DEFINING QUANTITATIVE MORPHOLOGICAL CHANGES IN LARGE RIVERS FOR A SUSTAINABLE AND EFFECTIVE SEDIMENTMANGEMENT APPLIED TO THE RIVER ELBE; GERMANY

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1 DEFINING QUANTITATIVE MORPHOLOGICAL CHANGES IN LARGE RIVERS FOR A SUSTAINABLE AND EFFECTIVE SEDIMENTMANGEMENT APPLIED TO THE RIVER ELBE; GERMANY Frauke Koenig 1, Ina Quick 2 and Stefan Vollmer 3 ABSTRACT To ensure a sustainable use of rivers an adequate and effective sediment management is necessary. The evaluation of natural changes and anthropogenic influences as well as a high degree of understanding of the morphological system are essential for an effective and sustainable sediment management. The Federal Institute of Hydrology in Germany developed methodologies to define and quantify hydromorphological indicators for an integrative sediment management. It will be described how changes in the sediment budget and morphodynamic processes could be quantified and evaluated using the example of the River Elbe. To quantify f. ex. bed level changes, coarsening of bed substrate or changes in the depth variability historical data were analyzed and compared with current values. Some stretches of the inland waterway Elbe show maximum erosion of 2 m in the last century. The knowledge gained shall be used to conduct a sustainable sediment management improving the ecological condition and ensure navigation. 1. INTRODUCTION Due to different reasons e.g. afflux, navigation, flood protection or in accordance to an urban setting the sediment budget and hydromorphological structures and processes of many rivers are eminently degraded with adverse effects for the ecology. F. ex. changes of the sediment composition have contributed to a loss of spawning habitats in many rivers (Noack 2012, Kondolf 2008). Riverbed degradation adversely affects not only the stability of river-training structures and bridges but also the functioning of ecosystems depending on groundwater like floodplains by causing irreversible losses of softwood and hardwood forests with its specific vegetation and fauna (SedNet 2006). Furthermore erosion as well as sedimentation processes could endanger navigation. The European Water Framework Directive (WFD) is the key initiative and legal basis aimed at improving water quality throughout the EU. A study carried out by European Environmental Agency showed that for 87% of the water bodies in Germany hydromorphological pressures and impacts are one of the most important risks of failing to achieve the directives objectives (EEA 2012). Hence to fulfill the legal requirements, to ensure ecosystem services and a sustainable use of rivers improvement of the hydromorphological conditions including an adapted sediment 1 Dr.-Ing. Frauke Koenig, Department M3 - Groundwater, Geology, River Morphology, German Federal Institute of Hydrology, Koblenz, Germany (frauke.koenig@bafg.de) 2 Dr. rer. nat. Ina Quick, Department M3 - Groundwater, Geology, River Morphology, German Federal Institute of Hydrology, Koblenz, Germany (quick@bafg.de) 3 Dr.-Ing. Stefan Vollmer, Department M3 - Groundwater, Geology, River Morphology, German Federal Institute of Hydrology, Koblenz, Germany (vollmer@bafg.de)

2 management is necessary. Therefore the evaluation of natural changes and anthropogenic influences is essential. Additionally there is a clear need to better understand hydromorphological and sediment dynamics and their interactions with human management and ecosystem functioning and services (SedNet, 2006, Noack, 2012, Vollmer et al., 2012, Quick, 2012). Knowledge of reference conditions regarding sediment budget and hydromorphological and hydrological conditions and processes is therefore necessary. Due to missing reference locations with natural or near-natural conditions the quantitative description of indicators characterizing the hydromorphological reference conditions and processes is difficult. However, the use of historic data provides good opportunities to evaluate the impacts of anthropogenic influences and improve system understanding. The German Federal Institute of Hydrology developed methodologies to define and quantify hydromorphological indicators which serve among others as a basis for an integrative sediment management. The presented method was developed in the framework of application of the module Valmorph (evaluation of MORPHology, one of the modules of the INtegrated FlOodplain Response Model INFORM of the German Federal Institute of Hydrology) for the River Elbe, conducted partly by an ad-hoc expert group of the River Basin Community Elbe (FGG, SSeM) (Quick et al., 2012, Rosenzweig et al. 2012). To quantify f. ex. bed level changes, modifications of the mean grain size or changes in the depth variability historical data was therefore analyzed and compared with current data. Thus, it could be described how changes of these hydromorphological indicators, which are representative for the sediment budget and morphodynamic processes, could be quantified and evaluated using the example of the Elbe River. Furthermore these indicators are well suited to describe impacts on the ecology due to anthropogenic influences. 