Tuesday, 4 February 2014

High Tides in Jersey: Answers from Correspondents

I'm grateful to my correspondent Adam Gardiner, who has ably furnished me with an explanation of how the tidal flow operates around Jersey's South coast, and also for Nicolas Jouault for some enlightenment on Facebook, and finally Sarah Ferguson for supplying a personal anecdote about the result of high tides.
Adam Gardiner on Tidal Flows
The latest is due to three factors. Spring tide + gale force southerly winds + a deep low pressure.
Tides are calculated on a year-on-year basis by the Hydrographic Office.
http://www.ukho.gov.uk/Pages/Home.aspx  which is annexed to the Admiralty.
It is a bit crucial for warships to know what depth of water is under their keel at any state of the tide! They are accurate only in the sense that they represent predicted mean level given a flat calm. If the sea has a swell then the depth of water under the keel will vary. The tide (both high and low) figures are ± from a measured mean of zero. Tides follow the cycles of the moon etc (as an astronomer you know all about this I am sure).
The problem is that in Jersey waters, being relatively shallow, the wind gets behind the swell and pushes it onshore, Like tea slopping in a saucer. It is recognised that sea levels are rising annually but the amount is very small - globally around 3mm per year. http://en.wikipedia.org/wiki/Current_sea_level_rise
If you have even a modest 1m swell that rise (as present) is a fairly insignificant factor. Even 100 years down the line if the sea level rises at the same rate, it will rise by 0.3 metre - or roughly a foot. The main issue for Jersey is that when you get the southerly winds and spring tides occurring together as we have experienced over the last 2 weeks, the swell it creates slops over the current seawalls. The water cannot drain away as its outflow is back into the sea. Soon as tide starts to drop it disappears very quickly.
We need to dissipate the swell and unfortunately that requires some unsightly civil engineering - like those concrete hexagons that encircle the reclaimed Waterfront area. I can see the 'shoreline groups' and particularly the St. Aubin Nimbys going for that! Raising the seawall will help - but then again it would need to be sufficient to contain the breaking waves...and by that I mean by several meters. No more view of the bays. Not a good solution. There is a further downside  is that the force of the waves on a solid stricture like a seawall is such that it will likely 'break' it and 'scour' undermine it. It's where those hexagons are different. A huge surface area contained in a relatively small space. It works - but ugly.
Flooding does appear to be happening with more regularity and I feel that the States have some sort of duty to examine what is available to them to minimise future risks.
The waterfront reclamation and its effect of tidal flow could well be a factor. The Severn bore is in part an example of how shape can effect the way water behaves given the right conditions. The Waterfront development has effectively constricted the width of the bay. I would say that it must have some sort of influence. The Venturi Effect needs to be considered. http://en.wikipedia.org/wiki/Venturi_effect
It's not just the high tides (spring tides) alone but that of low pressure, and particularly a strong southerly wind, all coinciding. It is the combination that causes us the problem. A high wind on a neap tide (or even ay low tide) will barely cause waves to reach our seawalls. Wind alone is no indicator. In the Great Storm of 1986 we did not experiencing flooding as it did not coincide with a spring tide. Had it have done we might have been in even more trouble!
Nicolas Jouault on Tidal Flows
A deep low pressure can push the tide higher around a metre higher, although the storm of 87 on a neap tide pushed it up well over a metre. The reverse occurs with a high (greater) pressure making the tide not as high as predicted but going lower a bonus for low water fishermen. A deep low pressure in the Atlantic like we have experienced this year is not unusual and these create the swell (like dropping a pebble in a still pool and the rings pulsate out). Because we are in the northern hemisphere the winds rotate around a low anti clockwise.
The wind/swell/ and springtide combination are rare and the biggest and most damaging one in recent times was I think in 2005 with the tide a little over 10 metres and an exceptional swell and some gale force winds combined, had the same conditions occurred with the recent tides a considerable part of St Ouen's bay would have flooded and walls and buildings would have been severely tested.
Some predictions give a 30 cm rise in sea levels in the coming decades so the possibility of extreme/severe coastal encroachment is very likely in say over twenty years or so if the conditions combine. Couple this with increased building on the island the possibility of flooding due to heavy rain at these times will also increase.
And (I hope she won't mind), an anecdote to finish on from Senator Sarah Ferguson:
In the late 70s/ early 80s or so the high tides broke the sea wall near Bel Royal and Victoria Avenue was flooded almost back to La Motte Ford. It delivered the coup de grace to my ancient VW golf!

1 comment:

James said...

Not all Volkswagens are the same...

My wife had a 15-odd year-old Polo she was driving home from Gloucester to Cheltenham as the waters rose during the July 2007 floods. The door seals were less than tight, and water did start to seep into the footwell: meanwhile the exhaust pipe was blowing bubbles in the water on the road. But while all the expensive BMWs and Mercs were stalling and being pulled over to the side of the road, the Polo just kept going... and got her home.

But that was a real flood, not a manufactured news event (when you lose mains water for three weeks, the army has to be called in to ensure the provision of drinking water for 500,000 people, and the floodwaters also come within 9 inches of taking out the main electricity substation for a third of a million people as well - well, you can imagine that we'd regard a few waves over the sea wall as pretty trivial...)