Weather effects on tides

[Peter Taylor]

This and my next post in this thread were prompted by remarks by Andy Dingle in another thread:
http://www.swallowyachtsassociation.org/smf/index.php/topic,1415.msg10335.html#msg10335

The tide tables are calculated for "average" air pressure. When the air pressure is higher than average the sea surface is pressed down, when it is lower than average the sea surface rises higher.  The change is 1 cm higher/lower for each 1 mb that the air pressure is below/above average pressure. This is called the "inverse barometer" effect.  Sometimes the tide tables tell you what was used for average pressure, in Southampton they say 1020mb (although I would have expected 1016mb to be more likely). Often the tide tables do not say, in which case use 1013 mb which is considered the "standard" air pressure. So if the air pressure is 1000mb you would expect the water level to be 13cm higher than shown in the tide table (or for Southampton 16cm to 20cm).  In normal conditions (no storm surge) applying these rules should give a reasonable estimate of air pressure effects.

In stormy weather the strong winds may result in additional changes in water level of similar order to or greater than the inverse barometer effect. Forecasting the resulting total "storm surge" is difficult and requires a computer model such as that described at the National Tides and Sea Level Facility (NTSLF) web site at:
http://www.ntslf.org/storm-surges/storm-surge-model.  This model is what is used by the Environment Agency for flood warnings, but the EA normally only issues these when they apply to the next high tide.  Using the NTSLF site you can get advanced warning of possible flood conditions one or two days ahead.

The NTSLF surge forecasts can only be accurate if the weather forecasts are accurate.  Errors may show up as changes in the timing of the surge. Tides are simply water waves with a long wavelength. The rate at which the peak of the tidal water wave travels along the coast depends on the depth. Thus the peak of the tide reaches Southampton before it reaches Dover and the time taken to travel between the two depends on the average depth of the English Channel.  If there is a storm surge the water will be deeper than normal, and this causes the tide to travel faster and arrive earlier. Thus storm surges are associated both with changes in the timing of the tides, and with changes in the depth of water at high tide, and each change effects the other. This is a non-linear problem and difficult to forecast.

Because of the shape of the North Sea, strong northerly winds can produce significant storm surges in its southern sector. Sometimes these propagate through the Straits of Dover resulting in a surge along the south coast some time after the storm has passed.  Similar events can happen in other coastal regions. It is a weird feeling when, on a calm day with average air pressure, you see the water level mysteriously rising to a level, or at a time, which does not conform to the Tide Tables! I'll give an example in my next post.

[Peter Taylor]

Following on from my previous post here is an example of a storm surge which was reasonably well predicted by the NTSLF model but which gave rise to warnings of flooding which mostly were not fulfilled. I'd previously studied this storm surge event and this thread gives me an excuse to publish it here! Any views expressed are my own, I don't know what the EA, NTSLF or National Oceanography Centre concluded about the event!

Over 13-14 January 2017 a storm surge in the North Sea led to the Environment Agency (EA) issuing severe flood warnings and the evacuation of people from their homes.  Figure 1 (attached) is map of the storm warnings and figure 2 a set of the storm surge residuals (departures of water level from the astronomical tide) for places down the North Sea coast and along the south coast as forecast by NTSLF ( http://www.ntslf.org/storm-surges/surge-forecast ). The plots are all on the same scale but have been offset vertically by varying amounts for clarity. The zero axis (no departure from astronomical tide) is marked with the location name.

What the plots show is that, for example, for Immingham a storm surge of 2m was expected but that it was not forecast for to coincide with high tide (shown by the circles). For the time of high tide the forecast surge was less than 0.5m. The successive Met Office weather forecasts gave different timings for the surge onset (shown by the scatter of the coloured lines) but the forecast time of peak surge remained consistent.