2. STUDY AREA The River Elbe runs from the Czech Giant Mountains to the Czech-German border and through central and northern Germany before discharging into the North Sea. The Elbe is one of the major waterways in Germany with a total length of 1094 km, a mean discharge of 862 m³/s into the North Sea and a river basin of km². The upper reaches of the River Elbe in the Czech Republic are characterized by a series of barrages and reservoirs whereas the river is free flowing from Usti-nad- Labem near the Czech-German border till the barrage at Geesthacht near Hamburg. This investigation is focused on the German part of the River Elbe till the tidal limit at the barrage at Geesthacht. Although the first morphological and hydraulic changes are dated back to the 12 th century the most appreciable river regulations were conducted in the 19 th century a great river training program to improve navigation was established (Rohde, 1971). Narrowing of the floodplains by dykes and river training by groynes etc. have increased the sediment transport capacity of the river. Whereas e.g. impounding of the Czech part and of major tributaries has drastically reduced the natural bed load and suspended load supply from upstream (SedNet, 2006). There is almost no sediment transport in the upper part of the River Elbe in Germany, from the Czech-German border till Elbe km 127 and the river bed is stable and armoured (Naumann et al., 2003). Downstream of this section, from Elbe-km follows the so called erosion stretch, a morphodynamic active river stretch with a local maximum erosion rate amounts to 2cm/year (WS, 2009, WSV 2009, WSA Dresden, 2011). Since 1996 artificial bed load supply, consisting of a sand gravel mixture should compensate the sediment deficits and reduce erosion (Faulhaber and Alexy, 2005, WSA Dresden, 2011). Another morphodynamic active river stretch is located downstream of the mouth of the tributary Havel (Elbe-km 428) till Elbe-km 521 with distinct bed forms (Nestmann and Buechele, 2002). The River Elbe has lost a majority of its floodplains. Those that remain are subject to annual floods and support wetland and floodplain forest habitats that have been internationally recognized and are unique within central Europe (Brunotte et al., 2009).

3 Figure 1 Location of the German part of the River Elbe and a view of the river close to Magdeburg (Hillebrand, 2009). 3. METHODS One component for an integrative sediment management plan of the River Elbe is the quantitative evaluation of different hydromorphological indicators like bed level change, modification of the mean grain size diameter or changes in the depth variability of the river. This improves understanding of the morphological system of the River Elbe and quantifies changes in the last century due to river regulation and river management. Therefore current characteristics of these parameters are correlated with historical data, which serve as reference condition (Quick et al., 2012). The indicator specific methodology and the database used are described in the following text Data basis A comprehensive and sufficient collection of hydrological and morphological data of the river Elbe from 1898 Der Elbstrom, sein Stromgebiet und seine wichtigsten Nebenflüsse ( The River Elbe, its river basin and major tributaries ) was used to describe the reference condition. The data collection does not reflect an unaffected natural condition of the River Elbe, but is the only available data basis for the whole study area of the Elbe River from Elbe-km 0 to 586 in the desired resolution. Furthermore these data describes a less affected river compared with the current hydromorphological conditions. The data collection provides different water levels corresponding to different discharges and the bed level for the whole part of the River Elbe in Germany with a longitudinal resolution of ~200m (cf. Figure 1). The historical data was digitalized and transformed to current reference systems.

