3.1.2 Risk of inundation in coastal cities of Denmark
Sea-level rise is dependent on various complex factors, each of which introduces varying levels of uncertainty. - Various estimates of mean sea-level rise (SLR) are dependent on the different scenarios of future GHG emissions ranging between 0.2 to 1.4m SLR[1] by the end of this century in Denmark (see Figure 54). However, there is a particular risk that sea level rises will become more intense due to unforeseeable cascading impacts. The Danish Meteorological Institute assesses that towards the end of this century (1981-2100), the water level around Denmark is expected to rise 0.1 to 0.6m for the most optimistic scenario (RCP2.6) and 0.3 to 0.9m for the highest scenario (RCP8.5) with the upper estimate by Danish Meteorological Institute (DMI) to reach up to 1.4m (Klimatilpasning, 2021).
Figure 54. (Top) The graph above shows the absolute mean water level around Denmark in metres for the years 1900-2100. The grey-shaded curve for the years 1900-2012 shows the observed annual mean water level measured by Danish water gauges, adjusted for isostatic uplift. The thin blue curve for the years 2012-2100 shows the IPCC’s best estimate of the mean water level in the North Sea for the RCP4.5 scenario, and the light purple shadow indicates the uncertainty of this scenario. The dotted line shows the Danish Meteorological Institute’s (DMI) estimate of an upper limit for water level rises for use in uncertainty calculations. To the right of the figure are shown the mean value and uncertainties for the period 2081-2100 for the four IPCC RCP scenarios as well as for the University of Copenhagen’s BACC assessment of the A1B scenario in grey (Olesen et al., 2014; DMI, 2018). Image credit: Olesen et al. (2014).
(Bottom) Map of Little Belt (Lillebælt) Denmark shows the change between 1981-2010 and the future period 2071-2100 in mean water level (cm) for the high emissions scenario RCP8.5. Change in mean water level: 54cm and uncertainty range: 10-99cm (Pedersen et al., 2020; DMI, 2022a). Image credit: Danish Meteorological Institute (DMI, 2022).
When the mean sea level rises, the maximum water level in the event of a storm surge can be expected to rise similarly or faster. The frequency and severity of storm surges are expected to increase, and the IPCC warned that a 100year storm surge event would likely happen every year by the end of the century if we carry on the B-A-U trajectory (IPCC, 2021a; 2021b). This is also the case for Denmark, where storm surges are expected to increase both in frequency and strength towards the end of this century. For instance, today’s 20-year and 100-year storm surge events can happen every one or two years in Denmark (Payne, Anker Pedersen and Fonseca, 2022), with a range of 1.62 to 2.9m in storm surge levels by the end of the century (for Vejle), as indicated in Figure 55 below (Kystdirektoratet, 2020).
10 year storm surge event (2021)
10 year storm surge event (2100)
Figure 55. (Top image) An increasing number of wilder storms in Denmark (Class 4 in red – classified based on wind speed and strength) within 130 years from the end of the 19th century. The recorded storms and hurricanes average 15 per decade, ranging in various storm surge heights (DMI, 2022c).
(Middle row image) An example of the growing storm surge risk is the coastal city of Vejle, where the range of storm surge could reach almost up to 3m by the end of the century. Image credit: Kystdirektoratet (2020).
(Bottom row images) Two maps of the city of Vejle with the impact of 10-year storm surge events for 2021 and 2100. By 2100 the bottom of the river valley where the city is located will be completely underwater compared to 2021, in which the water barely impacts the city. Image credit: Vejle Klimakort( n.d.).
(Extracted from Kumu Multiscalar map – Vejle Fjord node).
These estimates (see Figure 56) inevitably complicate implementing urban planning policies for coastal protection, which needs to take into consideration the minimum height of dikes and the elevation level of land reclamation projects[123]. For Denmark, the current elevation (called “kote” in Danish) level is set for 2.5m coastal protection and 3m coastal adaptation (Vejle Municipality, 2020a). There are concerns that these levels may be underestimated to keep up with constant worsening estimates for 2100. Many Danish coastal cities currently have hard concrete edge (bulkheads) conditions with no sea walls installed yet, as outlined in Figure 57. The elevation of the waterfront areas usually ranges from less than 1m to under 2m above current water levels (Klimatilpasning - KAMP, n.d.). However, if the sea level rises to a likely worst-case scenario of 0.5m-0.9m by the end of this century, then even a relatively frequent and mild storm surge event is likely to happen every year (such as Storm Malik in 2022, reaching 1.4m increase in water levels in Vejle – refer to section 1.2) would not only inundate all coastal cities of Denmark but cause damage to properties and a way of life (see Figure 57). Any storm surge event with a sea-level rise of 0.9m or more will negate most of the world’s current storm surge floodgates (Pilkey and Young, 2011).
Figure 56. A diagram showing the relationship between SLR and SS in its impact on inundating coastal cities in Denmark with Storm surge range for Little Belt Sea (Lillebælt where Vejle is). SLR alone will not cause inundation of coastal cities (even in a worst-case scenario), but SLR coupled with frequent and more intense SS has the potential to wreak havoc in a worst-case scenario situation. Image credit: Soo Ryu and Agnes Jarmund.
Figure 57. The current elevation (1-2m) above the normal water level in the form of fortified concrete bulkheads represents the hard edge conditions of many coastal cities in Denmark, such as Aalborg, Aarhus, Middelfart and Vejle. Furthermore, these typical urban coastal edge conditions are defined and segregated, the hard boundary between city and water that severs a closer and more tactile connection with the water and its life forms. The public space on the waterfront is mainly made of concrete surfaces fit for humans. Little consideration is given to terrestrial plants, and there is almost no designated space for interacting with the marine world. Image credit: The photos of the hard-concrete edge conditions of waterfront spaces were taken by the author in Aalborg (Top left), Aarhus (Top right), Middelfart (Bottom left) and Vejle (Bottom right) in Denmark during 2020-2022.
[122] According to DMI's Climate Atlas, it is expected that the average water level will rise by approximately half a meter towards the end of this century compared to today (RCP8.5). If the current political ambitions for emission reductions are achieved on a global scale (RCP4.5), sea level rise can be limited to approx. 35 cm.by 2100 (Payne, Anker Pedersen and Fonseca, 2022). However, for the future sea level rise, even fairly high increases cannot be completely ruled out. The uncertainty over the median rise is therefore large. Thus, the 95th percentile for rise in mean water level in the high climate scenario (RCP8.5) at the end of the 21st century is estimated at 1.4 meters (DMI, 2019). The rise in sea level after the year 2100 is more uncertain than the increases this century, but the SROCC gives the likely range of the global rise for the year 2300 at 2.3-5.4 meters in the high climate scenario (RCP8.5). It is expected that the increases around Denmark will be close to the global average (ibid.).
[123] Pilkey and Young (2011) recommends as a bare minimum to consider a minimal 0.9m for SLR in 50-100year planning horizon in coastal communities where the politics would not permit the consideration of more forward-looking coastal management. Pilkey and Young (2011, p. 179) also considers 2m SLR by 2100 being a real possibility.