Sediment Transport and Sediment Management in the Estuary, 11

HOLGER WEILBEER

SEDIMENT TRANSPORT AND SEDIMENT MANAGEMENT IN THE ELBE ESTUARY, GERMANY

ABSTRACT tidal exchange volume and the higher the morphological development. When choosing amounts of sediment that can be transported a disposal site, hydromorphological criteria as The tidal Elbe estuary in northern Germany with the flowing water mass. As a result, well as nature conservation aspects have to be serves as an example to show the kind of dredging operations become more intensive. considered. As a rule, the spreading of the data, modelling and analysis tools that are relocated dredged material is considered a required for a qualitative and quantitative In Germany, for example, more than desired effect, for instance, while description of sediment transport in an 45 million m³ of sediment material are resedimentation in dredged areas should be estuary. These methods can be used to dredged every year to maintain the required avoided since this would encourage repeated investigate sediment management options. fairway depths. With over 40 million m³, the sediment and dredging cycles. By choosing First a brief overview of the historic material dredged in sea waterways and suitable disposal sites and times, it might also development of the Elbe estuary is presented, seaports accounts for the major share of be possible to promote beneficial followed by a description of the modelling dredging operations. Considering these morphological developments. The evolution of system used in this study. Thereafter volumes it is clear that sediment transport the Wadden Sea, for example, is extremely measurements and model results for sediment processes in estuaries are of eminent important as it compensates for the impact of transport in the Elbe estuary are reported. importance. One of the main objectives is to the sea level rise. Lastly the article looks at specific model stop the continuing increase in the amount of applications for sediment management tasks. dredging material and rising dredging costs – An understanding of the hydrodynamic and This article first appeared in the journal Die even if the trend towards greater ship sizes morphodynamic influences in an estuary and Küste, 81 (2014) pp 409-426 and is reprinted continues unbroken. knowledge about the relationships with water here in a slightly adapted version with quality parameters are therefore an essential permission of the German Coastal Engineering Most of the dredged material is transported scientific prerequisite for the optimum Research Council (KFKI). to temporary disposal sites in the estuary. management of sediment. An example is the This means that the material remains in the upstream transport of sediments caused by system, thus influencing sediment transport tidal pumping and the influence of baroclinic INTRODUCTION rates and consequently also the estuary’s processes on the water flow and sediment transport in the estuary’s gradient zone. Both Estuaries are strongly influenced by tidal are crucial factors in sediment transport and waves on the one hand and salt water, mixed Above: A satellite image (USGS LANDSAT7) of the Elbe are the main cause of potential sediment water and freshwater zones on the other and estuary in northern Germany indicating the morphology cycles. Consequently, they need to be taken they transport large volumes of sediment of the mouth of the river with shallow areas, islands, adequately into account when designing and along with their alternating currents. The tidal channels and so on. The currents in estuaries implementing a sediment management deeper the water in fairways, the higher the transport large volumes of sediment. strategy. 12 Terra et Aqua | Number 139 | June 2015

THE ELBE ESTUARY – A BRIEF OVERVIEW The Elbe estuary is a very important German waterway. Its mouth is situated in the south- east of the German Bight, with the weir in Geesthacht defining the tidal limit. The entire length from the weir to the mouth, which has a width of approximately 15 km, is more than 160 km (Figure 1).

Over the centuries, the Elbe estuary has been modified several times to meet the changing requirements of maritime traffic. Between 1860 and 1999 the fairway was deepened up Figure 1. Geographic positions and denotations of the Elbe estuary, extending from Geesthacht through Hamburg to to 10 m. Furthermore, a range of measures, . such as the construction of the weir in Geesthacht, the cut-off of tributaries, the backfill of harbour basins, as well as diking In this article the tidal Elbe serves as an section, a brief overview of the historic and poldering, have been carried out in the example to show which kind of data, development of the Elbe estuary is provided last 50 years. Today the morphology of the modelling and analysis tools are required for a followed by a description of the modelling Elbe estuary is characterised by a deep fairway qualitative and quantitative description of system used in this study. The next section leading to the Port of Hamburg and a sediment transport processes in an estuary. presents measurements and model results for complex system of islands, tributaries and sediment transport in the Elbe estuary and branches in the landward section of the These methods can be used to investigate finally, specific model applications for estuary as well as extensive tidal flats and tidal sediment management options. In the second sediment management tasks are considered. creeks in the seaward section.

