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How Do Extreme Storms Impact and Control Coastal Evolution?

Postdoc-stipendium i Danmark | 02/05/2016

Coastal erosion at the Danish North Sea coast. The coastal cliffs are eroded during storms causing the coast to retreat rapidly. Photo: Mikkel Fruergaard

Extreme storms and storm surges have an inordinate impact on soft-sediment coastal environments, but little is known about how extreme storms affect coastal evolution on timescales ranging from centuries to millennia. This knowledge is significant in order to understand how coastal environments will respond and adapt to future climate changes that may cause an increase in the frequency of major storm events. In the research project COASTEVENT, we investigate the impact of major storms and storm surges on the long-term soft-sediment coastal morphology and coastal evolution. The main part of the research is carried out at the coast of the Cotentin peninsula along the English Channel, France where we apply a combination of sediment cores, near-surface geophysics, and high-resolution optically stimulated luminescence (OSL) and radiocarbon dating to investigate coastal spit systems. We expect that the results from COASTEVENT will allow us to identify and document the sedimentological signatures of extreme storms in the sedimentary records and to unravel how extreme storms have an impact on the long-term evolution of the investigated coastal system. This knowledge is very important in order to better comprehend future coastal evolution and to handle future coastal management challenges properly.

Extreme storms affect coasts and are a threat to people and infrastructure in the coastal zone. The photo is from the north coast of Zealand, Denmark during the storm Bodil in 2013. Photo: Mikkel Fruergaard

The purpose of the research project COASTEVENT is to investigate how extreme storms affect and control the evolution of the world’s coasts, and how a higher frequency of extreme storms in the future will influence the rate of coastal changes.

Storms are associated with large waves, strong currents and elevated water levels. When a storm hits a coast, it can result in both erosion and accumulation of sediment, which cause the coastline to retreat or build out. Improved knowledge of how coasts are affected by storms is of great importance for people living in the coastal zones.

More than one billion people live in coastal areas and therefore, the coastal zones are of great economic and recreational importance. In densely build and urbanised coastal areas, extreme storms can have severe consequences causing widespread destruction of buildings and infrastructure as well as losses of lives. Hurricane Katrina, the cyclone Nargis and to a smaller scope the storm Bodil, in Denmark (2013), are all examples of severe devastating storms.

"The direct impact of extreme storms on coasts is very apparent. However, the impact of storms on long-term coastal evolution is still not well-understood, and more research is needed before we fully understand the importance of storms on coastal evolution,” says Mikkel Fruergaard, who has received a postdoc fellowship from the Carlsberg Foundation to carry out the research project COASTEVENT.
“The latest report from the Intergovernmental Panel on Climate Change (IPCC), suggests that the frequency of extreme storms impacting the world’s coasts may increase during the 21st century. This report is in line with a number of other studies that indicate that large storms will become more frequent in the future. This emphasises the need for more research in how storms affect the evolution of coastal environments,” says Mikkel Fruergaard.

Storm Driven Coastal Response

What Controls the Impact of a Storm on the Coast?

Process intensity, duration and storm frequency control the impact of a storm on the coast. High wind velocities generate large waves, strong currents, elevated water levels and thus higher process intensity. The process intensity is the main parameter controlling the coastal impact of the storm. The duration of the storm controls the duration of the process intensity. Thus, a storm with a long duration has a larger impact on the coast compared to a storm with a short duration. The frequency indicates how often a storm with of a certain magnitude is expected to occur. If the storm frequency is high, the coast has less time to recover, which makes the impact of each storm larger.

Coasts are influenced by the impact of storms. A coastal profile attempts to reach a state of equilibrium with the hydrodynamic conditions. More storms will force coasts towards a new dynamic equilibrium by either eroding or supplying sediment.

An increased understanding of coastal responses to past storms is required to predict future coastal changes and to understand the mechanisms that control long-term coastal evolution better.

Therefore, the purpose of COASTEVENT is to answer this question: Will extreme storms control long-term coastal evolution, and will future climate changes accelerate the rate of coastal changes?



"Extreme events have an inordinate influence on coastal dynamics. There is a non-linear relationship between wave or tidal-current energy and the amount of sediment that can be eroded and deposited,” explains Mikkel Fruergaard. 

As a result, extreme storms can do more work than many decades of ‘normal’ coastal dynamics, and they can act in such a way that they either amplify or negate the results of day-to-day processes. Our knowledge about how high-energy events affect the long-term evolution of the coastal zone is, however, associated with large uncertainty, and this research area has never been explored in a systematic manner.

A Complete and Transnational Data Set

The aim of COASTEVENT is to give a comprehensive understanding of coastal changes caused by storms in France and Denmark. The main research site is the coast of Normandy at the peninsula of Cotentin along the English Channel. This area is exposed to low-pressure storm systems from the Atlantic Ocean, and it is therefore well suited for studying the impact of storms on coastal evolution. Besides, tidal processes heavily influence the area, as it is a macro-tidal environment.

