The Reporter
Issue 489, 24 March 2003
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Raindrops may keep falling, but that's not what causes the rivers to flood

 

Flooded fields

Widespread flood warnings seem to be a regular feature of our winter weather forecasts. We’ve become accustomed to the sight of towns and cities under water, of householders’ despair, submerged cars, and the messy aftermath of mud, gravel, sodden furniture and ruined carpets. University researchers have been finding out what’s to blame.

But where to lay the blame is not always clear: is it inadequate flood defences, climate change or a combination of the two? Or are floods just natural phenomena, which only seem more frequent because they’re more consistently reported in the news? Researchers at Leeds have been studying the figures and found that the reasons for this watery misery are not what they might seem.

Professor Stuart LaneSurely it’s an obvious cause and effect: it rains more, so it floods more. But the increase in flooding – which is real, not imagined – is not matched by an increase in rainfall. Professor of physical geography, Stuart Lane (pictured left) , is heading a major research project to identify what other factors are causing the flooding, and what action needs to be taken to reverse the trend.

An example close to the University shows the size of the problem. Flooding in York has increased from between ten per decade at the turn of the 20th century to nearly 50 between 1991 and 2000. Yet there has been no noticeable change in rainfall patterns during the last century.

Professor Lane said: “We’ve taken dry summers out of the equation and looked only at only winter rainfall. We’ve looked at the frequency of extreme rainfall events, but there is still no major change in the rainfall patterns since 1900 which can adequately explain the dramatic increase in floods. Something other than rainfall is causing the flooding, something which affects how the water drains off the land, where it can go and at what speed. That ‘something’ is the land itself, and how that land is managed.”

The research project is using Upper Wharfedale as the case study, to look at land management in the uplands where the rivers begin their journey and where the rainfall is high. Changes in agricultural practices have a knock on effect many miles downstream, in lower-lying areas where rivers can do a lot of damage in large towns and cities.

Blaming land management is not just speculation; the theory is borne out by the figures. The numbers of floods in York, for example, increase sharply first in the 1940s and then in the 1980s, and both of these relate to significant changes in land use.

“In the 1980s, the European Union started subsidising farmers per head of livestock. There was a massive increase in stock density in the uplands. Sheep and other livestock trample the ground, reducing infiltration and increasing the speed of run off, which has a direct impact on the amount of water getting into the rivers quickly following periods of rainfall, and so increases the likelihood of flooding,” said Professor Lane.

“Similarly, the increase in floods in the 1940s appears to be directly linked to the digging of drainage ditches, or grips, in the peat bogs, which was started at that time to create more grazing land. There is also a theory that worsening erosion is increasing the amount of sediment added to the river bed, raising the bed level, clogging up the river channel, and so reducing the amount of water needed to cause a flood.”

But water isn’t just a problem; it’s also a resource. Many of the rivers also provide drinking water for local populations and support complex ecological systems, all of which are also affected by changes in land use. The research project in Upper Wharfedale is also bringing water colour and quality, and environmental impact into the equation.

Hundreds of pieces of equipment are already in place in Upper Wharfedale measuring water level, colour and sediment discharge. Certain grips have been blocked to analyse the effect on where the water goes, and how quickly.

Hydrology specialist Dr Joe Holden said: “Very little is currently known about what is happening in these upland areas, despite their crucial role in what happens further downstream. Data collected over the next three years will allow us to start to fill in the blanks.”

cars struggle along flooded roadsThe data will be used to help develop a model for assessing the impact of different kinds of land management, not just on flood prevention, but on water quality, agriculture and the ecology of the area. Dr Chris Brookes is helping to develop the model which, although created for Upper Wharfedale, can be adapted for any kind of upland environment, whether forestry, peat bog or pasture.

“The effects of climate change are starting to be felt, but the true impact will take a while to show up in overall patterns of rainfall and flooding,” said Professor Lane. “While many of the decisions over changes in behaviour to tackle global warming may be out of our hands, how we use the land around us is in our control. Our model will enable people to have an effect at a local level, rather than waiting for others far removed to make those decisions for them.”

 

 
 


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