Coastal Defence Strategies for Sandy Coastlines as a Building Block for Integrated Coastal Zone Management

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1 Third Chinese-German Joint Symposium on Coastal and Ocean Engineering National Cheng Kung University, Tainan November 8-16, 2006 Coastal Defence Strategies for Sandy Coastlines as a Building Block for Integrated Coastal Zone Management Frank Thorenz Lower Saxony Water Management, Coastal Defence and Nature Conservation Agency (NLWKN), Norden frank.thorenz@nlwkn-nor.niedersachsen.de Abstract The Eastfrisian Islands are located in front of the German mainland coast in the North Sea. These islands, consisting mainly of dune areas in the north and saltmarshes in the south, are part of the Wadden Sea, which stretches from the Netherlands to Denmark. Islands and tidal flats can be characterized as part of a very dynamic system consisting of islands, tidal inlets, ebb deltas and tidal flats, where complex hydro-morphological processes induced by tides, waves and storm surges occur. In the past centuries these processes led to significant changes of the islands in size and shape up to their total disappearance. Beginning in the 19 th century, small villages on the islands developed into health resorts of steadily growing economical importance. Nowadays the islands are considered to be one of the most important sea resorts along the German North Sea coast. In order to safeguard settlement areas initial revetments and groynes were build around 1860 mainly located on the western parts of the islands. Strategies for coastal defence nowadays aim on flooding flood protection for settlement areas and infrastructure by a ring of protective dunes and main dykes as well as the continued existence of the islands as a legal obligation of the State of Lower Saxony. Coastal defence measures for dune areas depend on the sediment budget of the beaches and range from preserving the existing coastline to adaption to natural developments by relocation of protective dunes. As a necessary basis for coastal defence planning monitoring programmes and cooperated research are carried out. Integrated management is executed to harmonize the demands of coastal protection, tourism, development planning and nature conservation. 1 Introduction The coastline of Lower Saxony, a Federal State of Germany is located in the southern North Sea, a tributary of the Atlantic Ocean. This wadden sea coast with tidal ranges from 2 to 4 m can be classified as mixed energy / tide-dominated after

2 Hayes (1979). Off the coast between the Ems- and Jade-Estuary seven inhabited sandy barrier islands - the Eastfrisian Islands - are located (Fig. 1). The postglacial sea level rise formed predecessors of the recent islands whose age can be dated by more than 2000 years [Streif, 1990]. Between the islands tidal inlets are located, through which during each tide huge amounts of water flow in and out the southward the islands located tidal flats. Islands, tidal inlets and tidal flats represent a very dynamic system, where complex micro- to macroscale hydro-morphological processes occur. Erosion of beaches and the shoreface led in the past centuries to erosion and wash-overs of dunes up to complete dissimination of islands. Technical means and scientific knowledge were not sufficient to protect endangered settlement areas appropriately. The small villages have been destroyed by surges, relocated or abandoned. About 1800 the first sea resort on the German North Sea coast was founded on the island of Norderney. Nowadays tourism is the dominant economic factor for all islands. Norderney represents the most frequented sea resort in Lower Saxony. Beginning in the second half of the 19 th century, the first massive constructions to safeguard the growing settlement areas against storm surges and coastal erosion were constructed and continuously extended. Most parts of the islands are areas of unspoiled nature and of very high importance for nature conservation, protected by European and national laws [NWattNPG, 2001] which have be considered for coastal defence measures. Furthermore measures can only be carried out from late spring to beginning autumn due to weather conditions and safety reasons overlapping with the main season for tourism. Figure 1 Location of the Eastfrisean Islands.

2 Legal Basis for Coastal Defence and Coastal Defence System In Lower Saxony issues of coastal defence are regulated by a special law, the Lower Saxony Dyke Law [NDG, 2001].

