Influence of Changes in Sea Level Underestimated

A new study by the University of Siegen shows that the influence on storm surge and wave heights will in the future be stronger than previously thought - and therefore planning heights for protective structures, like dikes, should possibly be reconsidered.

The current storm flood on the German east coast shows our society’s vulnerability to extreme natural events. Therefore, a reliable determination of present and future storm tide and wave levels is of particular importance to coastal areas. However, along the North Frisian coast, higher coastal protection structures than previously assumed could be required in the future. This is the result of a new study by the international research team around Dr. Arne Arns from the Research Institute for Water and Environment (fwu) at the University of Siegen. The current results have been published in an article in the internationally renowned trade journal "Scientific Reports" of the "Nature Publishing Group".

Whereas the mean sea level rise has so far served as a basis for calculating protective heights, the new results show the necessity of a 1.5 to 2 times increase in the future

To date, experts have estimated future height levels of coastal protection structures, such as dikes, using scientifically based forecasts of the mean sea level. In past research, however, the sometimes strong changes in storm water levels and the emergent waves have not been considered. But as the scientist’s investigations now show, the interaction of those forces strongly affects the requirement for protective heights along some parts of the North Frisian coast.

In the current article, scientists have carried out comprehensive analyzes for the first time, which take into account the influence of the sea level on both storm-tide water levels and waves. "The research shows that dynamic and complex interactions between sea level changes and extreme water levels occur particularly in the shallow tidelands of North Frisia. As a consequence, storm tide water levels will rise slightly higher than the actual sea level. A rising sea level also has an elevating effect on the waves. In this way, in the future, the influences might not come from the tidelands so much, as rather from less strongly influenced movement towards the coast and protective structures, and therefore be larger than before," explains Dr. Arne Arns. His recommendation: "It is urgent to take these effects into consideration in future research. If we estimate the necessary levels of protection for projected sea level changes simply by means of projected sea level changes, it may lead to an underestimation of the necessary levels of protection." The researcher points out, however, that "the future development of the tidelands is difficult to predict. This could at least partially compensate for the above-mentioned effects."

Reference:
Arns, A. et al. Sea-level rise induced amplification of coastal protection design heights. Sci. Rep. 6, 40171; doi: 10.1038/srep40171 (2016).