We all know how much damage Supertyphoon Yolanda (international: Haiyan) brought to the Philippines back in November 2013. For instance, devastating storm surges wreaked havoc in many places where Yolanda made landfall, most notably in the coastal areas of Samar and Leyte.
But one surprising aspect of Supertyphoon Yolanda that hasn’t been widely discussed is that it was powerful enough move numerous large boulders for several meters. Using a combination of satellite remote sensing, field analysis, and numerical modelling, a group of scientists from the University of Cologne, Germany and the University of the Philippines Diliman studied one such boulder field in Eastern Samar.
These are really large boulders, as we can see from the images above. The top image shows two boulders, labeled ESA 8 and ESA 9, weighing approximately 22 and 180 tons, respectively. The dimensions of ESA 9 is also shown; for scale you can refer to the person standing beside the boulders. Using high-resolution satellite images, they mapped out the pre- and post-Yolanda positions of the boulders.
Did the storm surges and waves from Yolanda move the boulders? Turns out, it may be a bit more complex than initially thought.
To answer this question, the researchers first estimated how much energy it would take to move the boulders, using the weight of the boulders and how far each one was moved. They then simulated the storm surges and waves using available weather data as input to numerical models (the fancy term for this approach is “hindcasting”).
So, what did they find out? Surprisingly, their simulations showed that the energy from the storm surges and waves produced by Yolanda was not enough to move the boulders. A plausible explanation was that there was some unaccounted phenomenon in the equations they used. The authors hypothesize that a phenomenon known as infragravity waves may be responsible. Here’s a nice explanation of what infragravity waves are.
This is significant since, in the past, boulder fields like those in Eastern Samar were usually associated with geologic events like tsunamis. Their research provides compelling evidence that, under the right circumstances, extreme weather events like Supertyphoon Yolanda can produce enough energy to move objects weighing hundreds of tons.
This is particularly important since preparation for natural hazards, most especially for extreme events like Yolanda, require an understanding of the processes that control these phenomena. While significant progress have been made in the past years, there is still much to be learned. Each new knowledge produced could mean more lives saved from future disasters.
This study is published in the scientific journal Earth Surface Dynamics.
Bricker, J.D., Takagi, H., Mas, E., Kure, S., Adriano, B., Yi, C. and Roeber, V., 2014. Spatial variation of damage due to storm surge and waves during Typhoon Haiyan in the Philippines. Journal of Japan Society of Civil Engineers, 70(2), pp.I_231-I_235.
May, S.M., Engel, M., Brill, D., Cuadra, C., Lagmay, A.M.F., Santiago, J., Suarez, J.K., Reyes, M. and Brückner, H., 2015. Block and boulder transport in eastern Samar (Philippines) during Supertyphoon Haiyan. Earth Surface Dynamics, 3(4), p.543.
About the photo: M/V Eva Jocelyn was one of the ships washed ashore in Tacloban, Leyte by Yolanda. It has since been converted into a shrine that commemorates the lives lost during the disaster.