3 2 Legal Basis for Coastal Defence and Coastal Defence System In Lower Saxony issues of coastal defence are regulated by a special law, the Lower Saxony Dyke Law [NDG, 2001]. General objectives for coastal defence are defined as well as coastal defence structures and e.g. regulations for maintenance, utilisation, extension and inpection. Main coastal defence elements for the islands are: - Main dykes protect an area against flooding due to storm surges, - Protective dunes safeguard an area against flooding and continued existence of islands, - Groynes and revetments as protective elements for protective dunes and main dykes On the northern coastline of the islands mostly natural dune rows with a width of approx. 200 to 400 m are defined by decree as protective dunes. In the center parts as well as in the southward orientated areas of the islands marshlands dominate and mostly no dune rows exist. Here main dykes protect the islands. Dunes and main dykes form a ring of protection elements for settlement and infrastructure areas (Fig. 2). Single dune rows outside these rings are defined as protective dunes to guarantee the existence of the islands. In total more than 90 km of protective dunes and 35 km main dykes protect the islands. While storms surges the islands also function as a natural barrier system against storm waves swinging from the North Sea to the mainland coast and reduce wave energy significantly. Protective Dunes (12 km) Main Dykes (10 km) Figure 2 Coastal Defence System for the Island of Norderney. Eastern parts of many islands are not protected by coastal defences. Here priority is given to objectives of nature conservation such as allowing natural dynamics in the National Park Lower Saxony Wadden Sea. Coastal defence on the Eastfriesian Islands is task of the Federal State of Lower Saxony, executed by the Lower Saxony Water Management, Coastal Defence and Nature Conservation Agency, subordinated to the Ministery of Environment.

4 3 Coastal Defence Strategies for Protective Dune Areas Dune areas and adjecant beaches are characterized by very dynamic hydrological and morphological conditions. Long term strategies and technical concepts for guaranteeing the functionality of protective dunes as important coastal defence elements take this particulary into account. One important process consists in the litoral sediment transport along the island beaches showing a dominant west-eastern direction. Sediment transport is strongly influenced by the ebb and flood currents in the tidal inlets inducing a dynamic system of sandbars the ebb delta (Fig. 3). Sandbars reach the eastward island in a discontinuous pattern of high variability causing a varying sediment supply on the beaches and foreshore. West the meeting point often a long term negative sediment balance exists leading to foreshore and beach erosion and as a result cliff erosion of dunes. These areas are nowadays dominantly protected by revetments and groynes. Eastward the meeting point sediment supply often causes well-balanced or positive sediment balance leading to wide and high beaches and, as a result, stabile or growing dunes over a longer period. Technical management concepts for protective dunes and beaches therefore have to take sediment balances as well as short and long term development trends into account. Ebb Delta Tidal Inlet Norderney Juist Figure 3 Norderney Tidal Inlet and Ebb Delta. 3-1 Long Term Negative Sediment Balance Areas Protected by Hard Constructions The initial construction of groynes and revetments to counteract erosion of dunes and beaches started already in 1857/58 on the island of Norderney to protect the endangered city. Until the end of the 19 th century at the western ends of all islands

exept Langeoog and Juist such massive constructions were build. Causative hydro-morphological processes and their interaction with massive technical constructions were mostly unknown.

5 exept Langeoog and Juist such massive constructions were build. Causative hydro-morphological processes and their interaction with massive technical constructions were mostly unknown. Due to ongoing erosion and damages by storm surges continuous extension and reconstruction of these constructions was necessary [Erchinger & Thorenz, 1999]. Nowadays they form the major flood protection for the direct adjoining settlement areas and are as well intensively used for tourism as promenades. Main objective is to preserve these constructions and where necessary adapt them. The space in front of the existing buildings towards the revetment is part of the protective dune reserved for purposes of coastal defence, while other use needs permission according to the regulations of the Dyke Law. This ensures for all protective dunes reservation of space for future costal defence measures. These zones are also implemented in and harmonized with municipal building plans. Very restricted place for reconstruction of the revetment at the north-western end of Norderney necessitated special cost intensive constructions to reduce wave run-up and overtopping [Schüttrumpf et al., 2001](Fig. 4). Figure 4 Wave run-up reduction elements at the northern beach of Norderney. Additionally at Norderney beach nourishments are executed to protect constructions if the sediment volume falls below a critical threshold. Since beach and foreshore nourishments with a total volume of 4.7 million m 3 are executed. The loss of sediment averages m³ per year.