4 Water levels Bed level Figure 2 Bed- and waterlevels of the River Elbe from Der Elbstrom, sein Stromgebiet und seine wichtigsten Nebenflüsse 1898 The current characteristics of the investigated parameters were determined from sediment samples of the federal waterway of the river Elbe, digital terrain models from 2004 and corresponding water levels. The sediment samples of the upper part of the River Elbe in Germany, till Elbe-km 315 were taken in Downstream, sediment samples were only available from 1993 and All sediment samples are grab samples and were collected in the thalweg of the waterway with a longitudinal resolution of at least 5 km. The German Waterways and Shipping Administration provided echo sounding data of the federal waterway with a mean resolution of approx. 1 m which served as a basis for digital terrain models of the river Elbe from To compare historical and current water levels discharges need to be in the same order. Therefore the corresponding current water levels were calculated for the discharges of the historical dataset with the in-house water level information system Flys ( 2012). An investigation of discharges in Dresden (Elbe-km 56) and Darchau (Elbe-km 536) revealed that there has been no decrease in the mean discharges in the last 106/108 years. A slight increase of low discharges was observed which could result of the reservoir management in the tributaries and the Czech part of the River Elbe (Quick et al., 2012) Bed level change The evaluation of bed level changes enables a detection of river stretches with erosion or sedimentation in a certain time period and helps to understand long-term morphological changes. Furthermore it is helpful to gain information of pollution sinks and sources (Quick et al., 2012). The difference of the historical bed level from 1898 and the bed level from 2004 provides information of the bed level change of the River Elbe in a time period of 106 years. Negative values show erosion, positive values show sedimentation. To facilitate the analysis and identify trends a running average of the bed level change for equidistant river stretches of 5 km was determined (cf. Figure 3). For validation purposes the difference of historic and current water levels, the difference of low water levels in particular, was also calculated.

5 3.3. Depth variability The depth variability shows frequency and magnitude of changes in the water depth and allows conclusions regarding the habitat suitability in a certain river stretch. F. ex. studies of Jungwirth et al. (2003) on different river reaches in Austria showed that the variance of the maximum depth correlated with the number of fish species and fish species diversity. The historic water depth was determined from the mean water level and the bed level from The calculated corresponding mean water level (cf. chapter 3.1) and bed level from 2004 served as basis for the current water depth. For evaluation the standard deviation of the historic depth variability and the current depth variability for equidistant river stretches of 5 km was calculated Changes in the mean grain size diameter Grain size is the most fundamental physical property of sediment and the mean grain size is a representative statistical parameter describing the grain size distribution of a river bed. Numerous studies have identified the wide range of influences that sediment texture has on the habitat suitability (Noack 2012, Dirksmeyer and Brunotte 2009, Kondolf 2008). Degradation of spawning gravels is recognized as a primary contributing factor in the widespread decline of salmon and trout populations throughout North America and Europe (Kondolf 2008). To evaluate changes in the grain size distribution the historic and current mean grain sizes of the River Elbe are compared. The mean grain size diameter D M is determined with the equation from Meyer-Peter and Mueller (1948) where p i stands for the percentage of particles of diameter D i in the bed material (cf. equation 1). D M Di pi 100 (1) The mean grain size of the current condition was calculated from sediment samples of the federal waterway of the River Elbe (arithmetic values). For further processing mean grain size diameters for river stretches with a length of 5 km were calculated. Historic information of the grain size distribution and therefore of the historic mean grain size diameter was not available. Therefore the shear stress for the historic and current conditions was determined using equation 2, where τ is the spatially averaged bed shear stress, ρ is water density, g is the acceleration due to gravity, h is the water depth of mean flow and S is the water level slope (uniform, steady flow). The historic water depths were extracted from the given bed and water levels, the current water depths were a result of the digital terrain model and the calculated mean water levels (cf. 3.1). ghs (2) It was assumed that the bed shear stress has a significant influence on the grain size distribution, depending on the discharge. Simplistically a mean shear stress for stretches with a length of 5 km and the total width of the waterway as well as the mean grain size for the grain size distribution were used as representatives. In this regard the ratio of the historical and current shear stress with the same discharge was correlated with the ratio of the historical and current mean grain size diameter (cf. equation 3). That way the historic mean grain size diameter could be determined.