Anthropogenic measures have given rise to changes in tidal characteristics and sediment transport processes. Figure 2 shows the mean high water and the mean low water at a tidal gauge in St. Pauli, Hamburg, from 1900 to 2010. The tidal range has increased considerably (1 m in the last 35 years) owing to a fall in the low water level and a rise in the high water level. The asymmetry of the tidal curve is enhanced, i.e, the flood current has become shorter but now reaches higher velocities, the slack tide between flood current and ebb current has become longer, and the ebb current has also become longer with smaller current velocities. This hydrodynamic behaviour is probably the main reason for the enhanced “tidal pumping” of sediment upstream. Sediment is transported upstream with the strong flood current and then settles during the slack tide with less sediment mass being transported downstream with the weaker ebb current. The net transport of sediments depends on strength of the head water discharge (see below, ‘Sediment transport in the Elbe estuary: Model results’. Figure 2. Mean high water and mean low water from 1900 to 2010 at a tidal gauge in St. Pauli, Hamburg. Major anthropogenic measures are marked. Ongoing maintenance dredging of the Modified from chart at http://www.portal-tideelbe.de/Projekte/FRA1999/index.html. channel has been necessary in order to Sediment Transport and Sediment Management in the Elbe Estuary, Germany 13

HOLGER WEILBEER received a diploma and subsequently a PhD degree (2001) in Civil Engineering from the University of Hannover, Germany. Since 2001 he works at the coastal department of the Federal Engineering and Research Institute (Bundesanstalt für Wasserbau - BAW) in Hamburg, Germany. He has worked for 20 years in the development of three-dimensional hydro- morphological models and their application to hydraulic engineering and sediment management tasks. His work focusses on the Elbe Estuary and the Ems-Dollard- Estuary, modelling and analysing the historic developments, recent states and future scenarios. Figure 3. Amount of maintenance dredging in the Elbe estuary from 2000 to 2012. (Data from Baggerbüro Küste, GDWS-Nord).

guarantee the safety of shipping traffic. The to dredging amounts, but also for the to hydrostatic and non-hydrostatic free- amount of maintenance dredging has evolution of the Wadden Sea as an important surface flow and transport in water bodies increased significantly since the last deepening habitat and its meaning for coastal protection (Casulli and Walters, 2000; Casulli and in 1999, especially in the upper region of the and of course for the hydrodynamics of the Zanolli, 2002, 2005; Lang, 2005). estuary near Hamburg (see Figure 3). Figure 3 whole tidal Elbe because of the dissipation of • SediMorph is a software package for the also shows a change in the management tidal energy in the outer area. The two- or three-dimensional simulation of strategy for fine sediments. Since 2008, all morphological development of the last 40 fractioned sediment transport processes fine sediments dredged downstream of years is shown in Figure 4. It is strongly within the bottom and at the bottom Hamburg have been disposed on sites located influenced by the longitudinal dike surface bodies (BAW, 2005). in the turbidity zone (Figure 1) in order to “Kugelbake” north of Cuxhaven. On the one • DredgeSim can be used to take account of enlarge sediment cycles. As a consequence, hand, millions of cubic metres of sediment are dredging and disposal actions in free the amount of dredging near Osteriff has lost in some areas and, on the other, millions surface flows. This allows the increased. of cubic metres of sediment must be dredged anthropogenic influence on sediment every year to maintain the fairway. transport and morphology to be considered Another noticeable peak can be seen in a and maintenance strategies to be developed section in the outer estuary called “östliche All of these issues require monitoring, and evaluated focusing on different Mittelrinne”. The amount of dredged scientific investigation and system analysis of optimisation criteria (e.g., minimising sediments doubled in 2008 and more sediment transport processes using modern dredging costs). DredgeSim can be used in sediment also needs to be dredged in the numerical methods. A better understanding of two different modes. In each case the user neighbouring sections. The reason for this the cause-and-effect chain which has brought has to define dredging and disposal areas. strong increase can be found in the large- about these apparently anthropogenic driven The date, time and amount of dredged scale morphodynamics of the Outer Elbe changes to the estuary is required. material and its deposition is prescribed by caused by a chain of pronounced hydrological the user in the time controlled maintenance and meteorological events. In the winter MODELLING SYSTEM mode. In this criterion controlled season 2007/2008 the number of tidal high The model runs described below are maintenance mode dredging is initiated water events (Thw > 2.40 m NHN) was performed with the UnTRIM hydrodynamic according to prescribed dredge criteria, for significantly larger than usual (BAW, 2013). and suspended transport model in instance if a deposition area in the shipping Thus the hydrodynamic load on the shallow combination with the SediMorph channel affects navigability. This simulation areas in the Wadden Sea owing to wave and morphological model and – for some module was developed in cooperation with current actions was higher and more sediment applications – in combination with the BAW and the University of the Armed was moved. dredging module DredgeSim: Forces, Munich (Maerker and Malcherek, • UnTRIM is a computational model for 2007). Morphological development in the Outer Elbe solving a variety of two- and three- . is a very important issue, not only with respect dimensional differential equations relating The interaction of the simulation modules 14 Terra et Aqua | Number 139 | June 2015

