Wednesday, 6 October 2010

How high is high tide at Ramsgate?

With the various developments proposed and ongoing on the foreshore in Ramsgate, most particularly the proposed one on the harbour slipways, I have been looking at the various tidal flood risk assessments.

I spent some time today discussing the flood risk assessment for the slipways development with the chap who wrote it and as far as I can see there have been various errors made and problems relating to access of full and accurate information both about the development and the local tide statistics, by the person writing the flood risk assessment.

One thing that is petty simple here is that the development is to be built on three levels, undercroft, ground and first floor. Unfortunately it seems that the planning sheet showing the undercroft wasn’t passed on to the firm preparing the flood risk assessment.

Engineering hat now, so I would suggest just taking my word that people in the bar the ground floor may get wet feet, unless you have some patience.

OK for those who are following into the higher country of the mind here, I want you to think about just exactly how high, high is. Start with something big that is easy to think about, Mount Everest is at 29,002 feet or 8,840 metres high, you can check this if you want, but the question you may not have asked is higher than what?

OK if you said sea level, you are doing pretty well here, but the sea goes up and down so what sea level?

If you live in Ramsgate then you have probably have seen the levels marked next to the lock gate, between the inner and outer harbour so you may think that it is level that I am talking about.

It indicates that the harbour wall is about 6 metres above sea level. So you would assume that the top of Mount Everest is 8,834 metres above the harbour wall in Ramsgate.

I am afraid to say that you may be wrong the levels written on the harbour are in chart datum, but when in 1856, the Great Trigonometric Survey of British India established the height of Everest the surveyors used ordnance datum.

Ordinance datum is 2.58 metres (about eight foot six inches) higher than chart datum.

Architects use ordnance datum on their planning sheets and information about tides and sea levels is measured in chart datum, make a mistake about this and you may be in deep water.

This wasn’t the mistake that happened with any of the plans for developments on Ramsgate’s foreshore, but I think the confusion around it helped to cover some up.

Now to save confusion here I am going to take off my engineers hat and put on my sailors hat, so it’s time to say, “hello sailor.” If you didn’t think I could do this see http://thanetonline.blogspot.com/2007/12/friggin-in-rigging.html or http://www.michaelsbookshop.com/cryfromthesea/index.htm

All of the figures below are in chart datum.

The lowest indoor bit of the proposed development on the slipways is 6.18 metres above chart datum.

I think some of the confusion has come from, the figure 5.4m is the MHWS for Ramsgate, the flood risk assessment takes this to be about the highest astronomical tide, this actually stands for mean high water spring, i.e. the average height of a spring tide and not the highest astronomical height.

HAT - Highest astronomical tide is the one they should have used the HAT at Dover is 7.3m the difference for Ramsgate is 1.6 giving a HAT for Ramsgate of 5.7

The highest recent tidal surge locally was 14th Feb. 1989 at 10:00 hours this was 1.84 metres above normal high tide registered on the Dover tide gauge, as tidal surges are caused by low barometric pressure the level would be about the same for Ramsgate.

I would think it reasonable to assume that adding the HAT at Ramsgate to the surge would give the best idea of the highest possible spring and surge level here i.e. 7.54 metres.

We would be very unlucky to have a tide this high now, the tidal surge of 1953 was 8.22 m at Dover which works out at 6.62 for Ramsgate, this surge was combined with a severe storm, though fortunately not a spring tide.

In the 1953 tida1 surge 1,835 people were killed in the Netherlands and 307 were killed in the United Kingdom and about 250 people were killed at sea, many of them sailors.

A problem with the sea is that it has waves, I suppose the fact that it killed over 2.500 people in this part of the world during one storm, is a testament to this.

This means that anyone contemplating building on our foreshore needs to consider how high the waves here can get.

Several factors determine how high the waves will get in a storm, once again there are different ways of expressing the height of a wave, as I have my sailors hat on I will use the one called significant height, this is the height difference between the trough between the waves and the crest on the waves.

One factor is the strength (speed) of the wind, as a sailor I will express this in knots, nautical miles per hour, a nautical mile is a little bit longer than a statute mile.

In the days of sail sailors measured the speed of their ship with a line with equally spaced knots tied in it and a lump of wood on the end. One of them dropped the bit of wood over the side and counted how many knots passed through his hands, while another one timed this with a glass, something like a big egg timer with sand running through it.

Another factor is the duration of the storm, the longer the wind blows for the higher the waves, this has a saturation time after which the waves don’t get any bigger.

Another factor is the fetch of the sea, this is how far it is between the shores along the line of the wind direction.
There are other factors but I think we will stick to those, otherwise it is going to get scientific and didn’t wear a hat when I worked as a scientist.

As Michael Fish said there won’t be a hurricane so we certainly don’t need to worry about 11 metre waves on a worse case scenario 7.45 metre tide.

You will be glad to know that the depth of water mitigates the height of the waves.

The worst we have had in recent years was in 1953, because of the wind direction we were sheltered in Ramsgate, the waves were also mitigated here because the fetch was across the Straights so Dover and not along the Channel.

I would say about 7 metre waves on a 6.62 metre tide.

The environment agency say that new developments should be assumed to have a life of 100 years, the best predictions for the rise in sea level by 2110 are between 1 and 2 metres, they are working on just over a metre.

They are saying that by then we should be expecting static tides of about 8 metres above sea level here in Ramsgate and building now to accommodate for them.

Obviously there have to be compromises here a balance between the worst case scenario, which is a 2 metre sea level rise and 7.45 metre tide with 11 metre waves and what is likely to happen in reality.

Normally anyone producing a flood risk assessment will have considered the sea defences in front of the development and how far behind them the waves will be broken up enough to stop them damaging the building.

This isn’t an option with the slipways development as it is to be built on a small pier, so the waves will be under it as well as in front of it.
With the Pleasurama development there hasn’t even been a flood risk assessment, so no one has even bothered to consider the problem.

4 comments:

  1. seeing TDC are in the process of increasing the height of the sea wall at Margate due to the 1 in 100 year risk of flooding at the cost of a a couple of million (thankfully enviromental dept money not tdc's). It seems strange that they are accepting a shoddy flood risk assessment for the slipway development.

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  2. Very impressed by your last post, Michael. Wonderfully detailed.

    Speaking of rising levels, what impact do you imagine your post will have on the proposed building's insurance premiums?

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  3. 23.41 I don’t understand why Ramsgate wasn’t included in the SFRA, the whole of the eastern undercliff falls into an EA high risk zone. It could be that the council now stands to loose over £2m if they say that the Pleasurama development isn’t viable. They, the officers that is, have been put in this position because the cabinet elected not to determine the development agreement against the advice of a senior council officer.

    DH I am putting my thoughts together for my comments on the objection, glad you enjoyed the post, and sorry I never got to finish it properly, I am stuck trying to get an accurate figure for maximum wave heights in the outer basin of the harbour.

    Past experience suggests that the east pier will be breached, to a lesser or greater extent, in the next big storm as it has been before, so it is a bit of a variable when it comes to how much it reduces wave height.

    I have seen waves of about 1.5 metres there myself, and damage to the pontoons suggesting that wave heights must have been greater than that, with the east pier undamaged.

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  4. Seen sea water across the top of the cross wall before now. Can not remember the year, reckon in the last 40 approx'.

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