The Danish coastline comprises active coastal cliffs where the sea erodes material, as well as beach-ridge complexes where material is deposited in the lee of prevailing winds. About 80% of the Danish population lives in urban areas near the coast. A total of about 1800km of coastline is protected by dykes or other types of coastal protection.

Low-lying areas are especially at risk, since they are both exposed to increases in sea level and are under pressure from increasing surface runoff from land areas. In particular, many of the coastal towns near larger river estuaries or at the bottom of fjords may face complex problems. Merely building higher dykes, for instance, is not a long-term solution, as the problem of backwater flooding will just become greater as a result of river water being unable to flow freely into the sea. A long-term solution requires the involvement of river valleys further inland. 

Watch the film "Weathers will" by The Danish Coastal Authority

Water level
The tide varies from hour to hour along the coast. In winter time, the water level and waves are higher because it is generally more windy during winter.  Over the past several years, there have been more storms than usual.

The Danish Coastal Authority prepares statistics of the probability of a given extreme water level in 55 places throughout Denmark. These statistics are for use in coastal development planning and in the design of coastal defences. 

Below are the statistics of high water levels for the city of Åbenrå in Jutland based on almost 27 years of water-level measurements.

Water level relative to DVR90

The blue curve shows the statistical probability that a given water level will occur in Åbenrå. For example, statistically a water level of 160cm will occur every 30 years. Note that the horizontal x axis is a log axis.  The light blue, dotted lines indicate the statistical uncertainty.

The statistics are based on the water-level gauges which have been set up along Danish coasts, typically in harbours. These coastal water-level gauges are managed by harbours, municipalities, the Danish Meteorological Institute and the Danish Coastal Authority. To obtain good statistics, it is essential that the water-level gauges are calibrated on a regular basis. The statistics are updated approximately every five years.

Flooding
Today, water levels throughout Denmark are registered regularly by a number of water-level gauges managed by different bodies. The Danish Coastal Authority collects data from 55 water-level gauges, and on the basis of these data has prepared statistics (cf. above) on the frequency of different water levels.

Below is a map showing the water level that is statistically likely to occur once in every 50 years at the 55 locations throughout Denmark.

Risk of floods
When water levels generally increase due to climate change, the probability that a given water level will occur increases as well.  How much depends on the location.

Below is an example for the city of Korsør, where the present probability for a given water level is indicated by the light-blue line. Statistically, today a water level of 152cm occurs only once every 100 years. 

Note that the x axis is a log axis.  If, for example, the high water levels increase generally by 45cm up to the year 2100, the probability of a given water level in 2100 is indicated by the dark-blue line.  This means that a water level of 152cm is likely to occur once every three years in 2100. 

The risk of floods has not yet been mapped for all of Denmark. In connection with the implementation of the EU Floods Directive, maps of the current risk of floods in Denmark will be ready by the end of 2013. This work will take its point of departure in the national terrain model, which represents extremely detailed terrain surface measurements throughout Denmark.

In addition to rising sea levels, climate change will also lead to increased precipitation and more extreme downpours in winter. Far the majority of this precipitation will end up in rivers and watercourses. This will further increase the risk of flooding of coastal areas adjacent to the estuaries of rivers and watercourses.

Impacts on harbours and navigation channels
Harbours
Rising water levels mean that wharfs will no longer have the optimal height for loading and unloading. It will also mean water depths will increase in the harbour, if the increased sand volumes are not led into the harbour via the harbour’s breakwaters. A greater water depth would allow ships with greater draught to enter the harbour.

More powerful storms will render loading and unloading with cranes more difficult. Furthermore, more powerful storms will mean a greater wind effect on ships, and it will increase the height of waves as well as more powerful sea currents, which in combination will make it more difficult for the ships to enter the harbour.

This also means that harbour works will be exposed to greater forces. The outer works of the harbour will be exposed to increased wave and current loads. Harbours are dynamic places of work, and the functional requirements for the harbour change relatively often in a climate change perspective. The life span of for example a sheet pile is 20-30 years. The increased impacts from climate change will therefore not be significant during its life span.

Navigation channels
Today, navigation channels are dredged according to need. The need varies from year to year and in the course of a year due to variations in the climate. Higher water levels and more and more powerful storms will also lead to greater sanding up of navigation channels.

Possible action
The possibilities for ongoing climate adaptation in the Danish coastal zone are generally good. From an overall perspective, it is up to the individual landowner if he wants to protect himself against flooding or erosion. Therefore there are no laws or regulations determining whether coastal defences should be established, and, if so, the level at which the individual landowner must protect himself.

Planning and development
Climate change should be incorporated when planning the development of coastal areas, when this involves buildings and installations with long life spans, i.e. where climate change will have significant impacts on the buildings or installations during their useful lives. 

When planning the development of flood-prone areas, it is important to realise that the problem not much a rising sea, but rather insufficient terrain height, as illustrated below.

The illustration shows a situation where development is planned for the area behind a dyke. Two water levels must be taken into account here: the level of the sea in front of the dyke and level of the watercourse behind the dyke. Both levels are indicated by light blue. When the sea level rises and precipitation increases, the risk of flooding grows as well.

This challenge can be met in different ways. An obvious solution would be to decide that the area should not be developed for housing purposes and that the houses should be built on higher ground.

Another possibility would be to increase the terrain height by feeding the area with more sand. The terrain height could be adjusted to the desired level of security against floods. The level of security could be based on an assessment of the life span of the buildings; for example one hundred years’ security.  




A third possibility would be to construct the houses on foundations that are high enough to ensure the desired level of security against floods. The illustration below shows the extensions which should be considered when determining the foundation elevation of a house. When determining the extensions, several scenarios should be used to examine the vulnerability to climate change. The foundation may be designed in a traditional manner or the house may be constructed on piles or similar.