤Middle ages ≤Roman times (?) ≤Middle Bronze Age ≤Middle Ages ≤Neolithic ? ≤Neolithic
+/+ +/+ +/+ −/ο o/− −/− −/−
Abbreviations for landscape types: ORP = Offenburger Rhine Plain, FZL = Foothill Zone of Lahr-Emmendingen, KAI = Kaiserstuhl, FBE = Freiburger Bucht (Elz Catchment), BLF = Black Forest (Elz Catchment), MHC = Markgräfler Hill Country (Elz Catchment). Age of sediments. ≤ = younger or same age as. ? = uncertain. Data appraisal: +, good; o, sufficient; −, minimal.
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207 Table 3 Overview of the age and thickness of the Holocene sedimentary storage types in the Möhlin catchment Sedimentary storage type (landscape type)
Average Age of the thickness sediments
Data appraisal (quality/ quantity)
Colluvial sediments of the Mengener Brücke (FBM) Valleys with colluvial fillings in the Tuniberg (FBM) Hillwash colluvial sediments of the Tuniberg (FBM) Alluvial sediments with Black Floodplain Soil (MGR) Alluvial sediments without Black Floodplain Soil (MGR) Narrow floodplains (MHM) Floodplains of the Möhlin from Ehrenstetten onwards (MHM) Floodplains of the Neumagen (MHM) Colluvial sediments of the Markgräfler Hill Country (MHM) Floodplains of the Möhlin (BFM) Floodplains of the Neumagen (BFM)
350 cm
≤ Neolithic
ο/−
450 cm
≤ Neolithic
−/−
100 cm
≤ Neolithic
−/−
140 cm
≤ Neolithic
+/+
100 cm
≤ Neolithic
−/−
160 cm
o/−
80 cm
≤ Middle Ages ≤ Latène
o/o
80 cm
?
−/−
250 cm
≤ Neolithic ? − / −
105 cm
≤ Middle Ages ≤ Middle Ages
90 cm
o/o −/−
Abbreviations for landscape types: FBM = Freiburger Bucht (Möhlin Catchment), MGR = Markgräfler Rhine Plain, MHM = Markgräfler Hill Country (Möhlin Catchment), BFM = Schwarzwald. Age of sediments. ≤ = younger or same age as. ? = uncertain. Data appraisal: +, good; o, sufficient, −, minimal.
3.2. Assessment of the average thickness and volume of the Holocene sedimentary storage types After determining the spatial distribution of the Holocene sediments, the average thickness of each of the Holocene sedimentary storage types was assessed. This was carried out by evaluating all available data from previous research projects, which were mainly scientific studies by the Department of Physical Geography (University Freiburg) conducted within the DFG-Priority Programmes “Fluvial Geomorphodynamics of the Younger Quaternary” (Mäckel and Zollinger, 1989, 1995) and “Changes of the Geo-Biosphere During the Last 15,000 Years” (Mäckel and Friedmann, 1998; Schneider, 2000, Schneider et al., 2000; Mäckel et al., 2001, 2002,
203
2003). Further data on the thickness of the Holocene sediments were gained by means of our own fieldwork and by evaluating auger profiles from the mapping of the geological maps in the study area. In addition, information from archaeological sites within the DFG Research Training Group “Formation and Development of Present-Day Landscapes” was also considered for this study (Seidel, 2004; Mäckel et al., 2004). The available data for determining the thickness of the Holocene sedimentary storage types vary from 0 to 17 auger profiles and/or cross section through valley floors. For Holocene sedimentary storage types based on sufficient data, the thickness was obtained by determining the average base of the Holocene sediments from the auger profiles and excavations. In those cases, where there were only few or no data available, assessments about the thickness of the Holocene sediments were made by comparing data from Holocene sedimentary storage types from other landscape units with similar features. An overview of the gained results and an assessment of the age and an appraisal of the data base for both river catchments in the study area are shown in Tables 2 and 3. These tables also include a specification of the age of the Holocene sediments. This information was derived from more than one hundred 14C-datings, associated with field work in the course of this study and the research projects mentioned above. 3.3. Results of the sediment quantification The volume of the Holocene sediments was calculated by multiplying the average thickness with the area of the Holocene sedimentary storage types and adding these results. Table 4 shows an example of this procedure of the landscape unit “Black Forest” in the Elz catchment with five different Holocene sedimentary
Table 4 Results of the sediment quantification in the Black Forest (Elz catchment) Holocene sedimentary storage type
Ø Area Volume thickness (km2) (km3) (m)
Alluvial sediments in the Black Forest (without Brettenbach Valley) Alluvial sediments in the Brettenbach Valley Alluvial sediments in the Basin of Zarten Colluvial sediments in the Elz Valley Colluvial sediments west of the Black Forest main fault
0.9 2
54.42 0.048978 4.09 0.00818
0.9
10.74 0.009666
1.1 4
4.79 0.005269 0.86 0.00344
204
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
Table 5 Results of the sediment quantification
Area Total sediment volume Alluvial sediments Colluvial sediments Total area of Holocene sediments Area of alluvial sediments Area of colluvial sediments Ø thickness of alluvial sediments Ø Thickness of colluvial sediments
