The devastating floods in Cumbria and the North West have been awful to watch and, as an area prone to flooding we can empathise strongly with the local residents and businesses so badly affected. The media reports often only give part of the picture. So what causes these massive floods? Imagine that the ground is a sponge. Now sprinkle water on that sponge over a day and then keep on sprinkling and gradually the sponge will not be able to take any more water and becomes saturated. If you then sprinkle water onto the sponge at a much faster rate water will begin to pour out of it. In terms of land, how much water will pour out into the streams and valley will depend on the geology underneath the soil. If it is limestone for example, there may be large caverns and voids underground which can fill up with water thereby slowing the flood over a longer period. If the soil is clay over an impermeable rock then run off into streams will be very rapid.
When rain falls it does so over a wide area at the head of streams and rivers called a catchment. If it has been raining for a prolonged period and the ground is already saturated then all that excess water from the catchment which may be hundreds of hectares in size flows straight into the streams and rivers, then into valleys filled with houses.
To put the Cumbria flood into perspective, in Coalbrookdale our worst flood in recent times involved 68 mm of rain falling over a 36 hour period. A staggering 405 mm of rain was recorded at Thirlmere in the 36 hours to 6th of December and 341.4 mm recorded at Honister Pass, both breaking UK records! But this rain also fell heavily over a very large area of the catchment – see attached picture.
Research at Pontbren in mid Wales has shown that if this rain falls onto sheep pasture in the uplands then the flood pulse is much faster than if it falls on 5% wooded ground which can reduce the flood peak by 29% or 50% if its fully wooded. However a word of caution is that this was based on trees planted ten years before and are a projected simulation. But it makes sense. If water is having to make its way through a maze of tree roots the soil will have more voids to store water.
But the picture is very complex involving how much evaporation (water changing to a gas like the steam you see rising on a sunny day) and transpiration (plants and trees absorbing water through the roots and giving off water vapour through pores in their leaves) as well as abstraction (water taken out of rivers for our use) and rainfall is occurring in the catchment. It also depends on intensity of that rainfall. In the Boscastle floods 75 mm fell in only 2 hours onto saturated ground in the upper catchment.
Other issues are ways to slow down the flood pulse over a longer period and there are examples of debris dams (trees felled across streams in the upper parts of valleys) such as the recent ones in Stroud. The challenge in the Ironbridge Gorge is that stopping the water in steep sided valleys full of coal, silt and power station waste, prone to landslip can be dangerous as water bypasses the dams and erodes the banks, as well as depositing silt which causes water levels to rise up the saturated valley sides.
As well as flood defences what is needed UK wide is a very long term strategy for flood management that transcends the short term political ‘life of the parliament’ involving discussions between local communities, local authorities, farmers and landowners, the Environment departments. We also need more longer term research based on actual hydrology within catchments with on the ground experiments using forestry and permaculture (where not only trees but various layers of plants are used which may absorb more water as well as produce more food). The problem with the blame game is that everyone becomes defensive and that isn’t going to help flooded communities.