Vulnerabilities - German coastline

Germany’s coast extends over 3700 km on both the North (1600 km) and Baltic Seas (2100 km). Two-thirds of the 3,700 km coastline are eroding. The German coastline is mainly shallow, i.e., marsh, dune coast, or beach wall, while only approximately 11% of the coast (420 km) is steep. On the Baltic, more than half of the coastline belongs to the so-called Bodden Coast (Bodden are shallow bays and inlets cut off from the open Baltic Sea by islands, peninsulas, and narrow spits) (1).

Recent coastal morphological investigations have shown that approximately 75% of all (sandy) coasts are subject to erosion. On the Baltic coast, the average rate of shoreline retreat is approximately 40 cm/year (1); large sections of the Baltic Sea coastline are retreating at an average rate of 0.2-0.3 m/year with maximum rates up to 1.5 m/year (3). Approximately 70% of the coastline of the state of Mecklenburg-Vorpommern is permanently under retreat (4). The rate of active cliff erosion is 24 cm/year for Schleswig-Holstein and up to 30 cm/year for Mecklenburg-Vorpommern (5). Mean sea level rise for the German part of the Baltic Sea coastline is 1.1-1.3 mm/year (6); in addition, land subsidence of the southern part of the Baltic Sea in response to glacio-isostatic rebound is up to 2 mm/year (7).

Although additional investment in flood and erosion protection will be considerable (estimated at more than 500 million US$) this seems manageable for the national and regional economies. On the other hand, hard coastline defence and accelerated sea-level rise will increase ‘‘coastal squeeze’’ on the seaward side, endangering important coastal ecosystems such as tidal flats (Wadden Sea), saltmarshes, and dunes. Currently there is no strategy to remedy this increasing ecological vulnerability (1).

Vulnerabilities - Wadden Sea

Recent developments

Along the North Sea coast of the Netherlands, Germany and Denmark, a chain of barrier islands shelters an area of extensive intertidal flats and shoals, consisting of fine sand to silt, dissected by sandy tidal channels and creeks. This is the Wadden Sea, a UNESCO World Heritage Site. All sediment that settles into the channels and onto the intertidal flats is imported from the North Sea. Whether this area can keep up with sea-level rise, and the intertidal flats do not drown, depends on the amounts of sediments that channels and creeks import from the North Sea.

About 60% of the total extent of the Wadden Sea, approximately 300 km of Wadden Sea coastline, is part of the German North Sea coast. Recent developments of the German Wadden Sea, in terms of erosion and deposition, have been quantified based on all available data on the height of the intertidal flats and shoals, and the depth of the channels and creeks in the area (9).

Where future scenarios in scientific studies on the Wadden Sea show intertidal flats unable to keep up with sea-level rise, developments so far paint a completely different picture. In the period 1998 to 2016, almost all of the intertidal flats in the German Wadden Sea show an accretion of sediments. The majority of channels and creeks, on the other hand, have deepened over time. The intertidal flats accrete with rates ranging from 4 to 22 mm/year (9). These rates exceed the observed recent mean sea-level rise of 2.2 to 6.6 mm/year for the German Bight (10). This is in line with recent studies on the Dutch Wadden Sea (11) and on tidal flats in the Dutch Eastern and Western Scheldt estuaries (12). Also, the intertidal flats are expanding. The average deepening of the channels was 20.0 mm/year.

According to these results, the German Wadden Sea is accumulating sediment. The vertical and areal growth of the intertidal flats implies a gain of sediment in these areas that dominates over the sediment loss due to the deepening of the channels. According to the authors, this gain is about twice the amount of what would be needed for the intertidal areas to keep up with the current rate of sea-level rise. Apparently, there is an abundanceof sediment in the area while the combined effect of sea-level rise, tides and wave forcing results in an import of these sediments from the North Sea into the Wadden Sea (9).

Future projections

The most difficult and perhaps also the most crucial impact of future sea-level rise appears to be on coastal ecosystems. Along the North Sea, a decrease or total loss of coastal wetlands, such as tidal flats (Wadden) and salt marshes, seems likely as a result of ‘‘coastal squeeze’’ (the transgression of the sea across these wetlands, which are prevented from migrating landward by existing dike structures). It is possible that more than 2800 km2 of wetland areas might be lost in Niedersachsen and Schleswig-Holstein. On the other hand, a rising sea level could lead to an extension of wetlands along the Baltic coast of Mecklenburg-Vorpommern if no new obstructing dikes are built (1).

Adaptation strategies

Coastal protection measures are both soft (beach nourishments) and hard. In Schleswig-Holstein and in Lower Saxony structural erosion in sandy environments is typically compensated by artificial nourishments, particularly on barrier islands (8). Especially the west coast of Sylt and the island Föhr are nourished on a large scale. For the Baltic Sea coast there might be an increasing future demand of sand for beach nourishment due to the predicted sea level rise (6).

For the city of Hamburg, the loss of salt marshes is a major problem for the safety of the area against floods, especially along the Schleswig-Holstein coast. Therefore, specific measures have been initiated, like constructing revetments, structures that protect the marsh edges in order to prevent further cliff erosion; creating marshes by dumping dredged material, deposition of dredged material on inter-tidal areas along the mainland or Waddensea side of the barrier island; out banking of summer polders and opening of summer dikes in order to get a more frequent flooding of the area and higher sedimentation rates; and the construction of groins fields, creating of areas with reduced waves where accumulation of fine sediments is stimulated (2).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Germany.

1. Sterr (2008)
2. HafenCity Hamburg (2007), in: Walraven and Aerts (2008)
3. Schwarzer et al. (2003); Ziegler and Heyen (2005); Niedermeyer et al. (2011), all in: Jensen and Schwarzer (2013)
4. Harff et al. (2004), in: Jensen and Schwarzer (2013)
5. Ziegler and Heyen (2005), in: Jensen and Schwarzer (2013)
6. Jensen and Schwarzer (2013)
7. Meyer and Harff (2005), in: Jensen and Schwarzer (2013)
8. Niemeyer et al. (2016)
9. Benninghoff and Winter (2019)
10. Wahl et al. (2013), in: Benninghoff and Winter (2019)
11. Elias et al. (2012), in: Benninghoff and Winter (2019)
12. De Vet et al. (2017), in: Benninghoff and Winter (2019)