Elz catchment
Möhlin catchment
1502.60 km2 0.447 km3 0.161 km3 0.286 km3 270.49 km2 188.15 km2 82.34 km2 0.86 m 3.47 m
228.10 km2 0.073 km3 0.046 km3 0.027 km3 51.71 km2 42.09 km2 9.62 km2 1.09 m 2.80 m
storage types. An overview of the complete results is shown in Table 5. The sediment quantification in the Elz catchment added up to a sediment volume of 0.447 km3 with 0.161 km3 of alluvial sediments and 0.286 km3 of colluvial sediments, respectively. Thus, the greater parts of the Holocene sediments (63%) are accumulated in form of colluvial sediments. This can be explained by the fact, that wide areas of the Elz catchment are covered by loess (Kaiserstuhl Mountain, foothill zone of Lahr-Emmendingen). These loesscovered areas are characterized by thick colluvial sediments (up to 7 m) in the valley floors. In the Möhlin catchment, the total sediment volumes amount to 0.073 km3. 0.046 km3 are alluvial sediments and 0.027 km3 are colluvial sediments. In contrast to the
Elz catchment, the greater part of the Holocene sediment in the Möhlin catchment was deposited as alluvial sediment. The colluvial sediments make up only about 36% of the Holocene sediment volume, which is due to the relatively little loess-covered potential erosion surface (8 km2). 4. Calculation of average Holocene erosion in the study area The results of the sediment quantification were used to calculate average erosion for the Holocene. Generally, an erosion depth (E) is obtained by dividing the volume of the eroded material (V) through the potential erosion surface (A) according to Eq. (1). This equation also considers the sediment delivery ratio (SDR), since river catchments are “open systems”, in which eroded material is carried to the receiving stream to a certain degree. E ¼ V =Að1 SDRÞ
ð1Þ
The areas of accumulation are identical with the areas containing the alluvial and colluvial sediments shown on the 1:25,000 geological maps. The areas of erosion were derived from the digital elevation model (SRTM, 90 m resolution) of the study area. For the determination of the potential erosion surface, all areas outside the mapped Holocene sediments with slope over 2% were
Fig. 4. Relationships between the average erosion and the sediment delivery ratio (SDR) in the two river catchments.
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207 Table 6 Erosion amounts during the Holocene in the study area
Total volume of the Holocene sediments (km3) Erosion amount on potential erosion surface during the Holocene (cm) Erosion rates on potential erosion surface during the Holocene (m3ha− 1a− 1)
Elz catchment
Möhlin catchment
0.447
0.073
46–77
53–88
0.61–1.03
0.71–1.17
considered. According to the slope classifications of AG Boden (1994), these are areas with the slope classes N1 (slope of 2–3,5%) to N6 (slope N 36%). Beside the accumulation areas and the potential erosion surfaces, there are also sites in the study area, which were not or scarcely affected by erosion or accumulation processes, which is indicated for example in loess areas with uneroded luvisol profiles. Areas with a slope less than 2% (slope class N0 according to AG Boden, 1994) were classified as neutral areas regarding erosion processes. These are mainly areas covered with late glacial fluvial loams and mires, as well as relatively flat areas along the watersheds of sub-catchments, especially in the foothill zone of LahrEmmendingen. An indication of erosion and displacement processes within the accumulation areas could not be established, and therefore these processes cannot be regarded in this study. According to the classification above, the total potential erosion surface is 1108.5 km2 for the whole study area, divided into 970 km2 in the Elz catchment and 139 km2 in the Möhlin catchment. Eq. (1) for the calculation of the erosion amount includes two unknown variables (E and SDR) which can be set into the relationship. The result of this is shown in Fig. 4, in which the values determined for the sediment volumes (V) and the area of the potential erosion surface (A) in the study site are considered. The curve shows the erosion amount for both river catchments on the x-axis for different SDR values on the y-axis. The result resembles an exponential function. For low SDR values, the erosion remains around 1 m, but it rises quickly at SDR rates over 0.5 and draw asymptotically nearer to the y-axis. Unfortunately, no local studies on the SDR in the Upper Rhine Valley are available, and consequently an estimation of the SDR had to be derived from other studies. Leser et al. (1998) determined SDRs of 8–30% for different catchments in Germany and Switzerland. A study by Walling (1983) focusing on sites in the USA
205
