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Natural Science Forum / Earth Science / Oceanography / November 2006Tidal Surges - criteria ?
What are the criteria for large tidal excesses over astronomical predicted tides for the area around the UK ? Deep depression in the North Atlantic, yes. But for low barometric pressure induced sea level raising would it not be the area of sea affected being a major factor. Comparing 2 scenarios 1/ central low of 960mBar but 990 mbar isobar of 100 mile diameter 2/ central low of 960mBar but 990 mbar isobar of 500 mile diameter Would not 2/ create much more sea uplift than 1/? , so much greater volume to potentially come ashore, then magnifying the height anomaly by shelving/funneling. Is the timing significant? or is the effect independent of whether the principal (nearest the UK ) surge formation depression is, locally to itself, passing at high tide or low tide. From BODC data the highest surge to the South Coast for years 1992 to 2002 occured 30 October 2002 with maximum excess (residual) at Portsmouth of 1.12 metres, but not near local high tide so no flooding consequnces. The N Atlantic synoptic charts for the preceeding time window shows a double coupled low , one over the Shetlands and the other SW Iceland with a huge sea area contained within the 992mBar isobar stretching 45 degrees of longitude in the line of the 2 lows and a more normal 10 degrees of latitude span. Anyone know of a meteorological+oceanographic analysis of the 1953, 31 January Canvey Island/Dutch inundation? Look for the simple reason first. Last year prolonged Atlantic westerly winds produced a HW nearly 1m above the predicted height here on the Tyne. (and presumably elsewhere in the North Sea) It seems that water piles up against the continent given time and wind pressure. There's nowhere for it to go! > It seems that water piles up against the continent given time and wind > pressure. There's nowhere for it to go! Someone told me the other day that sea level on the French side of the channel is a metre higher than on the English side, and that this was due to the "banking up against a corner" effect of the water washing in and out of the North Sea round past Belgium, Holland, and northern Germany. It sounds almost plausible, but a metre also sounds like quite a difference. Pete >> It seems that water piles up against the continent given time and wind >> pressure. There's nowhere for it to go! [quoted text clipped - 6 lines] > > Pete There can be a couple of metres difference in height between each end of the Gulf of Corryvreckan. Alisdair The difference in tide ranges on the English and French coasts is due to the coriolis effect (earth spin) on the water. As it comes up the Channel, the water tends to turn South and pile up on the French side. Eddie >> It seems that water piles up against the continent given time and wind >> pressure. There's nowhere for it to go! [quoted text clipped - 6 lines] > > Pete > The difference in tide ranges on the English and French coasts is due to the > coriolis effect (earth spin) on the water. As it comes up the Channel, the [quoted text clipped - 12 lines] > > > > Pete The consistent apparent 1.5m difference in height http://www.southamptonvts.co.uk/tidesandweather.asp between the Bramble and dock head over the length of Southampton Water is presumably due to lack of someone clearing seaweed from the Bramble tide gauge > The difference in tide ranges on the English and French coasts is due to > the coriolis effect (earth spin) on the water. As it comes up the Channel, > the water tends to turn South and pile up on the French side. So why doesn't it turn North and pile up on the English side on the way back out? > The difference in tide ranges on the English and French coasts is due to the > coriolis effect (earth spin) on the water. As it comes up the Channel, the [quoted text clipped - 10 lines] > > out of the North Sea round past Belgium, Holland, and northern Germany. It > > sounds almost plausible, but a metre also sounds like quite a difference. And that would explain the Dublin tidal range compared to the Liverpool one? Except that these ports have tides at the same time. Who knows it might even be the correct one but it certainly sounds more fishy than plausible. Let's face it there is a hell of a lot of stuff missing from the riddle. >> The difference in tide ranges on the English and French coasts is due to >> the coriolis effect (earth spin) on the water. As it comes up the [quoted text clipped - 6 lines] > fishy than plausible. Let's face it there is a hell of a lot of stuff > missing from the riddle. You guys are just waiting for me to mention the A-word, aren't you? > What are the criteria for large tidal excesses over astronomical predicted > tides for the area around the UK ? [quoted text clipped - 8 lines] > to potentially come ashore, then magnifying the height anomaly by > shelving/funneling. I don't think so. I think the uplift height is determined by the pressure difference (between actual and normal), while the size of the low pressure area would affect the duration of the surge rather than its height. Statically each mb pressure difference corresponds to a height excess of 1cm (based on the fact that one atmosphere (1000mb) corresponds to the pressure of a water column of about 10m). Shelving/funneling would affect both short and long duration surges of the same height roughly the same, I'd have thought. > Is the timing significant? or is the effect independent of whether the > principal (nearest the UK ) surge formation depression is, locally to > itself, passing at high tide or low tide. I think the surge wave and tide waves simply superimpose independently, so, particularly if it is of long duration, a surge can be thought of as a temporary increase in *average* sea level, and hence of chart datum, and thus the predicted tidal levels simply need to have the surge excursion added onto them. Obviously if the duration involved includes a spring HW (and particularly if the surge peak coincides with the time of HW), then you will get exceptional flooding. In the storm of Jan 2005, the flood tide rose some 2m above the level normally associated with spring highs. It's not just the direct static effect of low pressure which brings these things about, but where the Atlantic depressions cause gales to persist in the same direction for long enough, the force of the wind "pushing" the sea along in front of itself also contributes a separate surge of its own. > > What are the criteria for large tidal excesses over astronomical predicted > > tides for the area around the UK ? [quoted text clipped - 38 lines] > the same direction for long enough, the force of the wind "pushing" the > sea along in front of itself also contributes a separate surge of its own. I'm particularly interested in distance effects where the point of arrival on the coast is hundreds of miles away from any storm conditions. That 2000 surge i mentioned, the local pressure on the S coast was 1090 mbar and no wind/storm conditions when the surge peak arrived. I've not yet back-tracked that surge around the other UK ports' BODC gauge data to see if it went mainly through the Irish Sea or the England West Coast, W Ireland or all possible paths. In another high surge of 0.86m on 21 Feb 1993 the local air pressure was a very high and stable 1024 mBar and the closest track of the 976mB centre of the originating N Atlantic depression was 900 miles away . At least I now know that tidal surges can come through the Dover straits to affect as far as the Solent and beyond. For anyone interested, these plots of actual, over predicted, will show the pulse for the next day before moving on. Shows the effect of the recent Low that moved N of Scotland into the North Sea Whitby tide guage plot http://www.pol.ac.uk/ntslf/pltdata_tgi_ntslf_v2.php?code=Whitby&span=1 Dover http://www.pol.ac.uk/ntslf/pltdata_tgi_ntslf_v2.php?code=Dover&span=1 Portsmouth http://www.pol.ac.uk/ntslf/pltdata_tgi_ntslf_v2.php?code=Portsmouth&span=1 Bournmouth http://www.pol.ac.uk/ntslf/pltdata_tgi_ntslf_v2.php?code=Bournemouth&span=1 Weymouth http://www.pol.ac.uk/ntslf/pltdata_tgi_ntslf_v2.php?code=Weymouth&span=1 disappearing by Plymouth http://www.pol.ac.uk/ntslf/pltdata_tgi_ntslf_v2.php?code=Plymouth&span=1 I found some info on the 1953 great flood surge data http://www.floodlondon.com/floodtb.htm assuming correct as sources are not given "To make sense of this, take some of the figures for surge residuals. Using the storm of 1953 for comparison, we see the surge residual as a long drawn out hump, the highest peak occurring near the beginning and measuring 2.59 metres at Sheerness/Southend. This is high for a surge but by no means the highest on record. At the same location, a surge in 1894 reached 2.9 metres; one in 1905, 3 metres; one in 1921 3.35 metres and one in 1943 a massive 3.66 metres." 2.59m surge is not exceptional , it just coincided with high tide. I unpacked the Sheerness BODC data for 1990 to 2002 the details where the excess surge peaks were greater than 2 metres. So it is only a matter of chance that the 21 Feb 1993 surge of 2.91 didn't occur 3 to 4 hours earlier to add to the high tide of 5.5m Ref No, Date, Time, Predicted Height metres, Residual metres(Excess over predicted) 3408) 1999/02/05 11:45:00 3.945 2.032 4932) 1993/02/21 08:45:00 4.843 2.941 8297) 1990/12/12 16:00:00 3.469 2.079 corresponding nearest high tides Sheerness on those days 1999/02/05 15.40 , Ht 5.5m 1993/02/21 12.04, Ht 5.5m 1990/12/12 21.18, Ht 5.0m |
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