6 3-1-2 Areas without Hard Constructions Most stretches of protective dunes are not secured by revetments or groynes. If it is necessary to stabilize a defined coastline due to settlement or infrastructure located directly behind the dune rows, beach and foreshore nourishments are executed to balance sediment deficits. Only in selected cases massive constructions often in combination with nourishments can be a suitable technical solution due to safety reasons. Long term effects on sediment transport and balance necessitate a comprehensive survey. Therefore in most cases nourishments as an environmentally sound and active coastal defence are preferred. A retreat of the coastline due to erosion is tolerated up to a limited extend as long as the required dune width to fulfil safety standards is ensured. Safety check of dunes is calculated by means of numerical dune erosion models in combination with morphological trend-analysis (Blum & Thorenz, 2005). Often a relocation of dunes by constructing a new naturally shaped dune row or a strengthening of existing dunes from the backside are necessary to guarantee the functionality of the protective dune. Therefor sand is excavated in beach areas with long term positive sediment. Against aeolian erosion the new dune is replanted with marram grass as a habitat sound bioengineering measure. Dune reinforcements and relocations are cost effective and environmentally friendly alternatives to fulfil coastal defence objectives. Table 1 shows the dune-reinforcements executed during the last 15 years on the islands and figure 5 a/b an example for the island of Langegoog. Table 1 Dune reinforcements on the Eastfrisian Islands Island Year Quantity [m 3 ] Length [m] Borkum Juist Langeoog Wangerooge Long Term neutral Sediment Balance Stretches of beaches, where bars from the ebb deltas reach the island are characterised by discontinuous phases of non sufficient sediment. This results often in intensive beach erosion and following dune erosion. In cases of relativly wide protective dunes, erosion is tolerated as long as the safety standard is guaranteed. The dune foot is rebuild by use of sand trapping fences to accumulate aeolian transported sand in phases of positive sediment balance. In case of an endangered protective dunes nourishments are executed to highten the beach

level and accelerate the bar migration towards the beach. The north-western beaches Langeoog Island were nourished six times since 1971 with a total amount of 2.9 million m3 Sand.

7 level and accelerate the bar migration towards the beach. The north-western beaches Langeoog Island were nourished six times since 1971 with a total amount of 2.9 million m3 Sand. Dune boundary1999 Sea Figure 5a Dune development between 1999 and 2002 based on laser-scanning survey at Langeoog Island with a retreat of up to 20 m. Figure 5b Reinforcement measure.

8 Eastward this sections regularly neutral sediment balance occurs. Sand trapping fences and subsequent planting of marram grass are used to equalize temporary sand losses as a result of storm surges. 3-3 Positive Sediment Balance In case of beaches with long term positive sediment balance, natural dune growth occurs. Normally no sand trapping measures are applied at the dune foot. Storm surges mostly do not affect the dune foot. 3-4 Dune Management Techniques Most protective dune rows on the Eastfriasian Islands are relatively narrow with maximum width of a few hundred meters. Maintenance of the dune corpus therefore is very important. Damages of dune vegetation cause eolian erosion increasing very fast from small spots to big gaps which can endanger the functionality of dunes seriously. Initial points are often illegal trails or burrows. Systematic planting of marram grass often in combination with sand trapping fences stabilizes such damages. Marram grass for coastal defence purposes is mostly gathered on natural dune sites and also cultivated for extended demand in a seedling nursery on Norderney (Thorenz & Schulze Dieckhoff, 1998). Systematic information and guidance of the numerous tourists is an important dune management technique. Information concerning the importance of top of dunes for coastal defence and nature conservation, paved well maintaned paths, viewing platforms on dunes in combination with inspection and information by dune wardens are important elements of this concept. Basis is a co-operation between costal defence and nature conservation agencies as well as municipalities. 4 Protection of the Marshlands The severe storm surges from 1962 and 1976 caused flooding of settlement areas on nearly all islands often from the low lying Wadden Sea side due to low crested dykes. Nowadays 24 km of 35 km main dykes on the island are reconstructed within a dyke strengthening programme. For future needs of coastal defence a 50 m wide zone behind the dyke is reservated by law and implemented in development plans. Usually the upper layer of dyke slopes consists of clay. On the islands no clay pits are available. By means of a long term management in cooperation with waterways-, coastal defence- and nature conservation agencies dredged mud from habour and navigation channel maintenance is stored and conditioned for later use in dike construction. Additional clay is transported from the mainland. 30 km of the island dykes are protected by a dyke foreland, a legally defined coastal defence element and as saltmashes of high importance for nature conservation. Groyne fields shelter approx. 14 km of the forelands against cliff erosion.

