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Flood Risk Assessments
PPG 25


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Recent extreme flooding events, from Easter 1998 to today, demonstrate that flooding is a real risk to many homes and businesses, even posing a threat to life.

'Nearly two million properties are in floodplains in England and Wales. Property, land and other assets are valued at 214 billion. In the past two years, 25 people have died as a result of floods and many thousands more suffered trauma and devastating damage to homes and valuables'. MAFF statement

Flooding is becoming more common. The reasons for this are complex, and include global warming which is affecting the weather patterns that shape our temperate climate. Other reasons are, however, closer to home.

When rainfall lands on a rural landscape, much of the water soaks into the ground, evaporates or is taken up by vegetation. The remaining water finds its way slowly to the watercourses and rivers. The result is that only 15-30% or so of the water ends up in the rivers, and this may be spread over a period of days.

Changes in agriculture have increased this runoff. A ploughed field will drain more quickly than a meadow. A well drained field will also remove the water much quicker, allowing less opportunity for infiltration and evapo-transpiration. The result is a greater volume of runoff at a greater rate of flow. Urbanisation is even worse..

Piped drainage systems will remove rainfall from roofs, roads and hardstandings in minutes, not hours. Losses are minimal, and 85% or so of the rainfall will end up in the rivers and watercourses.

Development in the flood plain makes things worse. Flood storage is lost and the capacity of the overland conveyance is reduced. Flood defence works reduce flooding in one area (that is their point) but this is lost storage. If the water has nowhere else to go, greater flooding will result elsewhere.

Modern drainage design can minimise the impact of development on rainfall runoff, but this will require a major re-think in the design of developments. Improvements can be achieved by Sustainable Urban Drainage Systems (SUDS), but major hurdles remain over long term maintenance of the necessary systems.

Following the Easter 1998 floods, the Environment Agency published requirements for developers in the upper Nene valley, requiring the rate and volume of runoff from a developed site to be less than the pre-development condition. These guidelines were largely superceded when the government published guidelines to planning authorities in relation to flooding and development as PPG25, extending similar requirements throughout England (but not the rest of the UK).

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