Wednesday, March 27, 2019

Calling a Ships Wash a Mini Tsunami Might Seem a Little Over the Top - However…

Unique Geography of Fjord Plays Havoc with the Physics of Wave Forms
Shipping News Feature
NORWAY – It might seem overly dramatic to refer to the wash from a passing ship as a 'mini-tsunami' but that is exactly the term used when Flaskebekk, a small community on the edge of the Oslofjord, started to suffer severe erosion of the foundations of its boathouses and docks.

This was no casual description issuing from the over-zealous fingers of a journalist, but a concise term used by author John Grue in a 2017 report for the Journal of Fluid Mechanics and published by the Cambridge University Press. The newly discovered phenomenon found that the waves move in their disruptive manner due to the geographical characteristics of the fjord, reaching up to 1.5 metres in height in a ‘wave period’ of 30-60 seconds..

Unlike normal ocean waves which are generated by wind or tides, tsunamis occur when large volumes of water are displaced. This type of wave is typically generated by earthquakes or rock slides. At Flaskebekk, however, the ‘mini-tsunamis’ seem to be caused by large ships passing areas of the fjord where there are significant variations in sea depth. A long wave entering shallow water will be reduced in length but will grow in height, compounding its impact when meeting the shoreline.

To better know what they were dealing with the community employed the services of Miros, a local technology company that specialises in measuring the ocean surface, providing sensors for environmental monitoring to the global offshore and maritime industries. Miros is measuring the wave height, wavelength and wave period of these ‘mini-tsunamis’ in order to better understand their causes and impacts. To measure these parameters Miros has installed its WaveWeather solution on the shore at Flaskebekk.

WaveWeather consists of a highly accurate radar-based wave measurement sensor, The Miros RangeFinder (SM-140), paired with multi parameter meteorological instrumentation. The solution not only measures real-time wave data and water level, but also provides wind speed, wind direction, temperature and more.

Dry mounting ensures that equipment is not impacted by the corrosive saltwater environment, the force of waves, floating objects or biofouling, and Miros CEO Andreas Brekke commented:

“We hope our contribution will help build a better understanding of these curious ‘mini-tsunamis’. This is a good example of how sophisticated technologies developed for major industrial applications can also be applied to the benefit of local communities.”

Photo: The picture postcard buildings that line the Oslofjord are under threat from the unusual wave forms.