and former USSR shows, that the SDR decreases with increasing catchment sizes. This is also observed by Van Rompaey et al. (2001) in the Dijle catchment in Belgium. Here the SDR decreases gradually to values of 20% when the upslope area exceeds 500 km2. Following these papers, the SDR in the Elz and Möhlin basins should be expected to be approximately 10%. In order to obtain an erosion amount on the basis of Eq. (1), the range of the SDR in the study area was considered to be between 0 and 40%. On the basis of the studies mentioned above, this seems a plausible range for the Elz and Möhlin catchment, since higher SDR values would also implement far higher erosion amounts, for which no evidence can be found in the study area. Assuming that the SDR ranges from 0% to 40%, the average erosion on the potential erosion surface during the Holocene could be deduced to be 46–77 cm in the Elz catchment and 53– 88 cm in the Möhlin catchment (Table 6). The average erosion rates for the Holocene range between 51± 13 mm ka− 1 in the Elz basin and 58.5 ± 14.5 mm ka− 1 in the Möhlin basin. 5. Chronological differentiation of the Holocene sediments On the basis of the available data, an area-wide chronological differentiation of the Holocene sediments has not been possible in the context of this study. The general problem is the fact, that datings in redeposited material such as alluvial and colluvial sediments only give information about the age of the material but not necessarily about the age of the sedimentation process. This is especially problematical in colluvial sediments, where cascaded transport and reworking of the material can occur (Lang et al., 2003b). Therefore the time of sediment deposition and the age of the dated material may not be coherent when using only 14C-datings. The evidence from fieldwork shows that the first erosion and accumulation processes took place in the loesscovered areas during the Neolithic. In less favourable landscapes, like the uplands of the Black Forest, the onset of erosion processes is marked by the colonization of these areas from the Middle Ages onwards. At some sites, it has been possible to establish a chronological differentiation of the alluvial sediments. On the fringes of the foothill zone of Lahr-Emmendingen in the northern parts of the Elz catchment for example, alluvial sediments with a thickness of 90 cm were deposited during the time of Neolithic to the late Bronze Age. These are overlain by another 60 cm of younger alluvial deposits, which date from the late Bronze Age to Modern Times (Schneider, 2000; Seidel, 2004). In the
206
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
basin of Zarten, the first alluvial sediments were dated to the middle Bronze Age. Data gained by field work show, that these sediments have an average thickness of about 50 cm (Seidel, 2004). An area-wide chronological differentiation of colluvial sediments is rather difficult, since colluvial deposits only provide information about local processes in the surrounding areas. The evaluation of the data on colluvial deposits has also shown that the chronological information derived from these sediments is partially inconsistent. Generally, the evaluated data indicate that the first thin colluvial sediments were deposited from the Neolithic onwards. During excavation works for a pipeline in 2002, relatively thin colluvial sediments (e.g. on Blankenberg Hill) could reliably be dated between the Neolithic and the Iron Age (Seidel et al., 2004). Qualitative statements can be made regarding the intensity of erosion and accumulation. The evaluated data show, that the greater part of these Holocene sediments were deposited during and after the Subboreal. Before this time, erosion and accumulation are at a rather low intensity. This changes from the Subatlantic onwards when the human impact on the environments increases noticeably (Mäckel et al., 2003). The evidence from colluvial sediments show, that most of the these sediments in the study area were deposited from the Middle Ages onwards (Seidel, 2004). 6. Discussion of results The approach used to determine the volumes of Holocene sediments in the study area by disaggregating them into different storage types does not allow us to specify an overall error range, since the errors determining the volume for each Holocene sedimentary storage type can add up or be compensated. Generally, the volumes of the alluvial sediments are more reliable than those of the colluvial deposits, since these sediments do not have a distinct relief, and the subsurface on the flood plains is usually relatively even. In contrast, colluvial sediments normally have a relief of their own, and information on the subsurface on which they were deposited is scarce and difficult to determine. This circumstance therefore implies a less precise determination of the sediment volumes regarding these storage types. A general problem is the fact, that there are several sites which were erosion sites first and subsequently covered with sediments, especially the lower slopes of the loess-covered areas. This is indicated by an eroded luvisol buried by colluvial sediments. Due to the lack of information on the dimension of these areas this case could not be considered in this study.
