Tidal Surge

Sunday 10th July 2011

Tidal Surge


Following Tim's fascinating description of the tidal surge that came up the Fowey and Lerryn Rivers, it is interesting to see the video clip of this same surge moving up the Yealm River just to the East of Plymouth.  Tim knows better than many about the "seiche" which surges up the Yealm when certain weather conditions affect the incoming tide. The seiche is a wave of variable height which is a wind and pressure event (i.e. it is not a function of normal tidal processes).  Click on the button to see this seiche moving up the Yealm on Monday 27th June.

It is possible that the strange events of Monday, 27th June were caused as a result of a landslide on the continental shelf 200 miles west of the Cornish coast.  Dr Martin Davidson, of the University of Plymouth, said the wave was probably caused by a sand or mud slide at sea. The tidal surge was witnessed all along the coast from Cornwall to Hampshire.

There were reports that people's hair stood on end when this tidal surge occurred.  There is a possibility that the static electricity was caused by the release of radon gas because of the stresses that build up when there is movement deep in the earth.  This radioactive gas ionises the air.

Another theory is that very old rocks that contain quartz can produce an electric charge if they are squeezed together by an earth movement such as might have occurred under the sea, an effect known as the "Piezoelectric effect".

Another possible cause for the tidal surge was put forward by Dr Simon Boxall, an oceanographer, who thinks it might have been a "seiche", a freak wave which can be caused by an area of very high or low pressure crossing an area of water, the effect of which can sometimes be seen on the River Yealm when certain weather conditions prevail.  He suggests that the static that built up had nothing to do with the wave but rather was caused by the low pressure weather system.  Southern England was hit by a number of powerful thunderstorms later that day and the air would have been charged with static.

Whatever the cause, it was an unusual event, and we are grateful to Tim for his description of the tidal surge in Lerryn.

Click here to read the "This is Cornwall" article on the mini tsunami.