Case 6: Norway 2 - Flash floods

The following is a preliminary assessment of how communities in Norway will be affected by climate change – the main hazard in focus is flash floods. The assessment will develop further as the project progresses.

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Photo: Lillian Tveit / Mostphotos

Norway’s national strategy for climate change adaptation

Norway is characterized by its mountainous topography which makes it prone to numerous hazards including landslides, floods, and avalanches. Floods, specifically flash floods, have been highlighted by the Norwegian government as a considerable risk especially as flash floods are a climate change-induced hazard (Hanssen-Bauer, 2017; Miljøstatus, 2020; Norwegain Government, 2012, 2021; Norwegian Water Resources and Energy Directorate, 2019). The number of rain-fed flash floods is expected to rise, given that the average annual precipitation level in Norway is projected to increase in the coming years. The following sections discuss flash floods in more detail, as well as their relation to climate change adaptation in Norway.

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Drammen river during flooding in February 2020. Photo: Peulle

What are flash floods and how are they related to climate change?

Flash floods are a type of flooding that occurs not only in Norway but also globally. Generally speaking, the World Meteorological Organization (WMO) defines flash floods as a “short and sudden local flood with great volume” (Associated Programme on Flood Management, 2012). Flash floods can occur in nearly any part of the world due to the fact they can be induced by a variety of different triggers. Some of the established flash floods triggers, as defined by the WMO, are “heavy or excessive rainfall, rapid snowmelt caused by sudden increases in temperature or rain on snow, or after a sudden release of water from a dam or levee failure, or the breakup of an ice jam” (ibid). Importantly, what distinguishes flash floods from other types of flooding is their limited duration, as well as their rapid onset, which typically occurs within six hours or less from the flood-inducing event(s).

In Norway, the Norwegian Water Resources and Energy Directorate (NVE) defines flash floods as “rapid flooding possibility occurring outside of the establish river channel network” (Peereboom, Waagø, & Myhre, 2011). Though the occurrence of flash flooding in Norway is relatively rare, there is the potential for widespread and damaging impact from their occurrence. A significant portion of Norwegian rivers are used for hydropower generation. Though such utilization of riverways increases their regulation, and consequently, decreases the probability of flood occurrence, there is nonetheless the potential for large-scale damaging impact, for example, in the event of a hydropower dam breach. Flash floods, particularly when they involve rivers on steep terrain, can induce landslides and other debris flows. In the event of such compound events, “debris dams” can occur at the bottom of the slope, which can result in a debris dam breach, and the potential for a rapid flooding event that can destroy critical infrastructure (e.g. hydropower plants), land, and life (Norwegian Water Resources and Energy Directorate, 2009).

 

It is important to note that flood risk in Norway has changed significantly in the last decades as land use within the country has changed. As land use has changed from more agricultural to industrial, the possibility for flood damages has also increased, due to more critical infrastructure being at-risk for damage. Therefore, there is now an arguably larger area in Norway that is potentially at risk now than previously for flood-related damages.

The potential for flash flooding is directly linked to climate change. As mentioned previously, flash floods are typically induced by large rainfall events and other meteorological-driven factors. According to NVE, despite the current projection that future floods will “decrease in size and quantity in large catchments areas (due to an expected decrease in snowfall and increases in temperature), smaller waterways will be more vulnerable to floods due to local rain events, which are expected to be more frequent in the future”(Peereboom et al., 2011). Therefore, flash flooding will continue in the coming years and decades, with an expectation that they will be a potentially recurring, but also devastating hazard, particularly at a local scale. Current research has shown that there are indeed localized trends regarding flash flooding in Norway: flash flooding prone areas are typically in the south of Norway and along the eastern and western coasts. In addition, areas of steep hills (with peaks between 800 to 1300m) are also prone to flash floods. Seasonal variations also exist, with springtime flooding more common in the east and fall and early winter flooding more common in the west, due to the polar front (Rauken & Kelman, 2010). Flash flooding damages often results in road and railway closure, infrastructure damage, property damage, and isolation of hit areas.

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Flooding in Finnmark, May 2010. Photo: Kristoffer Dybvik, NVE-HH

One of the largest challenges in addressing future flash floods, from a scientific perspective, is understanding not only when and how they will occur, but also in what manner. State of the art research on this topic shows that there is still indeed large uncertainty with regards to estimating the frequency and size of flash floods, particularly considering climate change, due to the large spectrum of potential floodwater levels in future years. However, there is consensus among the research community that compound events, such as flood-related avalanches need to be considered as part of future flash flood planning as well as incorporating potential debris flow trajectories into flood risk assessments since flash floods often take unexpected courses when they do occur (Orderud & Naustdalslid, 2020).

Billede af Chris Gallagher

Managing flash floods amidst climate change

Flash floods are a distinct type of flooding that requires a unique set of management strategies. Flood risk assessments, as well as flood management strategies for flash floods, should be tailored for flash floods rather than be adapted from strategies and practices developed for other types of floods. This is due to the rapid onset nature of flash floods, which often result in an unexpected and overwhelming load in the affected flood plain.

