Meyer, Bernhard

Rahlf, Holger

Wiegmann, Melanie

English

Hydraulic Engineering Repository

Conference Paper, Published VersionRahlf, Holger; Meyer, Bernhard; Wiegmann, MelanieBank Protection in Connection with the Fairway Adaptionof the Elbe EstuaryVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/106650Vorgeschlagene Zitierweise/Suggested citation:Rahlf, Holger; Meyer, Bernhard; Wiegmann, Melanie (2019): Bank Protection in Connectionwith the Fairway Adaption of the Elbe Estuary. In: Goseberg, Nils; Schlurmann, Torsten(Hg.): Coastal Structures 2019. Karlsruhe: Bundesanstalt für Wasserbau. S. 385-393.https://doi.org/10.18451/978-3-939230-64-9_039.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Abstract: Because of continuous erosion processes at the study location of “Altenbrucher Bogen” at the Elbe Estuary, bank protection was needed before fairway adaption starts in 2019. Bank protection at  “Altenbrucher Bogen” is a combination of groynes and contoured underwater depositional features (so called “submerged sand depot”. Standards in the design and dimensioning of the rubble mound groynes lead to rock weights in the armour layer that would be too heavy in comparison with the experience of local authorities. Based on the HUDSON-formula a required rock weight of 800 kg was calculated but reduced to 300 kg for construction. Keywords: bank protection, groyne, rock armour, fairway adaption, Elbe Estuary 1 Introduction Germany’s most important seaport is the Port of Hamburg, approx. 100 km away from the North Sea. Around 9,000 ship calls per year, almost 300 berths and a total of 43 kilometers of quay for seagoing vessels, four state-of-the-art container terminals, three cruise terminals and around 50 facilities specialized in handling roro and breakbulk and all kinds of bulk cargoes, along with about 7,300 logistics companies within the city limits – these are just a few of the factors making the Port of Hamburg one of the world's most flexible, high-performance universal ports. Around 8.8 million standard containers (TEU) crossed the quay walls at the container terminals. This makes the Port of Hamburg to the third largest container port in Europe (2017). In 2019 the fairway adaption of the 125 km long navigation channel to the seaport of Hamburg started after 20 years of planning. The environmental impact study shows, that river engineering measures are needed to reduce the negative impact of deepening and widening the navigation channel to the changes in abiotic system parameters of the Elbe Estuary like currents, water levels and salt intrusion. River engineering measures include the construction of groynes, submerged training walls and submerged depots of dredged material, which reduce the tidal amplification upstream in the estuary and will protect submerged sandy banks and embankments additionally. This paper is focused on the construction of groynes and an underwater depositional feature (submerged sand depot) in the lower estuary area at the location of “Altenbrucher Bogen” to stop submerged bank erosion (Fig. 1).  2 Fairway Adaption and River Engineering Measures in the Elbe Estuary After fairway adaption vessels with a draught of 14,5m are able to reach the Port of Hamburg dependent on tides. Therefore the target depth of the navigation channel will be increased by approx. 0.6 m in Hamburg up to 2 m in the outer estuary (Großer Vogelsand seaward of Cuxhaven). An extensive river engineering concept ensures that tidal amplification is reduced to an acceptable and minor degree. Therefore especially two large submerged sand depots are planned in the estuary mouth Bank Protection in Connection with the Fairway Adaption of the Elbe Estuary H. RahlfFederal Waterways Engineering and Research Institute (BAW), Hamburg, Germany B. Meyer & M. WiegmannFederal Waterways and Shipping Office (WSA-Cuxhaven), Cuxhaven, Germany Coastal Structures 2019  -  Nils Goseberg, Torsten Schlurmann (eds)  -  © 2019 Bundesanstalt für Wasserbau ISBN 978-3-939230-64-9 (Online)  -  DOI: 10.18451/978-3-939230-64-9_039385(Medemrinne Ost, Neufelder Sand) to increase the roughness for damping the tide. Smaller depots are planned at the river embankments to reduce bankside currents and submerged bank erosion.   Fig. 1.  Overview of the Elbe Estuary and the focus “Altenbrucher Bogen”. 3 River Engineering Measures at “Altenbrucher Bogen” 3.1 Bank Erosion The mouth of the Elbe River is an area with exceeding morphodynamic changes in the bathymetrie (Fig. 2). However, the historic deep channel of the river, which is used as navigation channel all the time, is in the area of “Altenbrucher Bogen” very near to the bank. Due to previous deepening and widening of the navigation channel in the last 150 years the tidal currents increased near the sandy bank at “Altenbrucher Bogen”, which becomes an eroding bank. River Engineering measures and naturally high morphodynamics in the area intensified this effect. With respect to coastal protection, the erosion of the sand flat and the bank could not be accepted anymore. The distance to the crest of the seadike (main coastal protection structure due to storm surges) was reduced from approx. 700 m in 1950 to 280 m in 2003. The erosion rate in this time is 4 m to 8 m per year (Fig. 3).       386 Fig. 2. Topography at the mouth of the Elbe River in the year of 1950, 1970, 1990, 2010.   Fig. 3. Measured cross-shore profiles show the erosion of the bank at Altenbrucher Bogen from 1950 – 2003. 