validated models from the Elbe estuary (BAW, concentration profiles in the same area. 2006, 2012). Measured sediment concentrations, transport rates and velocities are shown as cross-section SEDIMENT TRANSPORT IN THE ELBE integrated values in Figure 7. ESTUARY Measurements Cross-section at Hamburg Field data of sediment concentrations and The maximum flood concentrations at the transport rates, which can be used for entrance to the Port of Hamburg (Hamburg calibration and validation purposes, must be profile) are much higher than the maximum collected in special measurement campaigns. ebb concentrations. Peak values of mean A medium-term field programme was initiated measured concentrations of more than 0.4 g/l in 2006 (and repeated in 2010 and 2011) are reached during flood, followed by a with the aim of improving the understanding decrease to 0.05 g/l during slack water at of the suspended sediment regime and of high tide (Figure 7). The ebb current is slower building up a database for validation of the than the flood current owing to the significant numerical model. tidal asymmetry in this part of the estuary. Thus the sediment concentrations during ebb The field data collection programme covers are lower than those during flood and reach the entire Elbe estuary and provides data on maximum values of 0.2 g/l. The suspended solid concentrations and transport concentrations again decrease to 0.05 g/l rates. Acoustic Doppler Current Profilers during slack water at low tide. The calculated (ADCPs) are used for the measurements as sediment flux is a distinct indicator of the these can provide data over (nearly) the whole flood-dominated transport regime in this area: depth range in a temporal and spatial During flood current the transport of 27000 t resolution which is very suitable for numerical suspended sediment was measured, during models. Some results of measurements along ebb current only 13000 t. cross-sections are presented in this chapter. Further results and more detailed technical Cross-section at Rhinplatte information can be found in Maushake and In the turbidity zone of the Elbe estuary Aardom (2007) and Mol (2007). (Rhinplatte profile) the concentration in some areas sometimes increases to over 2 g/l, for The measurements were carried out in example at the sides of the navigation channel autumn 2006. The head water discharge in (Figure 6). Higher values probably occur near this period was below 400 m³/s. A total of the bottom, but the measurement method is three cross-section measurements have been not valid for this reach. Thus the real sediment performed, each representing a characteristic fluxes must be higher than calculated fluxes hydrographic regime (Cuxhaven: marine zone; based on measurements. Values of mean Rhinplatte: turbidity zone; Hamburg: fluvial (cross-section integrated) measured zone). In total, the cross-section concentrations of more than 0.6 g/l are measurements comprise more than 200 ship- reached during flood peak, followed by a mounted crossings on three transects, each decrease to 0.10 g/l during slack water at one covering the period of one tide with high tide (Figure 7). The sediment around 160 calibration sites and more than concentrations during ebb are higher than 300 water samples. during flood and reach maximum values of Figure 4. Morphological development in the Outer Elbe 0.7 g/l. This peak concentration at the end of from 1970 to 2009. Some measured distributions of suspended the ebb current is probably caused by a sediment concentrations for three cross- muddy area located a few kilometres sections are shown in Figure 6. In each case, upstream of this cross-section. This does not used in the applications is shown the maximum concentration during ebb and necessarily indicate an ebb-dominated schematically in Figure 5. The modelling flood currents has been selected for each transport regime because the length of the system contains all the necessary simulation profile. Transport rates and transport fluxes cross-section navigable with the vessel was modules which will enable it to be used as a can be computed directly from the collected shorter during the ebb and thus lateral areas tool for predicting estuary responses to datasets as ADCPs combine a current sensor with lower sediment concentrations are not proposed management options. Results and a SSC sensor in a single device which taken into account. During both flood and presented in this article are produced with provides velocity and suspended sediment ebb current the transport of 72000 t Sediment Transport and Sediment Management in the Elbe Estuary, Germany 15

