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Case 2: Denmark 2 - Cloudbursts

The following is a preliminary assessment of how small communities in the southern part of Denmark will be affected by climate change – the main hazard in focus, in this case, is flooding outside larger urban areas caused by cloudbursts and prolonged periods of precipitation. The assessment will be developed further as the project progresses.   

The case area

In this case, we focus on the area of Lolland-Falster, in the region of Southern Zealand (Figure 1), which holds two of the most vulnerable Danish municipalities in regards to climate change; Lolland Kommune and Guldborgsund Kommune (CONCITO, 2017).

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The case study explores communities in recreational housing areas, which are situated on the southern coast of Lolland-Falster protected from the risk of flooding from the sea by dikes, but vulnerable to the risk of flooding from cloudbursts and prolonged periods of precipitation due to their location in low-lying areas below or close to sea-level.  

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Figure 1 The location of Lolland-Falster

Hazard and exposure 

Extreme rain events that can cause flooding can be categories into three types of incidents:

  • Cloudbursts which are defined as short rain events where the amount of precipitation is more than 15 mm in 30 minutes.

  • Heavy rain which is more than 24 mm in 6 hours and lastly;

  • Interconnected rain (koblet regn in Danish) is a situation where cloudbursts or heavy rain occurs or interconnects, within a period of prolonged precipitation. This can create situations where the drainage capacity and absorption capacity of the soil is exceeded and thus lead to flooding.

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In a Danish context, heavy rain and cloudbursts are more likely to occur during the summer period (because of warmer temperatures), whereas interconnected rain events are more likely to happen during the autumn and winter periods (due to the likelihood of more precipitation). Periods of snowmelt can in some situations lead to similar results as interconnected rain events if a larger amount of snow melts over a short period of time while the absorptions capacity is low for instance due to frozen soil.

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Historical events

Flooding in Denmark is commonly associated with the risk of flooding from the sea. However, several extreme rain events during the last decade have created an increased awareness of the risk of flooding from precipitation. Most attention has been directed towards extreme rain events in larger urban areas. A cloudburst that hit greater Copenhagen in 2011 is estimated to costed approximately 800 million EUR in damages (6.2 billion Danish Kroner paid in insurance claims) making it the costliest single (natural) disaster in Europe in 2011 (Beredskabsstyrelsen, 2017).  

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Extreme rain events outside the larger urban areas have attracted less attention. During the summer months of 2011 areas of southern Denmark, Guldborgsund, and Lolland Municipalities received up to 600 mm of rain the equivalent of the normal annual precipitation. Combined with incidents of cloudburst and heavy rain in the month of august this led to severe flooding of agricultural land and housing areas (Guldborgsund Kommune, 2014; Lolland Kommune, 2016).

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The events in 2011 in Lolland-Falster did not have the same negative financial impact as the cloudburst in greater Copenhagen. However, it did highlight the vulnerability of communities in low-lying areas with inadequate or no drainage systems, such as recreational houses and summer cottages. This event has since formed a reference point for the municipality's work regarding flood risk management (Guldborgsund Kommune, 2014; Lolland Kommune, 2016).

Billede af Eutah Mizushima

Climate change scenarios

In Denmark, climate change is expected to increase the amount of precipitation during the spring, autumn, and winter and might lead to a decrease in the amount of summer precipitation (Olesen et al., 2014).

In general, the precipitation is highest in the western part of Denmark, however, changes in the amount of precipitation are expected to affect the southern areas of Denmark more, which is the research area in focus for this case (DMI, 2021).

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When estimating the impact of climate change in a Danish context two Representative Concentration Pathways (RCPs) scenarios are used by the Danish Metrological Institute (DMI). The RCPs describe different 21st-century scenarios or pathways based on among others the level of greenhouse gas (GHG) emissions (IPCC, 2014). RCP4.5 is an intermediate scenario where emissions are reduced in the latter part of the century, whereas, RCP8.5 is a high estimate sometimes described as business as usual (DMI, 2018) in regard to levels of emissions. Both scenarios can be used for planning climate change adaptation interventions in municipalities in Denmark.  

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Using RCP4.5 to estimate how climate change will affect Guldborgsund municipality it is found that the annual precipitation is expected to increase by 8% from an average of 1,71 mm/day in the reference period 1981-2010 to 1,81 mm/day in 2041-2070 (from 624 mm/year to 660 mm/year) (DMI, 2021). When assessing the changes, it needs to be taking into account that precipitation is mainly expected to increase during the winter, autumn, and spring months. For Guldborgsund municipality, there is an estimated increase of 9% in precipitation during the winter months (for RCP4.5 in 2041-2070) (ibid.).   

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Climate change is also expected to increase the severity and frequency of periods of precipitation in Denmark. As the average temperatures increase because of anthropogenic climate change the atmosphere can hold more moisture. An atmosphere with more moisture can produce more intense precipitation events. Consequently, cloudburst and extreme rain events are expected to increase in frequency and severity (Olesen et al., 2014).