3873.2 River Engineering Measures In workshops with the Authorities from the Waterway- and Shipping Administration and Lower Saxony Administration a common understanding of possible and useful local river engineering measures with following guidelines has been worked out: Protecting the submerged sandy bank against ongoing erosion is possible with • Group of groynes, because of good experience seaward of the investigation site, • Submerged depot as a current deflecting feature because of good experience upstream of the investigation site, • Dumping dredged material from maintenance dredging near the bank to balance the eroded material. To protect the revetment and embankment in front of the seadike it is useful to • strengthen the revetment, • arrange group of short groynes (safeguarding the revetment), • maintain artificial sandy beach as temporary sand depot in front of the revetment. For the layout of the measures it has to be considered that a local function concerning the embankment protection is ensured as well as a regional function for the reduction of tidal amplification and maintenance work for the navigation channel. Furthermore, it was important to keep in mind the environmental impact of the measures. At the end of the technical discussion a river engineering concept at “Altenbrucher Bogen” was recommended by the authorities, that consists of (Fig. 4): • 18 groynes (Altenbrucher Stacks) to protect 3 km of embankment westward of Glameyer Stack, • 6 groynes (Otterndorfer Stacks) to protect 1 km of embankment eastward of Glameyer Stack, • 1 submerged depot (59 ha area) to protect 2,5 km of submerged bank eastward of Glameyer Stack  • Initial dredging on the opposite (northern) side in the navigation channel to balance the measure-induced reduced cross section and to avoid stronger currents in the navigation channel.   Fig. 4. River engineering concept at Altenbrucher Bogen with groynes (Altenbrucher Stacks and Otterndorfer Stacks in red), submerged depot as a current deflecting feature (Unterwasserablagerungsfläche Otterndorf in yellow) and additionally dredging (Initialbaggerung Gebiet A und Gebiet B in orange) to avoid stronger currents in the fairway (Fahrrinne). 3883.3 Plan Layout 3.3.1 Groynes The Altenbrucher and Otterndorfer Stacks (Stacks = groynes) are a total of 24 groynes with the same basic design but various lengths. It was tendered as a core of geotextile containers without a fascine mattress but covered with a rubble mound armour layer (Fig. 5).  Fig. 5.  Longitudinal section of announced groynes 3.3.2 Submerged Sand Depot For the submerged sand depot a boundary dam with a core of geotextile containers was tendered (Fig. 6). The front should be covered with an armour layer of rubble mound. The enclosed area should be filled up with dredged material from the opposite side of the navigation channel (dredging is needed for hydraulic compensation in the cross section to avoid stronger currents in the navigation channel).  Fig. 6. Detail of cross section of tendered submerged sand depot enclosed with a boundary dam (the left side is in the direction of embankment, the right side is in the direction of fairway). 3.4 Design 3.4.1 Groynes The main loads of the groynes are caused by wind-waves (H1/3 ~ 1.0 m – 2.0 m), ship-waves (HS ~ 1.0 m – 1.5 m) and tidal currents (v ~ 1.5 m/s). Loads during storm surges are not crucial because of the high water levels. Then the crest of the groyne is submerged. In Germany’s EAK (2002) it is recommended to design the armour layer of rubble mound structures with the internationally well-established Hudson-formula. But it is also recommended that local experience and alternative design should be considered for a careful and economic design at all. The application of Hudson formula requires a max. armour stone weight of approx. 800 kg for the planned groynes. This is very overestimated in comparison to existing groyne armour layers seaward of Altenbrucher Bogen and local experience in maintenance work.  The reasons for the overestimation in the design procedure are given in CIRIA (2007). At “Altenbrucher Bogen” the main reasons are obviously for example (i) the real porosity and packing 389density in the armour layer, (ii) therefore a very high existing stability coefficient  kD, (iii) rounded shape of the crest.  The maximum armour stone weight was reduced to 300 kg and the contractor suggested to replace the geotextile containers in the core with a rubble mound core (Fig. 7) which reduced the costs appreciably. Fig. 7. Longitudinal (above) and cross section (below) of constructed groynes at Altenbrucher Bogen. 3.4.2 Submerged Sand Depot The main load of the submerged sand depot and its boundary dam is the tide current with maximum values of approx. 1.5 m/s (v ~ 1.5 m/s) parallel to the dam. Field measurements confirmed the status quo of currents and prediction of modified currents due to deepening of the navigation channel were taken into account. In BAW (2010) it is recommended to follow design guidance from PIANC (1987). As a result a core material of CP63/180 is necessary for the armour layer. Local experience with a test dam in the field and an existing sand depot upstream (Krautsand Nord) confirmed this result. The contractor suggested to replace the geotextile container in the core with a rubble mound core (Fig. 8) which reduced the costs appreciably. Fig. 8.     Design of cross section of constructed submerged sand depot enclosed with a boundary dam (the left side is in the direction of embankment, the right side is in the direction of fairway). 