6 historic current D D M historic M current (3) 4. RESULTS The results reflect a general trend of hydromorphological alterations for the inland Elbe like a lower depth diversity and bed degradation in the most sections. Moreover stretches with significant erosion or coarsening of bed substrate could be identified and relationships f. ex. between bed and water level could be recognized Bed level change Figure 3 depicts the results of bed level and water level changes of the inland Elbe between 1898 and Some stretches of the Elbe River show maximum erosion of 2 m in the last century. The medium flow and low flow water levels follow largely the bed level change. The deviations between Elbe km 0-60 are due to methodological flaws, f. ex. missing data from the Czech part of the River Elbe. Downstream of approx. Elbe-km 530 the water levels are influenced by the weir in Geesthacht (Elbe-km 586). The so called erosion stretch between Elbe km can be clearly identified as well as a second river stretch with maximum bed level change of 2 m close to Magdeburg ~Elbe km 326). In order to gain more water depth in 1935 the Elbe channel was narrowed from 170 to 145 m and deepened by approx m between Elbe-km (Simon, 2012). Between Elbe-km the comparison of water- and bed levels suggest an increase of the water depth, downstream of the mouth of the Havel River a decrease of the water depth. The results match to the investigations of Faist (1992), which showed f. ex. a decrease of the low water level in Magdeburg-Rothensee (Elbe-km 333) of 1.42 m from 1893 to 1964 and 0.16 m from 1964 to Downstream of Elbe-km 428 the results show wide fluctuations of the bed level change and sedimentation in a river stretch with an approx. length of 40 km which could result from the sediment supply of the tributary Havel (mouth: Elbe-km 428).

7 Figure 3 Bed and water level change of the inland Elbe from (Quick et al. 2012) 4.2. Depth variability Figure 4 shows the deviation of the water depth of the stretches of the River Elbe for a historic and a current condition. As expected, the depth variability of the current condition is in general lower and less heterogeneous. A particularly conspicuous aspect is the significant increase of the depth variability downstream of the mouth of the Havel River at Elbe-km 428. A reason for that might be the occurrence of bed forms with a higher magnitude in 1898 than in 2004 due to river maintenance and dredging activities. The river stretch is characterized by a high morphodynamic. Rohde (1971) described the river bed between Elbe-km as mobile sand with dunes with a velocity of m per year. Furthermore a significant regulation to improve navigation conditions in this river stretch did not start before the Second World War. Hence the historic data from 1898 probably represents an almost natural condition. Figure 4 Depth variability of the River Elbe in 1898 and 2004 (Quick et al. 2012) 4.3. Mean grain size diameter The results depicted in Figure 5, show mean grain size diameters for different time periods along the inland Elbe. A mean discharge was used to determine the historic mean grain size diameter (cf. 3.4). Calculations of the historic mean grain size with a higher discharge showed a good correlation. Till Elbe-km 115 the results show in general a coarsening of the riverbed between 1898 and Downstream of this section the historic and current values of the mean grain size diameter vary in the same range. Reasons for this could be in fact only slight differences in the grain size distribution of the sandy material between 1898 and 2006 or methodological flaws. Since the calculations were based on simplified assumptions it might be not possible to recognize small scale difference in the mean grain size. The similar trend of the different mean grain size diameters is particularly a result of the methodological approach. The upper part of the River Elbe, till Elbe-km 115 shows a river bed consisting of coarse and