The forecast surge was larger further down the coast reaching 3m at Sheerness, but at most places the peak surge did not coincide with the times of high tide. Only at Cromer and Lowestoft was the forecast surge still large when the tide would be high;  about 1.2m and 1.5m respectively.  In general the flooding which the EA warned about did not occur and this was blamed on a "change in the winds effecting the surge timing". In reality the surge was consistently forecast to miss the high water period and hence avoid flooding at most locations.  However, given such large forecast surges, I suspect the EA were playing safe in issuing the warnings. Surges of this magnitude are rare so there is little experience of how well the model forecasts them. The timing of the forecast surge would only have to be out by 2 or 3 hours and one would have had disastrous flooding.

On the south coast their was a surge directly associated with the storm during the night of 11th to 12th. However the North Sea surge surge associated with the northerly winds following the storm occurred on the night of 13th to 14th. It propagated through the Dover Straits and can be seen traveling westward along the south coast in Figure 2. It arrived at Portsmouth around midnight on the 13th slightly after the time of peak high tide. The NTSLF forecast of this surge is shown in figure 3.  A surge of about 0.4m was forecast to occur about 2 to 3 hours after the time of high tide, observations at the time suggested the forecast was accurate. Figure 4 shows the astronomical tide at Portsmouth (red) and the tide gauge observations (blue crosses). The effect of the surge was to boost the peak water level to about 5.3m. 

Although the surge reaching the south coast was much smaller than when it was in the North Sea, even a small surge can do damage if it floods your house!  Had it arrived a couple of hours earlier the tide would have exceeded 5.4m which is the trigger for implementing the Community Flood Plan in the area of Southampton where I live.  The EA did not issue a flood warning for Southampton and the NTSLF model was suggesting that we would escape flooding. Nevertheless, I slept on Seatern over the night of 13 to 14th January and, around midnight, kept checking the level of her hull compared to the decking of my jetty until I was finally sure that the water level was falling and no one would be flooded!

[Andy Dingle]

Peter. Thanks again for the very informative thread.

Please do not think my previous comment was in any way less than respectful to you and your meteorological colleagues! And I do apologise if it were taken as such.
I am hugely interested in this subject and not just that it affects my (and all of our) daily sailing interests, particularly those of us who sail in shoal waters.

It occurred to me that we although we talk of 'storm surges' and hence dangers of flooding, the same principles surely apply on an every day basis with air pressure and wind affecting the heights of tides. A tide height at a given location and time would therefore vary considerably from predicted - the effect of wind 'pushing' or 'holding back' the tide, and the air pressure depressing or raising the height as well. And in 'real life' this often proves to be the case as I'm sure we are all aware, people often talk of the 'tide not making' today etc.

A lot of differing factors that surely should be taken into consideration when passage planning, making it appear to me an 'inexact science' (!!).
For example - we do our calculations of tidal heights, which are based on predictions made a year or more previously. I can work out that I should be able to make into a drying harbour, or cross a sand bar at a certain time, with a predictable depth below me.
But this isn't the case surely? I should be considering that there (perhaps) has been a blocking high of say, 1040 hpa (nice settled weather so we are out sailing), but depressing the tide height, but it's northerly anticyclonic wind is pushing the 'tidal wave' down the north sea (in my case), earlier than predicted. So I try and make an entrance to find there is less water, and this shallower water is ebbing sooner than I would want.
The opposite applying to a low pressure system, which will lift the predicted height but with it's (often stronger) southerly cyclonic wind holding back the north-to-south 'tidal wave'. (Again, the guys in the club are saying 'tide didn't make today.. And I'm out there in full view of everyone hastily winching up my centreboard !?').
I would think as well that these factors also will have a not inconsiderable effect upon tidal currents.. which I am sure we all have noticed, when the tide doesn't 'turn' when you hoped it would?

I wonder if there is a web site that can give the tidal height 'adjustments' given the current barometric pressures and wind direction and speeds over our country?

Am I right in my thinking? Or should I just shut up and get out more!

