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.
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.
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).
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 😉
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.
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.
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.