 

Flash flood management therefore typically is comprised of two measures, structural and non-structural (Associated Programme on Flood Management, 2012). Structural measures aim to regulate and reduce the potential impact of a flash flood via modifying, limiting, redirecting, and/or delaying the flow of floodwater via various construction activities. Non-structural measures refer to policy, management, and community activities that are focused on increasing flood awareness and reducing flood vulnerability via various land planning measures, community preparedness, and rehabilitation activities (i.e., flood insurance schemes as well as flood compensation). Importantly, flood forecasting and early warning systems are types of non-structural measures that are particularly important in flash flood management, due to the rapid, and often severe, onset of flash floods.

 

One of the main challenges in flash flood management amidst climate change is the effect of the uncertainty of the occurrence of future climate change scenarios on flash floods. Flood forecasting often relies partially on historical data, in conjunction with meteorological and hydrological data, to derive probability estimates for future flood occurrences. However, uncertainty regarding future climate change scenarios exists, particularly with regards to projected future rainfall, when makes having accurate and reliable flood forecasts not only challenging, from both a long-term spatial and temporal perspective. In addition, general circulation models (GCM) which are often used to assess climate change rainfall trends on a global scale, can have limited or invalid results when applied at a local scale (Chang et al., 2013; Field, Barros, Stocker & Dahe, 2012)

 

Nonetheless, despite the limitations of flash flood forecasting models, the central challenge that exists in flash flood management under climate change is not only technical but largely rooted in decision-making under uncertainty. Ultimately, flash flood management requires managers as well as potentially affected citizens to decide which flash flood management strategy to use, based on limited and potentially unreliable data, due to the varied impact climate change may have in a flash flood-affected community. Furthermore, new communities, who were previously not affected by flash floods, may become prone to these events, requiring them to invest time, money, and energy in dealing with a new local phenomenon.

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Flooding in Narvik, May 2010. Photo: Kristoffer Dybvik, NVE-HH

Case area

The exact case studies have not yet been chosen but two or three municipalities in Norway will be chosen for closer scrutiny as part of this project. Due to the project’s focus, those municipalities should be, besides prone to climate change-induced flash floods, also relatively small rural or semi-rural communities. The national competent authority in this field is the above-mentioned Norwegian Water Resources and Energy Directorate (NVE). A meeting with NVE was organized in April 2021, and the cooperation will continue to map the most suitable municipalities for the project’s purposes.

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References

  • Associated Programme on Flood Management (2012). Integrated Flood Management tools Series Management of Flash Floods. Retrieved from https://library.wmo.int/doc_num.php?explnum_id=7337

  • Chang, H.-K., Tan, Y.-C., Lai, J.-S., Pan, T.-Y., Liu, T.-M., & Tung, C.-P. (2013). Improvement of a drainage system for flood management with assessment of the potential effects of climate change. Hydrological Sciences Journal, 58(8), 1581-1597.

  • Field, C. B., Barros, V., Stocker, T. F., & Dahe, Q. (2012). Managing the risks of extreme events and disasters to advance climate change adaptation: special report of the intergovernmental panel on climate change: Cambridge University Press.

  • Hanssen-Bauer, I., Førland, E.J., Haddeland, I., Hisdal, H., Mayer, S., Nesje, A., Nilsen, J. E. Ø., Sandven, S., Sandø, A. B., Sorteberg, A., & Ådlandsvik, B. (2017). Climate in Norway 2100 – a knowledge base for climate adaptation. Retrieved from

  • Miljøstatus (2020). Norges nasjonale miljømål, klima. Retrieved from https://miljostatus.miljodirektoratet.no/miljomal/klima/

  • Norwegian Government (2012) Climate change adaptation in Norway — Meld. St. 33 (2012–2013) Report to the Storting (white paper) https://www.regjeringen.no/en/dokumenter/meld.-st.-33-20122013/id725930/

  • Norwegain Government (2021). Norway’s comprehensive climate action plan. Retrieved from https://www.regjeringen.no/en/aktuelt/heilskapeleg-plan-for-a-na-klimamalet/id2827600/

  • Norwegian Water Resources and Energy Directorate (2019). NVE’s strategy for climate change adaptation 2015-2019. Retrieved from https://www.nve.no/Media/6535/klimatilpasningsstrategi_eng_260816_page.pdf

  • Norwegian Water Resources and Energy Directorate (2009). Planlegging og utbygging I fareområder langs vassdrag. https://publikasjoner.nve.no/retningslinjer/2008/retningslinjer2008_01.pdf

  • Orderud, G. & Naustdalslid, J. (2020). Climate change adaptation in Norway: learning–knowledge processes and the demand for transformative adaptation. International Journal of Sustainable Development & World Ecology, 27(1), 15-27.

  • Peereboom, I. O., Waagø, O. S. & Myhre, M. (2011). Preliminary Flood Risk Assessment in Norway. Retrieved from Norwegian Water Resources and Energy Directorate (NVE): https://publikasjoner.nve.no/report/2011/report2011_07.pdf

  • Rauken, T. & Kelman, I. (2010). River flood vulnerability in Norway through the pressure and release model. Journal of Flood Risk Management, 3(4), 314-322.