Flood Risk and Sustainable Drainage

12 02 2014

With all the flooding that is happening across the south of the UK, it is interesting to look at how The Stilwell Partnership has been actively advising our clients of Government Planning Policy with respect to Flooding and Flood Risk.

Our Technical Bulletin in 2007 explained how Planning Policy Statement 25: Development & Flood Risk required Flood Risk Assessments to be prepared for new developments: http://www.stilwell-ltd.co.uk/html/bulletin_02_07.htm

Later that year, another event brought flooding into the spotlight.

In July 2007, after a long spell without rain, the ground was dry, hard and effectively impermeable. Then, areas of the country, including the south west, suffered heavy continuous rainfall.

The rainwater, unable to soak into the hard ground caused extensive surface water flooding. Minor watercourses rose rapidly and spilled into houses and gardens. Major roads were blocked by surcharging drains, drivers either slept in their cars or abandoned them. Even short journeys were difficult, trying to find a passable route.

The foul sewerage system was overloaded by rainwater. Foul sewers overflowed into brooks and watercourses, or worse, foul sewage surcharged from the downstairs toilets flooding houses in low lying areas.

After the initial 24 hours, people tried to get back to normal and clear up. But it was not over, the River Severn in Gloucestershire started to rise as the rainfall arrived from its outer catchment areas.

The Severn Trent Water company discovered that their newly refurbished and extended water treatment works in Tewkesbury was not protected against this level of river flooding. As river water spilled into the pump houses, the plant was abandoned and water supplies for the thousands of households were lost. Water was distributed to the surrounding communities by the army and volunteers. Water bowsers appeared on street corners.

If that wasn’t enough, a key electricity substation on the outskirts of Gloucester was threatened with the flood water. It was only the round-the-clock work by the army and emergency services, that electricity supplies to the south west were not lost. Flood water was apparently only 2 inches below the hastily erected sandbag walls around the substation..

According to the local gold commander, if the power supply had been lost, mass evacuation of the south west was a real possibility.

So, how had that happened? Our Technical Bulletin on The Pitt Report on the Summer 2007 Floods set out to explain: http://www.stilwell-ltd.co.uk/html/bulletin_07_08.htm

So what did all this mean in practice? Part 3 gives an example of how the Stilwell Partnership helped a local authority implement the recommendations of the Pitt report, and minimise the potential threat of future flood events.


Cheltenham, Gloucestershire was severely affected by the 2007 Floods. This was only the latest and worst flooding event that was suffered by the Borough. We were appointed to help implement the Pitt report recommendations, and proposed a number of measures covering management, technical issues and development.


Flooding can be caused by any number of sources, e.g. surface water, river or sea, groundwater, etc. and in any season. Currently the country is faced with a number of severe flood events.

After the flood levels have peaked and subsided, the specialist drainage engineers of the Stilwell Partnership can help find sustainable drainage solutions to mitigate future flood events.

See our next post for the presentation that we made to a number of Building Developers, bringing them up to date on Flood Risk according to the latest NPPF Planning Policy.

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