For Guldborgsund municipality, the frequency of cloudburst is expected to increase by 31% (for RCP4.5 in 2041-2070) and the frequency of days with more than 20 mm of rain per day is expected to increase by 40% during the winter months (for RCP 4.5 in 2041-2070) (DMI 2021).   

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Whereas the impact on cloudburst and heavy rain in terms of frequency and intensity can be estimated it is more difficult to estimate the changes in the occurrence of interconnected rain events. These events, which are critical for flooding outside urban areas, can also be expected to increase in frequency since the intensity of prolonged rain is also expected to increase as well as the days with heavy rain (ibid.). In addition, the increase in precipitation is also expected to lead to an increase in groundwater levels, which again can increase the risk of flooding.       

Flooded fields and road due to heavy rainfall.jpg

Vulnerability

It can be difficult to predict the exact location where a cloudburst and/or an incident with heavy rain are going to impact. However, areas that are vulnerable to flooding from those types of hazards can be identified. When focusing attention on areas outside larger urban settlements areas most prone to flooding from precipitation are low-lying areas, with high groundwater levels, and no or limited drainage capacity. This is the case for many rural areas in the southern part of Zealand in Denmark (CONCITO, 2017).

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The landscape of this region with the two islands Lolland and Falster is dominated by agricultural land and smaller villages and towns. The land is low-lying and large parts are below sea level and protected from the sea by dikes that run along large stretches of the coast. Water pumps are in many locations used to drain reclaimed land for agricultural production. For instance, one-sixth of the Lolland is drained continuously and two-thirds of the rainwater falling in the municipality is pumped to the sea (Baron & Petersen, 2015). This makes this area of Denmark vulnerable to flooding from the sea and precipitation-caused flooding.

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The open land, such as rural and agricultural land, is often less vulnerable to extreme rain events, than the urban areas due to amongst others the absorption capacity of the soil. However, in the open land, interconnected rain events, such as the one in 2011, can have severe consequences. On agricultural land, it can damage the yield of the harvest. Thus the financial and economic risks associated with climate change are estimated to be high for a municipality such as Lolland where a large share of the industry and jobs a depended on agriculture (Hansen, 2014).

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For landowners and homeowners in areas with less developed infrastructure, such as areas of summer cottages, climate change and increase in precipitation is likely to worsen an already existing problem as many of these cottages are built with little or no ability to drain excess surface water away from their properties (Gregersen, 2014).

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This can lead to consequences related to building damages and loss of income from rent and prospective sale value of the cottages, and for many communities loss of considerable cultural and social value that is tied to their ‘second home’ and the location. As many of the recreational areas have been identified as representing a high (financial) value the municipalities have increased their attention on flood risk management in these areas (e.g. Guldborgsund Kommune, 2014:26)

Picture 1 Flooded wheat fields on Lolland during summer. © Peter Ege Olsen.jpg
Picture 2 Flooded turnip fields on Lolland during summer. © Peter Ege Olsen.jpg

Flooded wheat fields (left) and turnip fields (right) on Lolland during summer. Photo: Peter Ege Olsen

Challenges and solutions

From the outset management of flooding from precipitation can seem like a simple problem. There are several well-known technical mitigation solutions with regards to improving drainage systems, management of surface water, etc., as well as solutions targeting preparedness, response, and recovery actions (Figure 2). There is also data for flood risk management and practical guides available for both public officials in local governments as well as private landowners (e.g. DMI, 2021; Niras, 2018).

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Figure 2 Relevant actions to flooding from precipitation in the disaster management cycle. Adapted from Hoffmann & Baron, 2018:19

Nevertheless, when looking more closely, flood risk management in a Danish context can often resemble a complex problem rather than a simple problem; solutions to mitigate the risk of flooding often necessitates numerous stakeholders taking collective action, it involves blurred lines of responsibility and despite the seeming abundance of technical solutions the path to common understanding and agreement on problems and what constitutes the right solution can sometimes be a difficult path to tread for the stakeholders involved (Baron, 2020; Baron & Petersen, 2015).

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In Denmark, it is common to have citizen engagement in the management of public welfare services and for civil society organizations to form around local problems. There is a long and strong tradition for community organizations to form and invest in the management of dikes and pumping stations as a response to floods (Hoffmann & Baron, 2018). It is important to note that in a Danish context the landowners legally carry the main responsibility for water management on privately owned land, however, it is not uncommon to find local governments involved with local communities in finding ways to solve problems with regards to flooding. There are examples from a Danish context on how collaboration between the municipal, national authorities, and other relevant private stakeholders can be an effective way of finding innovative and solutions to flooding (Raahauge et al, 2015; Sorensen et al., 2018).

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However, large collaborative multi-stakeholder processes are not necessarily a recipe for success in terms of successfully implemented solutions (Thaler et al., 2019). It might be that they can lead to a greater sense of common understanding concerning the challenge, e.g. climate change, and possible solutions, but it is not given that the identified solutions are implemented.

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The CliCNord research project will further expand on – and explore how these and other challenges can be dealt with in the local context.

Åbn bøger

References

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