The obtained results from this research project can be compared with two recent studies within the RheinLUCIFS framework. Hoffmann et al. (2007) calculated a simplified sediment budget for the whole Rhine catchment and averaged an erosion rate of 38.5 ± 10.7 mm ka− 1 for the Holocene. This result does not take colluvial sediments on the hill slopes into account and it is based on a SDR of 10% and a mean floodplain thickness of 1.6 m. Therefore the results for the erosion rates are rather low (Hoffmann et al., 2007), but they correspond quite well to the results obtained for the Elz and Möhlin catchment in this study. Houben et al. (2007) conducted a detailed field-based empirical sediment budget in a small river catchment (10 km2) in the loess area of the Wetterau basin near Frankfurt/Main. These results show an averaged Holocene erosion amount of 50–70 cm and a catchment SDR of approximately 30%. The major part (∼ 90%) of the Holocene sediments remaining in the catchment are stored as colluvial slope deposits. That empirical study shows, that the SDR in a relatively small catchment is at the upper limit of what was considered to a plausible range for the Elz and Möhlin catchments. Another study by Rommens et al. (2005), who established a sediment budget in a small catchment in the Belgian loess belt, derived SDR between 20% and 42%. The results from these two studies and the fact, that the SDR decreases with increasing catchment size (see above, Section 4) therefore support the thesis, that the SDR in the two examined river basins in the Upper Rhine Valley are most likely to be noticeably less than 40% and within the range of what Hoffmann et al. (2007) assumed as SDR for the Rhine catchment (∼ 10%). 7. Conclusion In this study, the volumes of Holocene sediments in two river catchments (Elz and Möhlin) tributary to the River Rhine were deduced from the thickness of the sediments in combination with data on the spatial distribution of these sediments. This proves to be a promising approach to quantify Holocene sediments in mesoscale river catchments. The sediment volumes obtained were used to calculate integral denudation amounts and erosion rates for the Holocene. Though there were more than 100 14Cdatings and several archaeological findings available, only a limited amount of Holocene sediments in some storage types could be referred to certain cultural phases or time spans. The problem is, that the time of deposition of the Holocene sediments cannot be determined accurately by using information from 14C-dates and archaeological findings. These datings only indicate a terminus post
J. Seidel, R. Mäckel / Geomorphology 92 (2007) 198–207
quem of the sedimentation process. In order to establish a chronological differentiation of the alluvial and colluvial sediments, dating techniques like OSL would have to be used to determine more reliable information about the course of sedimentation. The evidence from field work points out, that the greater part of the erosion and accumulation processes took place from the Subboreal onwards, so the determined erosion rates of 51 ±13 mm ka− 1 in the Elz basin and 58.5 ± 14.5 mm ka− 1 in the Möhlin basin are not linear. It can be assumed, that the erosion rates were much lower at the beginning of the Holocene and rose to higher rates than stated above. Acknowledgements The authors wish to thank the Deutsche Forschungsgemeinschaft (DFG) for the support of this research project (Ma 557/15). We would also like to thank Jens Vogel for comments proof-reading. References AG Boden, 1994. Bodenkundliche Kartieranleitung, 4th edition. Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover. Bork, H.-R., Bork, H., Dalchow, C., Faust, B., Piorr, H.-P., Schatz, T., 1998. Landschaftsentwicklung in Mitteleuropa. Wirkungen des Menschen auf Landschaften. Klett, Stuttgart. Dikau, R., Herget, J., Hennrich, K., 2005. Land use and climate impacts on fluvial systems during the period of agriculture in the River Rhine catchment (RhineLUCIFS) — an introduction. Erdkunde 59, 177–183. Friedmann, A., 2000. Die spät-und postglaziale Landschafts-und Vegetationsgeschichte des südlichen Oberrheintieflands und Schwarzwalds. Freiburger Geographische Hefte 62 Freiburg. Hoffmann, T., Erkens, G., Dikau, R., Houben, P., Seidel, J., Cohen, K.