KM-R-2: Land protection dykes without safety deficit – case study | Umweltbundesamt

KM-R-2: Land protection dykes without safety deficit – case study

In Schleswig-Holstein land protection dykes protect the adjacent coastal lowlands from inundation over a total length of 433 kilometres. Given that the rising sea level increases the danger of flood events, these dykes are continuously adapted to increasing challenges. Recently, the proportion of land protection dykes without a safety deficit increased to 81.7 %.

Land protection dykes meet increasing protection standards

The impacts of climate change put increased pressures on coasts and coast protection structures. The adaptation to climate change is indispensable in order to keep any negative impacts on human health in socially acceptable proportions; the same applies to impacts on the environment, on cultural heritage, on economic activities and, not least, on coastal protection. In Schleswig-Holstein this challenge is met to a large extent by land protection dykes. These dykes currently extend over a total distance of 433 kilometres, thus protecting more than 90 % of potentially flood-threatened coastal lowland areas of the country.

The case-study indicator presented in this report in respect of land protection dykes without safety deficit was developed in 2019 in Schleswig-Holstein as an indicator for monitoring the implementation of UN sustainability objectives. This indicator 47 entitled ‘Generalplan Küstenschutz’ of Action Field 5 ‘Infrastructure and climate protection’ is evidence for the fact that the climate change adaptation strategy laid down in the Master Plan for Coastal Risk Management (GPK) has been taken into account in Schleswig-Holstein.

Sea level rise and the future development of the storm scenario in the regions of North Sea and Baltic Sea are crucial factors to be observed in planning and in respect of the continuous adaptation of measures for coastal protection and safeguarding in Schleswig-Holstein. The protection standards to be met and their underlying design principles are part and parcel of the declared strategy laid down in Schleswig-Holstein’s GPK. As a core planning and management tool, the GPK pools essential principles, guidelines and measures in respect of coastal protection. Approximately every 10 years, this plan is updated and adapted to current circumstances. This process is concurrent with a safety inspection of the land protection dykes: Purpose of this inspection is – by means of a procedure complying with the current state of technology and as far as possible long-term and current time series of annual maximum high water levels – to verify whether the land protection dykes would be able to withstand a storm surge with a statistically determined annual probability of 0.5 %. If that is not the case, the dyke segment is entered into a list of land protection dykes to be reinforced.

The reinforcement measures take climate change fully into account. Their implementation adopts the concept of ‘Klimadeich’ which has evolved in the course of the past two decades. Already since 2001, the dykes in Schleswig-Holstein (for the first time in the world) have been extended in height as a protective measure to offset an expected sea level rise by another 0.5 metres. Furthermore, they have been fitted since 2009 with a wider dyke crest and a more gradual outer slope. This measure is intended to offset a sea level rise up to a total of 1.0 metre. By making the outer slope more gradual it will be possible to facilitate a subsequent structural adaptation, thus ultimately – in the course of several construction phases – offsetting a total sea level rise by approximately 2.0 metres compared to the present sea level. This demonstrates that the planning measures are indeed taking into account the current projections regarding the most unfavourable sea level rise.

The indicator shows the percentual proportion of land protection dykes without safety deficit compared to the total length of all land protection dykes in Schleswig-Holstein. The indicator thus reflects the current condition of land protection dykes with reference to the hydrological loads imposed by storm surges. At present, 81.7 % of land protection dykes do not show any safety deficits. This is a higher proportion than the value of 76.5 % determined by the previous safety inspection in 2011. Nevertheless, as a result of the severe storm surge known as Xaver in 2014, a 5.6 kilometre stretch of the dyke line was additionally categorised as unsafe. The strong decline in 2012 is due to the hydrological safety inspection in 2011 of land protection dykes based on a higher safety criterion applied than in 2001 (annual probability of 0.5 % instead of 1.0 %). This was the main reason for extending the length of dykes to be reinforced from 72.7 kilometres again to a total of 101.5 kilometres which equates to 23.5 %.

As can be seen from the chart, there was no dyke reinforcement completed in the years of 2006 or 2015. It would be incorrect to take this as evidence that no dyke reinforcement took place in those years. Usually, dyke reinforcements take several years to complete; this is why in some years the work is not completed despite ongoing construction work. By the same token, the fluctuations should not be directly related to the availability of financial resources as having caused these fluctuations. In fact, the specific costs of dyke reinforcements are dependent on several factors.

Based on developments so far, it appears as though all land protection dykes in Schleswig-Holstein were reinforced once after 120 years according to the Klimadeich (climate dyke) principle. Whether the actual interval extends over longer or shorter periods depends, above all, on the human and financial resources made available. Another challenge continues to be the increasing complexity of planning and application procedures. In early February 2022, precisely 60 years after the last flood disaster on the German North Sea coast, the government of Schleswig-Holstein adopted a resolution on the fifth update of the GPK including another safety inspection of land protection dykes. As far as the length of the land protection dykes to be reinforced is concerned, there have been no fundamental changes.

The aim of this indicator is to illustrate a technically appropriate and up-to-date strategic consideration of climate change and associated impacts in the regular updating of the GPK. However, it would not be appropriate to set a concrete target for this indicator, as its development is also strongly influenced by several external framework conditions, such as the budget funding made available by state parliament, Federal government and the EU; it is also important to remember the influence of the stochastic nature of extreme storm surges in this context.