But thank goodness for a variable depth centre board eh?!

I saw this on the excellent 'visit my harbour' website which I found interesting.

http://www.visitmyharbour.com/articles/3145/the-tidal-wave-as-it-travels-around-the-coasts-of-the-uk


Regards

[Peter Taylor]

Hi Andy, don't worry I was not assuming any disrespect (although as a scientist you get used to being told you are wrong 'cause often you are)!

You are correct in that the meteorological effects on tides occur all the time, not just when there is a storm. The air pressure effect will operate everywhere in a similar manner, except that, as I said,  a different average pressure may have been used for the tide tables for different places.  The reason air pressure is often neglected is that for high pressure it will normally only lower the depth by 20cm or less. The major changes due to pressure are increases in water level during storms and in those conditions you shouldn't be in shallow water anyway!

The effect of wind is much more variable from one place to another. In an enclosed channel the wind will tend to cause the water to slope upwards in the direction it is blowing. For example a strong northerly wind will cause increased tidal heights in the southern North Sea. If there is a strong northerly you do NOT leave your car in the Blakeney Harbour car park! However elsewhere the effect of the wind is different from what one might expect and you need "local knowledge". 

For major ports such as Southampton, the port tide tables (published by the port operators, ABP for Southampton) will include a "Meteorological Correction table".  For Southampton it shows that for a SE wind blowing up Southampton Water the water levels are actually depressed, not increased! The opposite of what one might expect. 

However, I'm suspicious of port tide tables. I suspect that the Port Hydrographers are more worried about effects that lower the water depth than ones that increase it.  Container ships coming into Southampton sometimes have less than 1m water under their keel. It is very embarrassing if they run aground. In contrast for those of us who live by the water, it's the times when the tide level is increased, with the possibility of flooding, that are most important!

Peter

[SteveWD]

Fascinating stuff. And then there's the fact that after a surge and strong winds and tides the shallow entrance channel you are approaching (e.g. Christchurch Run) may be deeper or shallower or in a different place than before the blow, just to complicate matters even more.

[Andy Dingle]

I've been re studying the dark arts of secondary port calculations - as one does when one's life is as dull as mine.
I was very interested to read that one of PBO's contributors and RYA Instructor 'Sticky' Stapylton in his work on secondary ports points out this phenomenon and gives a table of adjustments to be made to the Secondary Port tidal graph basically, exactly as Peter says. Given an 'average' of 1013 mb, for low pressure (below 1013 mb) add 10 cm, per 10 mb and conversely for high pressure (above 1013 mb) deduct the same. (www.sail.help.co.uk)
He states that barometric pressures, wind and geographical features can affect the depth of water by up to a meter and timings of HW or LW by as much as an hour difference - which is quite significant!
A useful table/graph to apply all the differentials can be downloaded from his web site.

All this is probably not a good chat-up discussion, but I find it strangely interesting ...!

Regards..

[Graham W]

This has nothing much to do with sailing but there's an interesting tidal effect in the Dee Estuary.  During spring tides and as long as the air pressure is low and the wind blows from the NW, there's a big tidal surge that floods the salt marshes on the Wirral's north bank.  As it doesn't happen very often, it causes rodent pandemonium - all the mice and voles that had previously had a nice dry home try to escape the mini-tsunami.  A lot of them climb up grass stems, where they are picked off by Hen harriers, Short-eared owls, Merlins and Kestrels.  Peregrines exploit the confusion by stooping on displaced ducks and waders.  It's quite a spectacle but can be disappointing if the air pressure and winds aren't right.  Peter's link to the surge forecast (in this case for Liverpool) will help to determine whether it's worth going to see or not.

In the photo below, the conditions were not quite right but when everything comes together, the salt marsh grasslands in the foreground are completely covered.  The car ferry in the background is the 'Duke of Lancaster'.  It has been mouldering in its current location on the Welsh side for nearly 40 years.