9 5 Monitoring and Research In the very dynamic system of islands, tidal inlets and tidal flats permanent changes occur, which can affect directly the functionality of flood defences and therefore safety. Also the interaction of measures and their environment are of special interest. Survey and analysis of hydrological and morphological data by monitoring programmes are a necessary basis for evaluation and prognosis of relevant processes. Regularly monitoring actions are: terrestrial and nautical survey of reference profiles, airborne laser-scanning of beach and dune areas, tidal flats and forelands, airborne photography of ebb-deltas, survey of hydrological data by gauges and bouys also as basis for numerical wave modelling and storm surge forecast. These measurements are a necessary basis for guaranteeing safety of coastal defences as well as for sustainable planning and construction. Applied research in the field of coastal engineering is conjointly organized between the Federal States and the Federal Republic and coordinated by the German Coastal Engineering Research Council KFKI [Thorenz & Eggert, 2004]. 6 Conclusions Protection against flooding and erosion is a necessary basis for the Eastfriesian Islands as a settlement, economical and cultural area of high ecological importance. Flood protection is formed by a ring of protective dunes and main dykes as legally defined flood defence elements. The technical concept to guarantee the functionality of protective dunes depends on sediment supply of the beaches and specific protection objectives. Where massive constructions are present, the existing coastline is preserved and constructions are if necessary reinforced to protect the settlement areas. In purely dune areas more flexible concepts are applied. They range from dune relocation and nourishments to bio-engineering measures depending on the sediment balance of the certain beach stretches. Special zones only for coastal defence purposes are an important planning instrument to ensure feasibility and flexibility for future coastal defence measures. This demands in integration of coastal defence, development planning and nature conservation (EC, 2002), (CPSL, 2005). Hydrological and morphological monitoring programmes are executed as a basis for analysis of the dynamic system, design, safety check and sustainable future planning.

10 7 References Blum, H. and F. Thorenz. Risk Assessment for the Island of Langeoog. Die Küste Heft 70, CPSL. Trilateral Expert Group on Sustainable Coastal Defence for the Wadden Sea, Solutions for Sustainable Coastal Protection in the Wadden Sea Region, Wadden Sea Eco System, No. 21, Erchinger, H. F. and F, Thorenz. Schutz der Ostfriesischen Inseln (Protection of the Eastfrisian Islands). In: Kramer, J. et al.: 1000 Jahre Leben mit dem Wasser in Niedersachsen Band II, Leer, EC. Recommendation of the European Parliament and the Council to Implement a Strategy for Integrated Management of Coastal Zones in Europe (002/413/EC), Hayes, M. O.. Barrier Island Morphology as a Function of Wave and Tide Regime, in Leatherman, S. P. ed., Barrier islands from the Gulf of St. Lawrence to the Gulf of Mexico: Academic Press, New York, NY, pp 1-29, NWattNPG. Gesetz über den Nationalpark Niedersächsisches Wattenmeer (Lower Saxony Wadden Sea National Park Law) vom 11.Juli 2001, Nds. GVBl. S. 443 zuletzt geändert durch Artikel 3 des Gesetzes vom 23. Juni 2005, Nds. GVBl. S. 210, NDG. Niedersächsisches Deichgesetz (Lower Saxony Dyke Law) vom , Nds. GVBl. S. 83, Schüttrumpf, H., H. Oumeraci, F. Thorenz and J. Möller. Reconstruction and Rehabilitation of a Historical Seawall at Norderney. Proc. Coastlines, Structures and Breakwaters, London, Streif, H.. Das Ostfriesische Küstengebiet (The Eastfriasian Coastal Area). Sammlung Geologischer Führer 57. Berlin, Stuttgart, Thorenz, F. and M. Schulze Dieckhoff. Dune Management in Lower Saxony, Coastline, 4, Thorenz, F and D. Eggert. Forschung für das Küsteningenieurwesen im KFKI (KFKI Coastal Engineering Research). Jahrbuch der Hafenbautechnischen Gesellschaft, 54. Band, 2004.

Protection of Coasts

Protection of Coasts [Coastal Zones] Jan van de Graaff Delft University of Technology 1 Coastal Zone: many definitions here: rather narrow strip of land adjacent to sea or large inland water (say: 150