Areas of high risk in the UK
The map on the left shows the areas of England and Wales considered to be most at risk from flooding in the UK. It highlights the extent to which those areas are likely to flood from overflowing rivers and exceptionally high seas if there are no flood defences in place. Flooding is a naturall occurrence, which cannot always be prevented in advance. If our climate changes as many experts predict, bringing fiercer storms and wetter winters, along with a rise in sea level, the likelihood of floods will increase.
Hydrologists try to forecast the likelihood of futrue flood events using past records. The data they use include river discharge records in relation to precipitation, and flood recurrence interval graphs. These graphs calculate statistically the probability of flooding in the future based on past records. The further back flood records go, the more accurate the prediction.
Records of a river’s discharge are ranked over the longest period available, from highest peak discharge to the lowest recorded. The following formula is used to calculate the recurrence interval:
recurrence interval (years) = number of years on record +1 / ranking of flood being considered
When the recurrence interval is plooted against discharge as a scatter graph on semi-logarithmic graph paper, it is possible to use the line of best fit to predict when the next flood of a particular magnitude might occur. This is called the flood return period. The Department for Environment Food and Rural affaris (DEFRA) and the Environment Agency are the main organisations responsible for flood management of major rivers like the Thames and the Severn in the British Isles and they use fllod recurrence interval graphs to plan flood defence strategies. They recommend that densely populated urban areas are sufficiently protected agianst a 1 in a 100 year flood events but that grassland and low productivity agricultural land should not be protected at all.
In addition to studying the likelihood of flooding on an annual basis, hydrologists use past data records showing the regime or yearly pattern of discharge in relation to annual precipitation patterns. In this way the likelihood of seasonal flooding can be assessed.