The causes of coastal flooding, UK January 2014

The approximate contributions to coastal flooding, SW England, early January 2014 are shown in Figure 1. Any contribution from sea level rise over the last 100 years is minuscule compared with the other factors.

Figure 1 Approximate contributions to coastal flooding, SW England, early January 2014. Eustatic sea level is that component of sea level rise that may be attributed to melting glaciers and warming oceans. Image from Poleshift.

In their Final Briefing report on the 2014 winter storms, the UK Met Office provide a good description of tide, wave and storm surge factors in the main body of the report. However, in the summary, which is the part most read, they chose to highlight the possible contribution from rising sea levels using data that appears to have been fabricated. I am still awaiting a response from the Met Office explaining the provenance of their sea level data and an explanation of why this was highlighted to the exclusion of the real causes of flooding.

Note there are two versions of the Final Briefing at large. The original is here and the version with amended comments on sea level is here.

Tide

I have used Newlyn in Cornwall as an example of the tidal contribution in SW England. Figure 2 shows an exceptional high tide of 5.4 m in the evening and low tide 0f 0.7 m at Newlyn on 5th January 2014. I have taken the difference between the two (4.7 m) to illustrate the tidal contribution relative to a low tide baseline. See Figure 4 for an illustration of how tide and storm surge may combine to cause flooding.

Figure 2 The tides at Newlyn Cornwall, 5th January 2014 from this source.

Storm surge

A storm surge has two components. One is barometric, due to low pressure in the eye of the storm. The second is a wind driven wave that is much larger than the barometric component (Figure 3).

Barometric

A report from the National Oceanography Centre says that 1mb pressure change can result in a change on water level of 1 cm.  It also says that average barometric pressure along the South coast of England is about 1016mb. The eye of the storm of 5th January had pressure below 950mb. Using a pressure difference of 66mb I have allocated 0.66 m to the pressure component of the storm surge at the eye.

Figure 3 Illustration of a storm surge showing pressure (barometric) and wind driven wave components. From Wikipedia.

Figure 4 Cartoon from the UK Met Office illustrating how tide and storm surge may combine to cause coastal flooding. I’d be interested in comments on this cartoon 😉

Wave

Assessing the wave component is more tricky. All I have to go on is the Met Office Final Analysis report where the main body is actually quite good in parts, which says this:

Throughout the development of the storm that affected the UK on 5th, 6th and 7th January, the Met Office ocean and wave forecast models were giving very useful guidance. On the global scale, significant wave heights in excess of 16m were predicted to the South west of the UK, consistent with other estimates of wave heights exceeding 15 m (50ft) (Figure 6, left panel). Higher resolution forecasts using the UK 4 km model showed that these waves would reach UK shores as a strong, very long period swell.

Absent other guidance I have allocated 10 m for the wave component. This number may be in significant error.

The storm surge was of course linked to a very large storm. The Final Briefing said this:

The combination of significant wave height and peak period is likely to mark out the storm as a one in 5-10 year event in the southwest of the UK, based on experience of waves over the last 30 years.

Sea level

Changes in global sea level with time are by comparison with the foregoing, minuscule. They are also broken down into two components. That due to the rise or fall of the land called the isostatic component and that caused by the change in the volume of ocean water called the eustatic component. This is what most people think about when glaciers melt.  The Newlyn tide gauge has shown a total sea level rise of 18 cm in the period 1910 to 2010 (Figure 5). But the land is going down at 1.1 mm / year hence over 100 years 11 cm is due to isostasy and 7 cm is due to the eustatic change in sea level that may be attributed to climate change, most of which is likely to be natural.

Figure 5 Map and chart from Roger Andrews. The Newlyn tide gauge record is from SW England. The blue and red lines show the Met Office version of future sea level rise as published in their penultimate Final Briefing.

Conclusion

Coastal flooding of SW England, early January 2014 was caused by a storm surge linked to a large storm combined with exceptional high tides. The contribution from rising sea levels was negligible.

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6 Responses to The causes of coastal flooding, UK January 2014

  1. Euan,

    storm surge itself was enhanced by climate change,

    best,

    Alex

    • Euan Mearns says:

      Alex, that is your opinion which you should not state as fact unless you have evidence to back it up. The Met Office says this:

      As yet, there is no definitive answer on the possible contribution of climate change to the recent storminess, rainfall amounts and the consequent flooding. This is in part due to the highly variable nature of UK weather and climate.

      and this:

      The combination of significant wave height and peak period is likely to mark out the storm as a one in 5-10 year event in the southwest of the UK, based on experience of waves over the last 30 years.

      • Hi Euan,

        yes, I have read that as well. But I think that we “only” don’t know the exact contribution of climate change to the storm surge. At the same time, if would be strange if *any* of the weather events were not affected by enhanced greenhouse effect, since all the weather patterns are now created under warmer atmospheric conditions (stronger storms, more energy to fuel them, stronger winds during the storm, etc.)

        But I agree that this is uncertain, however rising sea level is one of the least adabtable (and stoppable) effect of climate change in the long-term. Simply people cannot live in water. Even if CO2 concentration were to stabilize or decline, sea level rise will continue for decades after that.

        Best,

        Alex

  2. Willem Post says:

    Euan,

    That is similar to what happened in the Netherlands in 1953; I was there!!

    Strong winds from the southwest, bottled up melt and rain water from the various rivers flowing via Zeeland, combined with a very high tide, overcame the dikes, drowned 2,000 people, made about 200,000 homeless; all this just after WW II.

    Absolutely no one blamed it on rising sea level, or global warming, as a similar flood had occurred in the 12th century, with minor ones in between.

    The Dutch embarked on the 55-year DELTA Plan, completed about 10 years ago, which is designed for a 10- thousand year flood. Hello!!

    The UK has been underinvesting in all types of infrastructures for decades. It is time to get going.

  3. Roger Andrews says:

    Euan: You’re still “awaiting a response from the Met Office explaining the provenance of their sea level data and an explanation of why this was highlighted to the exclusion of the real causes of flooding.”

    Well, they have more or less already given the provenance. It was the UKCP09 sea level report, Table 2 of which shows 11.4 – 16.0 cm of sea level rise between 1990 and 2030 for London, close to the 11 cm and 16 cm for the English Channel listed in the 2014 report. The original version of 2014 report, however, failed to mention the 1990 start date, so everyone naturally assumed the increase was relative to the present.

    In a comment on mygardenpond Richard Betts then lowered the numbers to 5-7cm, which are more or less correct relative to the UKCP09 report, and the Met Office issued a “corrected” version of the 2014 report to clear the matter up. But the revised wording (“a further overall 11-16cm of sea level rise is likely by 2030, relative to 1990”) can still be interpreted to mean 11-16cm between now and 2030 depending on how one interprets “further”. What it should have said was “a further overall 5-7cm of sea level rise is likely between now and 2030”.

    However, a 5-7cm or even an 11-16cm sea level rise is totally insignificant in the context of 50-foot waves, so why was sea level rise even mentioned, leave alone highlighted, in the Met Office report? Because it’s the only variable in the equation that can be in any way linked to man-made climate change. Without it the Met Office would have had nothing to talk about.

    I’ve summarized the Met Office’s fixation on sea level in the graphic below. I hope you don’t mind me playing games with your pie chart but I think the results are instructive. 🙂

    http://oi60.tinypic.com/35n6azq.jpg

  4. Hi Euan,

    I like the pie chart. It gives a perspective on the US policy to throw Appalachian coal miners out of work to try to reduce the purple sliver.

    Dave

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