The project is carried out in collaboration with researchers at Le laboratoire Morphodynamique Continentale et Côtière (M2C), University of Caen, France and the Geological Survey of Denmark and Greenland (GEUS).

The main research site is the coast of Normandy at the peninsula of Cotentin in the English Channel, France (1). This coast is impacted by low-pressure storm systems from the Atlantic Ocean. Data from the coast of Cotentin and the Danish Wadden Sea (2) will be compared in order to understand how large scale (regional) coastal evolution is affected by storms.

“The coast of Normandy is complex, both in sedimentology and coastal morphology, and a complementary data set is required to evaluate the coastal evolution in relation to storms. Especially the result from optically stimulated luminescence (OSL) dating (see fact box) is expected to provide useful information on the coastal evolution,” Mikkel Fruergaard explains.

DEM-Hillshade model from the French research site. The complex morphology of the area is composed of dunes, spits, salt marshes, and tidal channels. It is still uncertain what controlled the evolution of the area. The current hypothesis is that changes in meteological storm patters may have played a key role. Data from the Institut National De L'Information Géographique Et Forestière.

OSL dating can be used to determine when clastic sediments last were transported and deposited. The clastic sediments consist of rock fragments that have been eroded by physical and chemical processes and transported by water, wind, or ice to the place where they are deposited and buried. OSL dating has been intensively and successfully applied to date the sediments in the Danish Wadden Sea, but it has never been used at the study site in France. “I expect that the results from OSL dating will provide us with substantial new knowledge about this area,” Mikkel Fruergaard says.

In addition to the study sites in France and Denmark, sedimentological data from Greenland, United States and Argentina will be applied in the research. This data will be used to do a more comprehensive evaluation of the effects of extreme storms on the world’s coasts, and because of this large regional data set, solid conclusions can be drawn from the project.
“I believe that by using different methods and data sets from several different localities, COASTEVENT will set the course for future research in how storms influence the coastal zones on a regional scale. I expect that the results of this project will contribute to a better protection and management of our coasts. This is particularly important in a time where climate changes most likely will cause coastal changes to occur faster than we have seen in the past. New and improved knowledge about our coasts is pivotal in order to face and handle coastal management challenges as a result of climate changes,” concludes Mikkel Fruergaard.

The Carlsberg Foundation and COASTEVENT

The research project COASTEVENT is financed by the Carlsberg Foundation. About the importance of the Carlsberg Foundation for the research, Mikkel Fruergaard says:

“Climate changes have really increased the public awareness of the importance of coastal research, and this research area is extremely significant for improving the way we deal with the consequences of climate changes in the coastal zones. The financial support I have received from the Carlsberg Foundation allows me to improve the knowledge of how the coasts of the world will develop in the future. In addition, the support from the Carlsberg Foundation is crucial for me in the process of establishing an independent research career. The research at a foreign research institution allows scientific immersion, and this is decisive in building a strong research network and an independent research profile.”

What is Optically Stimulated Luminescence (OSL) Dating?

In Mikkel Fruergaard’s research project, it is fundamental to know the time of sediment deposition. The most used dating method is radiocarbon dating, also known as 14C-dating. This method is used to date organic material, e.g. wood, bones from humans or animals, and peat. However, this method cannot be applied to clastic sediments, which most soft-sediment-coasts are composed of, because of the low content of organic matter in these deposits. Instead, clastic sediments can be dated by OSL dating. This method can date quartz and feldspar sand, which is an important component of most soft-sediment-coasts. Quartz and feldspar sand grains function as “rechargeable batteries” that are charged with energy from naturally occurring radiation in the ground. The amount of energy stored in a grain is proportional to the time of burial. If the sand grain is eroded, transported and exposed to sunlight, the energy stored in the grain is released. In the laboratory, the sand grain is illuminated, and the stored energy is released as a light signal, which is called the luminescence signal. The luminescence signal can be converted into an age corresponding to the time of deposition of the grain.

Age-depth diagram with dated sediment samples from a barrier spit. The green squares show that an approximately 7 metre thick sediment body was rapidly deposited shortly after the most catastrophic Danish storms in 1634. The blue squares are sediment samples from underlying older deposits. Modified from Fruergaard et al. (2013).

 

Further Reading

  • Fruergaard, M., Møller, I., Johannessen, P.N., Nielsen, L.H., Andersen, T.J., Nielsen, L., Sander, L. & Pejrup, M. 2015: Stratigraphy, evolution, and controls of a Holocene transgressive-regressive barrier island under changing sea-level: Danish North Sea coast. Journal of Sedimentary Research. 85 (7), pp. 820-844. DOI: 10.2110/jsr.2015.53.
  • Fruergaard, M. & Kroon, A. in press: Morphological response of a barrier island system on a catastrophic event: the AD 1634 North Sea storm. Earth Surface Processes and Landforms. DOI:10.1002/esp.3863.