10. The Risk of Tidal Flooding in London
TJ Marsh

Number of times per year when the Thames Barrier was closed to avoid a risk of flooding.

Specific Indicator
The Thames Barrier is closed when high water levels at Southend and the flow of the River Thames at Teddington Weir (the tidal limit of the Thames) reach critical levels. The specific indicator is the number of times per year that the Thames Barrier is closed to avert a risk of flooding.
[Source:Environmant Agnecy, Thames Barrier, Eastmoor St., Charlton, London SE7 8LX]

Relevance
The Thames Barrier was commissioned in 1982 and cost ,530 million. It was designed to provide protection for London from tidal flooding until 2030 (and, by raising the gates, beyond). Over 150 km2 of London lies below high tide level and the homes of 750,000 Londoners are at risk from a major storm surge. Flooding would result in immense disruption to the capital=s commercial activities and could cause direct damage equivalent to around ,20 billion, threatening London=s future as an international centre for trade and commerce.

Sensitivity to climatic and other factors
Historical records of rising tide levels in London reflect the fact that SE England is tilting downwards at around 30 cm a century, and that settlements have narrowed the river - the width of the Thames at Westminster is now about one-third of its width in Roman times.

Currently, the major flood threat to central London is from storm surges - when meteorological conditions (primarily atmospheric pressure and wind) exaggerate tidal peaks. Global warming and the resulting rise in sea levels (involving both thermal expansion and contributions from ice melt) will increase this risk. Climate change may also increase the frequency of synoptic patterns which give rise to dangerous storm surges.

Decisions on whether to close the Barrier also take into account the amount of water flowing in the River Thames. Thus, changes in rainfall and evaporation which alter the flow of the Thames (particularly during the winter) may also affect the number of closures of the Barrier. Any future changes in the operating rules could also influence closure frequency.

Change over Time
Historical references to tidal flooding in London extend back at least until the eleventh century. Pepys refers to >all of Whitehall having been drowned= in 1663. A continuous record of high tides at London Bridge is available from 1780, showing that the level of the highest tides (relative to the land) has been rising steadily over the centuries, totalling over 1.5 metres. About 40% of this rise is attributed to the land sinking.

However, the peak level registered during the extremely damaging tidal surge in 1953 has not been closely approached during the period since the Barrier was constructed. Nonetheless, the tendency over the last 17  years has been for the closures to become more frequent. Closures over the 1993-99 period greatly exceed those for the preceding 10 years. The 1990s were characterised by significant year-on-year variability. Nine closures were required in 1993 but none in 1997 when water levels in the Thames were low following prolonged drought - high tides which would normally have triggered a closure required no action. The six closures during 1999 all occurred during December.

Because the Thames River Barrier is now subject to different operating rules, it may be less useful as an indicator. The barrier is now closed to retain water in the Thames River as well as to lessen the risk of flooding. (It was closed on 9 successive tides at the start of 2003.) Thus, the number of closures has increased greatly in recent years. This indicator would only be useful if it were possible to distinguish the number of closures made specifically to lessen flood risk. The EA officer at the barrier would be able to assess what is possible.