A barrier to understanding?

People don’t seem to embrace global measures of temperature rise (~0.2ºC/decade) or sea level rise (> 3mm/yr) very strongly. They much prefer more iconic signs – The National Park formerly-known-as-Glacier, No-snows of Kilimanjaro, Frost Fairs on the Thames etc. As has been discussed here on many occasions, any single example often has any number of complicating factors, but seen as part of a pattern (Kilimanjaro as an example of the other receding tropical glaciers), they can be useful for making a general point. However, the use of an icon as an example of change runs into difficulty if it is then interpreted to be proof of that change.

With respect to sea level, the Thames Barrier is a concrete example that has been frequently raised.

The trends in its ‘raising’ have been linked to increasing sea levels and storm surges. But how often is it being raised? why? and does it give us any real insight into sea level rises on a wider basis? Looking into it, I was fortunate to get an exceptionally comprehensive set of data on the closings and reasons for them from Anthony Hammond at the Environment Agency in the UK. The results are interesting, but complicated….

The background to the Thames barrier is available here, but suffice to say it started operating in 1983, and is raised whenever there is a forecast danger of high tides, river flow and storm surges combining to threaten London. It has been raised over 100 times operationally since being constructed.

Lest you think this isn’t really a problem, I recall a friend’s car being parked near the boathouses in Putney. After a particularly high tide (this would have been in 1992 maybe), she returned to the car to find it filled with river water. No amount of cleaning ever got rid of the rather pungent odour. Flooding along the riverwalk in Chiswick near where I lived at the time, is frequent. And of course, the city has been flooded many times in the past – most recently in 1953 when over 300 people died.

The raw data on the closings (per winter season – the 2007 number is for Mar 2006 to Mar 2007) is seen here:

It is clear there has been a strong upswing in closings over time. The last year alone there were almost 3 times as many closing as during the first 5 years of operation put together. The three-year running mean is possibly a little clearer, showing two definite periods of more frequent closings, 1992 to 1995 and 2002 to the present.

Is this a sign of increased sea level, increased storminess, increased river flow, or changes in river management policy? As always, local factors in short records are important. Over the two and half decades of barrier operation, understanding of the Thames river hydrology has grown and models are now more accurate than they used to be, allowing for more precision in decisions to raise the barrier. The decisions depend on three main factors, the river flow at Teddington (which is where the first weir is), the forecast high tide and the (more uncertain) accompanying surge. Thus if the river flow is strong, an unexceptional high tide could cause problems, while even an exceptional tide might not if the river is particularly low. In the data, closings are distinguished by whether they are due to tidal issues, or to the combined effect of tides and high river flow (fluvial), but there is not necessarily a clean distinction.

In the 25 years of operation, global sea levels have risen around 5 cm, but it’s not at all clear that such a change would be registered in the very noisy extreme surges. The two peak years for closure (2001 and 2003) had very strong river flows, making the tidal threshold for closure much lower than normal.

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