8 very coarse gravel with small cobbles and a varying mean grain size between 30 and 80 mm. Between Elbe-km the mean grain sizes ranges between the size fractions very fine and very coarse gravel (3 45 mm). Downstream the mean grain size decreases and shows mainly fractions of sand and gravel. Compared with the descriptions of Rohde (1971) the results show for the historic as well as for the current conditions a slightly coarser mean grain size diameter, especially downstream Elbe km 115, with a lower proportion of the sand fraction (0,063-2 mm). Anyway he also described a big variation of the individual values of the mean grain size with sharp increases. Figure 5 Mean grain size for different time periods of the inland Elbe in Germany. 5. DISCUSSION The uncertainties of the presented methodologies shall be taken into account. In the historical data some information is missing, f. ex. it is not known where in the cross profile of the waterway the measurements were carried out. It was assumed that the waterway was not delocated and that all measurements were conducted in the thalweg. Moreover the results are dependent on the length of the investigated river stretches and the subset size of running averages. Otherwise, the historical data collection from 1898 provides a rarely data basis with a high level of detail. It does not reflect natural river conditions but the reasonable results confirm that the hydromorphological condition of the River Elbe in 1898 is much more natural and less affected than the current condition. It should further be kept in mind that the last morphological effects of river regulation occur much later. To reduce uncertainties and measuring errors a comparison of data over a longer period, f. ex. 106 years is in general recommended. Further information like temporal development of discharges and water levels, measurements of bed load rates and literature research were used to validate the results. Furthermore it should be noted that the measurements of bed- and water level as well as sediment samples are snapshots and the riverbed constantly changes by nature especially after certain events like floods. This applies in particular to morphodynamic active stretches with bed forms. However the results show clearly hydromorphological and sedimentological changes of the

9 River Elbe in the last century, reveal the magnitude and expose key regions of concern. It was possible to identify morphological trends. The information of all parameters supports ecological investigations and biological monitoring. Changes of the habitat suitability could be detected and hence provide knowledge which helps to protect endangered species. As required by the Water Framework Directive the large scale investigation for the total length of the inland Elbe in Germany improves and complements preliminary local investigations. In order to obtain further information of the temporal hydromorphological development of the River Elbe due to different periods of river management, data from different time periods should be compared. Investigation of more sediment samples from the waterway of the River Elbe as well as from oxbow lakes could provide further information to evaluate changes in the sediment budget. However these investigations are very time consuming. 6. CONCLUSION On the example of the inland waterway Elbe in Germany a comparison of current and historical data of the hydromorphological indicators mean grain size diameter, bed level changes and depth variability as a component for a sediment management plan is described. Therefore historical data from 1898 and current echo sounding data from 2004 and sediment samples from 2006 are used. The results reflect a general trend of hydromorphological alterations for the inland Elbe like lower depth diversity, coarsening of the grain size in the upper part and bed degradation in the most sections. Some stretches of the inland waterway Elbe rivers show maximum erosion of 2 m in the last century. The investigation improves the understanding of the morphological system and quantifies morphological changes of the River Elbe in the last century. The results serve as a component in the development of a