Figure 5. Simulation modules of the BAW modelling system. The most important modules used for this study are shown in red.

suspended sediment in each tidal phase was results. However, flocculation processes, compared with measured data, i.e. the measured. which are probably responsible for the strong hydrologic history is inherent in the model decrease in sediment concentrations during results. Overall the model fits the measured Cross-section at Cuxhaven slack tides, have not yet been considered data well. This model is used for the The lowest concentrations were measured in satisfactorily in the model. Despite this morphodynamic application described under the marine transect near Cuxhaven. The weakness, mean sediment concentrations and “Modelling in dredging situations” below. maximum of the ebb current is located in the transport rates calculated from the model are western area of the profile and during flood qualitatively and quantitatively in an In further specific model applications only the the maximum current velocities occur at the acceptable range for the entire Elbe estuary, head water discharge is varied (Q = 180, 720 eastern area of the profile. Thus the pattern i.e., the model reproduces, in a broad sense, and 1260 m³/s) in order to investigate the of suspended sediment concentration looks the general nature of sediment dynamics in influence of head water discharge on different (Figure 6). Because of the high the estuarine system. hydrodynamic and sediment transport current velocities in this area, the morphology processes. All other model steering is mainly affected by sand transport. The Figure 8 shows a comparison of measured parameters, including initial conditions such as averaged measured sediment concentrations and modelled current velocities and sediment sediment and salinity distribution and other are small and vary between 0.02 g/l and 0.11 concentrations at the cross-section Glückstadt boundary conditions and values, were uniform g/l with a tidal-averaged value of 0.07 g/l / Rhinplatte. In general, measured values are for all model runs. (Figure 7). During flood current as well as ebb more scattered and modelled values are current the transport of 30000 t suspended smoother. The vertical distribution of sediment The model runs cover a four-week simulation sediment was measured. concentrations is more pronounced in the period (June 2 to June 30 2006). The analysis measurements. High concentrations near the of the model results starts after nine days of Model results bottom seem to be underestimated by the simulation (June 11 2006 to June 25 2006). These data were used in a calibration process. model, but on the other hand measured data During this period (one spring-neap-cycle) all The site-specific model was first calibrated are often not valid in this region. model results are stored every ten minutes. In using bathymetric and hydrologic conditions several post-processing steps these data are from 2006. During the calibration process the Results of measurements and model results analysed to calculate the minimum, mean and influence of settling velocity formulations was for a similar set-up of the Elbe model 2010 maximum values of water level, current also investigated. This process cannot be are shown in Figure 9. Measured and velocities, salinities and sediment described here in detail. One of the main modelled current velocities, sediment concentrations for each element. In addition, findings was that none of the tested settling concentrations, discharges and sediment the sediment transport across defined cross- velocity formulations works well for the whole fluxes at the cross-sections at Elbe-Km 689 sections and along defined long-sections is estuary. The model set-up using two are averaged over the cross-section area. Note calculated for each model run. This analysis suspended sediment fractions each with that with this calibration procedure the model provides a set of metrics which are useful to constant settling velocities delivered the best ran already nearly 6 months until it was describe the system behaviour and provides a 16 Terra et Aqua | Number 139 | June 2015

Figure 6. Maximum measured sediment concentrations at the cross-sections Hamburg / Blankenese, Glückstadt / Rhinplatte and Cuxhaven during flood and ebb current. The scale at the profile Glückstadt / Rhinplatte was adapted to the higher sediment concentrations in this area.

basis for comparison between model runs. flushes the suspended sediments to the sea. suspended sediments. Together with the Higher sediment concentrations occur in these knowledge about absolute transport rates, Figure 10 shows, on the right side, suspended runs. On the left side the ratio between the this ratio constitutes an important criterion, sediment concentrations along the fairway. suspended sediment transport rates during for example, for assessing placement sites. These are time-averaged values of three- flood current and during ebb current is The highest values of this ratio occur near dimensional model results for one spring- shown. This value does not provide any the Port of Hamburg at low discharges. neap-cycle. A turbidity maximum exists in all information about quantities, but instead model runs. A higher fresh water discharge characterises the transport regime of There are two hydrodynamic reasons for this Sediment Transport and Sediment Management in the Elbe Estuary, Germany 17

