Supertides – again!

Flooding in Christchurch on 28th October 2015 at high tide
Flooding at Quay Road, Christchurch 09:45 (approx. 10 minutes after high water) 28th October 2015 (taken by Matthew Wadey). Air pressure ~1007mb, predicted tide: 1.9mCD

During 27th-28th October 2013, the so-called ‘St Jude Storm’ registered gusts of over 100mph at the Needles, Isle of Wight. Surprisingly, this storm caused flooding at nearby Yarmouth (surprising, because flooding coincided with small or ‘neap’ tides, almost unheard of for the UK), which raised the actual sea level to almost 1.4m above the predicted high water.

About a month ago (and earlier this year), the media reported upon extreme spring ‘Supertides’ when seasonal and inter-annual cycles of the sun-moon alignment and proximity to the earth’s surface allowed for exceptional high waters around our coast. Interestingly, this week we have again seen large spring tides – in most locations almost as large as those in late September, and in some locations (e.g. Portsmouth) the biggest over an 18 year period. However, the sea surface air pressure is lower this week (than it was a month ago) hence allowing the water surface to be higher than predicted due to the ‘inverted barometer effect’.

Today’s predicted tidal high water was 3.2mCD (at 10:26 GMT) at Yarmouth, Isle of Wight – almost 0.6m higher than the predicted (neap) tide during the morning of 28th October 2013, although as we know the storm raised the actual level then to almost 4mCD. However, if the biggest tide and biggest surge theoretically combined (fortunately at many locations this is of low probability or even physically impossible) this could have generated a very extreme sea level (approximately 1 in 10,000 years probability!).

Extent of flooding at Quay Road, Christchurch at approximately 09:45 on 28th October 2015 (taken by Matthew Wadey).