sustainable sediment management plan of the River Elbe in Germany with the aim of improving the ecological condition and ensure navigation at the same time. ACKNOWLEDGEMENTS The authors are grateful to the German Waterways and Shipping Authorities Dresden, Magdeburg and Lauenburg for providing data. We would also thank our colleagues Christian Svenson, Nathalie Cron, Soenke Schriever, Doreen Graetz and Klauda Kroetz for their work in this investigation as well as Martin Helms from the Karlsruhe Institute of Technology who determined the discharges corresponding to the historic water levels. Special thanks also to the ad-hoc expert group SSeM of the River Basin Community Elbe. REFERENCES Brunotte, E.; Dister, E.; Guenther-Diringer, D.; Koenzen, U. & Mehl, D. (2009) Flussauen in Deutschland. Erfassung und Bewertung des Auenzustandes. Naturschutz und Biologische Vielfalt 87, BfN, Bonn-Bad-Godesberg Dirksmeyer, J. and Brunotte, E. (2009) Sediment textures and hydrogeomorphological characteristics of salmon and sea trout spawning habitats in Germany a contribution to river ecology. Z. Geomorph N.F. 53, Berlin-Stuttgart, DOI / /2009/ EEA European Environmental Agency (2012) State of water assessment. Peter Kristensen. Integrated Water Assessments. EEA-European Topic centre-inland, coastal, marine waters. Faist, H. (1992) Zur Sohlenerosion der Elbe, Zeitschrift für Binnenschiffahrt, No. 6, March 1992

10 Faulhaber, P. and Alexy, M. (2005) Artifical bed load supply at the River Elbe investigation and realization. Large Rivers Vo. 15. No.1-4. Arch. Hydrobiol. Suppl. 1551/1-4 Kondolf, M. G., Williams, J.G., Horner, T.C., Milan, D., (2008) Assessing physical quality of spawning habitat, American Fisheries Society Symposium 65, Meyer-Peter, E. and Muller, R. (1948) Formula for bed load transport. Proceedings of the Second Meeting. International Association for Hydraulic Research, Volume 6. Naumann, S., Schriever, S., Moehling, M., Hansen, O. (2003) Bedeutung der Nebenfluesse fuer den Feststoffhaushalt der Elbe, BfG No. 1382, Koblenz. Nestmann, F. and Buechele, B. (2002) Morphodynamik der Elbe Schlussbericht des BMBF- Verbundprojektes FK , Karlsruhe Netzband, A., Reincke, H., Bergemann, M. (2002) The River Elbe A Case Study for the Ecological and Economical Chain of Sediments. Journal Soils $ Sediments 2 (3) Noack M. (2012) Modelling Approach for Interstitial Sediment Dynamics and Reproduction of Gravel Spawning Fish, Institute for Modelling Hydraulic and Environmental Systems, University of Stuttgart, Germany Rohde, H. (1971) Eine Studie über die Entwicklung der Elbe als Schifffahrtstraße, Mitteilungen des Franzius Instituts für Grund- und Wasserbau der Technischen Universität Hannover. Sednet (2006) Sediment Management an essential element of River Basin Management Plans. Report on the SedNet Round Table Discussion. Venice november Simon M. (2010) Untersuchungen zu anthropogenen Beeinträchtigungen der Wasserstände am Pegel Magdeburg-Strombruecke. Potsdam Institute For Climate Imapct Research (PIK). No Potsdam Quick, I. (2012): Sediment management concept with special regard to hydromorphological aspects. In: Die Elbe und ihre Sedimente. Magdeburger Gewaesserschutzseminar 2012, Tagungsband. Hamburg. [in print] Quick, I., Koenig; F., Svenson C., Cron, N.; Schriever, S.; Vollmer, S. (2012) Hydromorphologische Bewertung und Praxisprojekte mit Schnittstelle zur Ökologie, 1. Oekologisches Kolloquium. Bundesanstalt für Gewässerkunde, Koblenz Vollmer, S., Quick, I., Moser, H., (2012) Sedimenthaushalt und Managementaspekte Binnenwasserstraße Elbe Aspects of sediment balance and management fort he inland waterway Elbe, In: Die Elbe und ihre Sedimente. Magdeburger Gewässerschutzseminar 2012, Tagungsband. Hamburg. [in print] WSA Dresden (2011) Erosionsstrecke der Elbe Bewertung der Geschiebezugabe und ergänzende Untersuchungen , Dresden, Koblenz, Karlsruhe. WSV (2009) Sohlstabilisierungskonzept für die Elbe von Muehlberg bis zur Saalemündung. Projektgruppe Erosionsstrecke Elbe. WSD Ost, WSA Dresden, BfG, BAW. Magdeburg, Dresden, Koblenz, Karlsruhe, /1/2012