3903.5 Construction Each groyne is a simultaneous land-based, land- and waterborne construction. From the seaside grabs, barges and stone dumping vessels were applied, especially during the tide phase of high water levels. From landside backhoes were applied and essential for profiling the armour layer (Fig. 9). The working time for the backhoe was restricted by high water levels and only possible during low water levels. Fig. 9. Backhoe profiling the armour layer (left) and crane barge with grab placing the core material (right). Two construction periods were needed for completion (respectively April – October 2011 and April – October 2012), because in winter time (respectively storm period) it is not allowed to run a construction area in front of the seadike (safety aspect). Every groyne was constructed as rubble mound in three steps: (i) placing scour protection with geotextile, (ii) placing and profiling the core with coarse particles (CP 60/200), (iii) cover the core with armour layer of light mass blocs (LMB 40/300) and profiling the layer. Fig. 10 illustrates the construction site as an overview. Fig. 10. Overview of construction site for the groynes at Altenbrucher Bogen in August 2012. The submerged sand depot was completed within 6 months. At first the boundary dam was erected with about 430 000 000 kg of rubble mound material (June 2011 – August 2011). In September and October 2011 the sand depot was filled up with dredged material from the initial dredging site at the opposite area of the fairway as explained in chapter 3.2. Fig. 11 gives an impression of work. 391 Fig. 11. Loading, transport and dumping of the dam material for the submerged sand depot with side stone dumper . 3.6 Conservation and Evidence One of the essential aims of the river engineering measures was to stop the bank erosion at “Altenbrucher Bogen”. During still water phase in the groyne fields the sedimentation starts directly after completion of the groyne. This effect was expected and within 6 – 7 years maximum sedimentation of up to 4m is observed (Fig. 12). The appropriate spacing of groynes and head line of groynes prevents strong currents at the embankment and deposition in the groyne fields is facilitated. No failure or damage of the armour layer is observed so far.  Fig. 12. Example of measured changes in the topography in the groyne fields between 2012 and 2018 at Altenbrucher Stacks number 4 – 5a. The submerged sand depot is placed upstream in line with the groyne heads. The conceptual design of the submerged sand depot allows natural morphodynamic processes on the top area of the sandy depot, because only the dams were built with non-erodible material. Erosion and deposition is monitored and the changes to the artificial sand flat due to natural morphodynamics on the top of the depot. The non-erodible dam in the line of upstream and downstream groyne heads ensures that no bank erosion is possible (Fig. 13) and that no strong currents are in front of the revetment. 392 Fig. 13. Measured cross-section of submerged sand depot at Altenbrucher Bogen upstream the groynes. All authorities agree that the river engineering measures at Altenbrucher Bogen are a great success. It is obvious, that the local bank erosion, which was monitored over decades, now is stopped or reversed into a positive trend. An investment of 53.5 Mio Euros was necessary for that. 4 Conclusions A combination of groynes (in the Elbe estuary so-called Stacks) and submerged sand depot was constructed in 2011 and 2012 at “Altenbrucher Bogen” in the Elbe estuary. The river engineering measures are needed (i) to avoid local further bank erosion and (ii) to mitigate the tidal amplification stemming from the fairway adaption in the Elbe estuary that began in 2019. The design of the armour layer of the groynes with the well-known Hudson formula leads to an overestimation of required stone weights. In addition to reasons, which are documented in the Rock Manual, it is very probable, that the actual wave attack on the structures deviates from theory. The very high wind waves usually coincide with high water levels and are not relevant for the main load of the structure. In contrast, the long-period ship waves are relevant, because this load is very strong when the water level is low. But wave transformation in front of the structures and at the flats between the groynes leads to reduced loads. It was acknowledged in this project, that local experience in the maintenance of comparable structures is very important for an economical design of conservancy measures.  References BAW, 2010. Grundlagen zur Bemessung von Böschungs- und  Sohlensicherungen an Binnenwasserstraßen (GBB). BAWMerkblatt Ausgabe 2010, Karlsruhe CIRIA, 2007. The Rock Manual. CIRIA C683, London EAK, 2002. Empfehlungen für Küstenschutzbauwerke. Die Küste (65) PIANC, 1987. Guidelines for the Design and Construction of Flexible Revetments incorporating Geotextiles for Inland Waterways. Report of the Working Group 4 of the Permanent Technical Committee I Supplement to Bulletin No. 57, Brüssel Wiegmann, M., Meyer, B., 2015. Ufersicherung im Altenbrucher Bogen. In: HANSA International Maritime Journal, 152. Jahrgang, 2015, No. 1, pp 76-77. 393

Bank Protection in Connection with the Fairway Adaption of the Elbe Estuary

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Bank Protection in Connection with the Fairway Adaption of the Elbe Estuary

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