-M., 2007. Holocene flood plain sediment storage and hillslope erosion within the Rhine catchment. The Holocene 17 (1), 105–118. Houben, P., Burggraaff, P., Hoffmann, T., Kleefeld, K., Zimmermann, A., Dikau, R., 2007. Reconstructing Holocene land-use change and sediments budgets in the Rhine system. PAGES Newsletter 15 (1), 17–18. Lang, A., Bork, H.-R., Mäckel, R., Preston, N., Wunderlich, J., Dikau, R., 2003a. Changes in sediment flux and storage within a fluvial system: some examples from the Rhine catchment. Hydrological Processes 17, 3321–3334. Lang, A., Hennrich, K., Dikau, R., 2003b. Concepts and approaches to long term ans large scale modelling of fluvial systems. In: Lang, A., Hennrich, K., Dikau, R. (Eds.), Long Term Hillslope and Fluvial Aystem Modelling. Concepts and Case Studies from the Rhine River Catchment. Springer Verlag, Berlin, pp. 9–15. Lechner, A., 2005. Paläoökologische Beiträge zur Rekonstruktion der holozänen Vegetations-, Moor-und Flussauenentwicklung im Oberrheintiefland. Dissertation at the Faculty of Forest and Environmental Sciences, University of Freiburg, Freiburg. Leser, H., Prasuhn, V., Schaub, D., 1998. Bodenerosion und Landschaftshaushalt. In: Richter, G. (Ed.), Bodenerosion. Analyse
207
und Bilanz eines Umweltproblems. Wissenschaftliche Buchgesellschaft, Darmstadt, pp. 97–109. Ministerium für Umwelt und Verkehr Baden-Württemberg and Landesanstalt für Umweltschutz Baden-Württemberg. Wasser und Bodenatlas Baden-Württemberg. Mäckel, R., Friedmann, A., 1998. Wandel der Geo-Biosphäre in den letzten 15000 Jahren im südlichen und mittleren Oberrheintiefland und Schwarzwald. Freiburger Geographische Hefe 54 Freiburg. Mäckel, R., Zollinger, G., 1989. Fluvial action and valley development in the central and southern Black Forest during the Late Quaternary. Catena Supplement 15, 243–252. Mäckel, R., Zollinger, G., 1995. Holocene river and slope dynamics in the Black Forest and Upper Rhine Lowlands under the impact of man. Zeitschrift fur Geomorphologie N.F. Supplementband 100, 89–100. Mäckel, R., Friedmann, A., Seidel, J., Schneider, R., 2001. Natural and anthropogenic changes in the palaeoecosystem of the Black Forest and Upper Rhine Lowlands since the Bronze Age. Regensburger Beiträge zur Prähistorischen Archäologie 7, 143–160. Mäckel, R., Schneider, R., Friedmann, A., Seidel, J., 2002. Environmental changes and human impact on the relief development in the Upper Rhine Valley and Black Forest (South-West-Germany) during the Holocene. Zeitschrift für Geomorphologie, N.F. Supplementband 128, 31–45. Mäckel, R., Schneider, R., Seidel, J., 2003. Anthropogenic impact on the landscape of Southern Badenia (Germany) during the Holocene — documented by colluvial and alluvial sediments. Archaeometry 45, 487–501. Mäckel, R., Steuer, H., Uhlendahl, T., 2004. Gegenwartsbezogene Landschaftsgenese am Oberrhein. Berichte der Naturforschenden Gesellschaft zu Freiburg i. Br. 94, 175–194. Rommens, T., Verstraeten, G., Poesen, J., Govers, G., Van Rompaey, A., Peeters, I., Lang, A., 2005. Soil erosion and sediment deposition in the Belgian loess belt during the Holocene: establishing a sediment budget for a small agricultural catchment. The Holocene 15, 1032–1043. Schneider, R., 2000. Landschafts-und Umweltgeschichte im Einzugsgebiet der Elz. Dissertation an der Geowissenschaftlichen Fakultät der Universität Freiburg, http://freidok.uni-freiburg.de/volltexte/125. Schneider, R., Friedmann, A., Mäckel, R., 2000. Hangsedimente und Kolluvien in den Lößgebieten Südbadens. Berichte der Naturforschenden Gesellschaft zu Freiburg i. Br. 88/89, 1–16. Seidel, J., 2004. Massenbilanzen holozäner Sedimente am südlichen und mittleren Oberrhein. Dissertation an der Fakultät für Forst-und Umweltwissenschaften der Universität Freiburg, http://freidok.ub. uni-freiburg.de/volltexte/1565. Seidel, J., Faustmann, A., Rauschkolb, M., Sudhaus, D., 2004. Untersuchungen zur Landschaftsgenese entlang der TENP-Trasse im Raum Freiburg von 2001 bis 2003. Berichte der Naturforschenden Gesellschaft Freiburg i. Br. 94, 151–173. Sudhaus, D., 2005. Paläoökologische Untersuchungen zur spätglazialen und holozänen Landschaftsgenese des Ostschwarzwaldes im Vergleich mit den Buntsandsteinvogesen. Freiburger Geographische Hefte 64 Freiburg. Van Rompaey, A., Verstraten, G., Van Oost, K., Govers, G., Poesen, J., 2001. Modelling mean annual sediment yield using a distributed approach. Earth Surface Processes and Landforms 26, 1221–1236. Walling, D., 1983. The sediment delivery problem. Journal of Hydrology 65, 209–237.