Figure 7. Measured current velocities, sediment concentrations and sediment fluxes at the cross-sections Hamburg / Blankenese (upper graph), Glückstadt / Rhinplatte (middle graph) and Cuxhaven (lower graph). The values are averaged over the cross-section area. 18 Terra et Aqua | Number 139 | June 2015

Figure 8. Elbe model 2006: Measured and modelled current velocities (left graph) and sediment concentrations (right graph) at the cross-section Glückstadt / Rhinplatte.

Figure 9. Elbe model 2010: Measured and modelled current velocities, sediment concentrations (left graph) and discharges, sediment fluxes (right graph) at the cross-sections at Elbe-Km 689. The values are averaged over the cross-section area.

transport behaviour – baroclinic effects owing the long term in this part of the Elbe and lead mid-term or long-term morphodynamic to density gradients and tidal pumping, both to an increase in the amount of maintenance simulations. indicated in Figure 10. The influence of dredging. baroclinic processes on the sediment transport Usually only information about the yearly is high. The strong dominance of the near- Monitoring of dredging activities amount of dredging volume for certain bed transport in flood direction between Dredging and disposal actions lead to changes sections of the waterway are available (Figure Cuxhaven and Brunsbüttel, which is also of morphology, of sediment concentrations 3). A detailed spatial and temporal analysis of visible at higher head water discharges, may and thus to changes in the net transport of these data is not possible and this kind of indicate further sediment cycles in the Elbe sediments. The spreading of sediments and data is therefore not appropriate for use in a estuary. Furthermore the model runs for low changes in sediment concentrations may offer numerical simulation. or mean head water discharge predict distinct economic and ecological criteria for sediment transport in flood direction, at least comparing and evaluating realistic dredging Dredging data which can be used for a model upstream of Brunsbüttel. This transport and disposal scenarios (location, tidal phase, run must describe the dredging and disposal characteristic is caused by tidal asymmetry, sediment properties and such). Detailed data action in great detail: already described above in the overview of of real dredging and disposal actions, or at • A polygon to describe the dredging area the Elbe. Owing to these transport least information about the applied sediment • Date and time of dredging characteristics fine sediments accumulate over management strategy, are also needed for • Volume and density of dredged sediments Sediment Transport and Sediment Management in the Elbe Estuary, Germany 19

Figure 10. Flood/ebb (F:E) ratio of suspended load transport (left) and mean suspended load (right) for discharge Q = 180 / 720 / 1260 m³/s. 3D data, averaged over one spring neap cycle. Specific features are marked by red circles.

• A polygon to describe the disposal area • Date and time of disposal • Several identification numbers for a distinct description of the dredging cycle.

If these data are available, dredging and disposal actions can be considered in detail during a numerical simulation. Since 2009, most of the dredging vessels working in the Elbe estuary have been equipped with the sensors needed for operational monitoring purposes and the data is now available for further investigations. If the morphodynamic model is only driven by dredging data, only that part of the bottom evolution becomes visible which is influenced by dredging operations. Figure 11 shows dredge polygons Figure 11. Dredge polygons near Osteriff and resulting bed evolution based on monitoring data from the year 2010. 20 Terra et Aqua | Number 139 | June 2015

Figure 12. Schematic view of the management concept for fine sediments, applied 2005-2011.

Figure 13. Spreading of fine sediments from the disposal site at Elbe km 738, indicated by maximum sediment concentrations of coarse silt eroded from the disposal site. Sediment Transport and Sediment Management in the Elbe Estuary, Germany 21

Figure 14. Modelled bottom evolution in the mouth of the Elbe estuary for the year 2010. Result of a three-dimensional model run. All known dredging and placement operations are included. Dredging sites as well as disposal sites are recognisable. Black lines indicate the borders between dredging sections according to Figure 3.

