Flash floods

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  • Flash floods
    27.07.2007 14:06

    Britain's chaotic summer continues, with more rain on the way before the month is out. However, there is a suggestion of some drier weather on the way as we head into August; more on that in our next story Signs of Summer?.

    Meanwhile, it’s becoming clear that all sorts of rainfall records are going to be broken. Over the three months up until July 23rd, on average around 387mm (15.2in) of rain has fallen over England and Wales, which is almost double the average of 187mm (7.3ins). The first half of the summer, if we may divide it up, is looking like the wettest since records began in 1766. The last big event was 1789 when almost 350mm (13.8ins) fell. Interestingly, those same records show that the month of July has been following an overall drying trend, one of the more remarkable and consistent trends of the last century and a half. That is, until now.

    The reason for the huge amount of rainfall is the position of the jet stream, the belt of strong, high-altitude winds that normally, or at least often, steer low pressure north of the UK, during the summer. By moving northwards, the jet stream usually allows high pressure to build from the south-west to bring drier, more settled conditions. The UK is caught in a southern loop of the jet stream, and as a consequence depressions are being steered across us, and slowing as they do so. The rain is, therefore, not only frequent and heavy but unusually persistent for July. Conditions have remained this way for such a long time that the ground has become saturated. This means that any new rainfall cannot be absorbed by the ground and hence it becomes what is known as "effective rainfall".

    Effective rainfall produces overland flow that pours straight into rivers and streams, and with heavy downpours it can result in flash floods. This is a term that is much in vogue at the moment, but flash floods are distinguished from "regular" floods by emerging in less than 6 hours. Flash floods are extremely dangerous due to their sudden nature, which can catch people unaware. This is particularly true when a distant deluge, such as a mountain thunderstorm, causes a wall of water to pour down a valley, wadi or gulch, where it may well be sunny. 

    Another problem with flash flooding is that it can cause landslides. Sulawesi in Indonesia has suffered a recent landslide as a result of days of heavy downpours, bringing about at least 40 fatalities and displacing over 20,000 people from their homes.  It’s a similar story in Sudan, where rain and flash floods have killed 59 people and damaged over 35,000 homes.

    While parts of the world have been suffering from intense rainfall and some of the worst flooding in decades, the south-east of Europe has been experiencing the opposite problem: that of prolonged extreme heat. Unusually high temperatures of over 40C (104F) have been affecting parts of Greece, Turkey and the Balkans, with a high of 45C (113F) on Monday in Bulgaria. This has resulted in more than 30 deaths, widespread forest fires and scorched harvests.

    By: Claire Austin