near Osteriff and the resulting bed evolution the incoming tidal energy. Possible measures areas. This enables the sedimentation based on monitoring data from the year include construction of hydraulic structures, processes to be steered in the estuary and the 2010. the enlargement of shallow water areas in the entry of fresh sediments in polluted harbour Outer Elbe and the construction of additional basins to be avoided. Currently, most dredged APPLICATION TO SEDIMENT tidally influenced areas in the upper part. material is disposed of within the estuary MANAGEMENT TASKS (Figure 12). Furthermore, the Hamburg Port Sediment management concept Further, optimised dredging and disposal Authority has had temporary permission to Currently, the maintenance concept for the strategies are aspired to reduce volumes of dispose of a certain amount of fine sediments Elbe estuary is a sediment management dredged material. This includes identifying in the . As a result, around 1 million system combining different hydraulic and enlarging or destroying dredging cycles, m³ of fine sediment was removed from the engineering activities. It includes construction where dredged material which is disposed in estuary every year between 2005 and 2011. measures and optimised dredging actions and the estuary is transported back to dredging A smaller amount, polluted by different was developed in cooperation with all the sites. This can be achieved by disposing of fine contaminants which are transported and associated authorities (HPA and WSV, 2008). sediments in a more ebb-dominated transport accumulating in the harbour from sources This concept is in a continuous process of regime. Coarse particles, in contrast, can be upstream, has to be treated and deposited ongoing development and completion (BAW, used in construction measures. Further, the landside. 2013, BfG, 2014). influence of the head water discharge as well as the time-dependent change of the flow This management concept still requires the Construction measures are mainly investigated regime can be used to optimise dredging and treatment and deposition of polluted to overcome the negative effects arising from even more disposal activities. sediments. The improvement of the water flood dominated tidal characteristics and quality of the river Elbe and its sediments is associated sediment transport. This can be By constructing several sediment traps, the still an important goal and one which can only achieved by creating a bigger tidal volume sediments are further forced to settle at be fully achieved by stopping the emission of upstream and by increasing the dissipation of certain stages to keep them out of critical contaminations along the upstream 22 Terra et Aqua | Number 139 | June 2015

Figure 15. Difference topography for the year 2011-2010. Dredging sites are indicated as green polygons. The bottom evolution in the vicinity of the disposal sites is obviously influenced by disposal operations.

tributaries. This is supported within the marked sediments shows the extent of Modelling dredging activities estuary by removing polluted sediments in the influence of the investigated disposal site. This Finally the modelling system (see above) was harbour. information, together with knowledge of applied to simulate hydro- and dredging and disposal strategies, can be used morphodynamics for the year 2010, whereby Investigation of disposal sites to recognise sediment cycles. all known dredging and disposal operations The modelling system was applied to were included. Figure 14 shows the modelled investigate the function of the disposal sites This method was applied not only to all recent bottom evolution in the mouth of the Elbe used in the Elbe estuary. The set-up of the disposal sites in the Elbe estuary, but also to estuary. Dredging sites as well as disposal sites model for this application is nearly identical to potential new sites, which may be more are recognisable. Some larger scale the studies described above in “Model convenient (BAW, 2012, 2013). An example morphological trends correspond well with Results”, but in addition to the initial from this investigation is the spreading of observed changes (Figure 15), but overall the sediment inventory three more sediment coarse silt from the disposal site at Elbe km model set-up used for this run appears to fractions are taken into account. These 738 as shown in Figure 13. The preferred overestimate erosion in some areas. sediment fractions (fine, mean and coarse silt) transport direction is indicated by maximum Nevertheless the model is an indispensable have physical properties which are identical sediment concentrations of coarse silt eroded tool for evaluating different sediment with the background inventory. These from the disposal site. This is a proper site management strategies. It can be used to test sediments are located at disposal sites and from a hydraulic engineering point of view management options and the difference can be eroded and transported by the tidal because most of the sediments are between two model runs shows the impact of current. transported in a south-east direction towards the variation. shallow areas, but not in the fairway, This kind of model application allows for indicated by the black line. Thereby the Figure 15 shows the difference in topography detailed analysis of the transport behaviour of formation of the Wadden Sea is supported or from the year 2011-2010. Dredging sites different types of sediments from different at least the erosion of sediments from this known from monitoring data are indicated as locations in the estuary. The spreading of the region is compensated. green polygons. The bottom evolution in the Sediment Transport and Sediment Management in the Elbe Estuary, Germany 23

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