Flooding

How could the gravel pit and concrete plant affect local flooding?

The gravel and sand pit proposed by Hills lies mostly on the Thames floodplain between the rail bridge south of Culham station and Clifton Hampden bridge. As we know, this area often floods together with Clifton Hampden High Street and the road between the Barley Mow and Long Wittenham. A large proportion of the proposed site (~80%) lies within the flood risk zones as defined by the Environment Agency.

This area of floodplain is vital as it is able to absorb large volumes of water as and when flooding levels rise. Once the Thames has burst its banks, any subsequent rise in water levels is able to expand over an increasing area of floodplain. Therefore, this floodplain acts as a vital release valve to take up excess flood waters. Any development that reduces the capacity of this floodplain will lead to greater flooding in Clifton Hampden, Long Wittenham, Appleford and surrounding areas, and the frequency of flooding will increase. As Clifton Hampden bridge acts as a constriction to the flow of the Thames (all water flowing from upstream of the bridge has to flow through the bridge) flooding further upstream around Culham bridge is also likely to be affected.

In both the winters of 2012/2013 and 2013/2014, the High Street in Clifton Hampden flooded and the Clifton Hampden bridge was closed for several weeks. This affected local businesses, such as the Barley Mow and the Clifton Hampden village shop. It also affected residents of the Bridge Caravan Park and the High Street, in addition to all traffic wishing to travel between Long Wittenham and Clifton Hampden (and beyond). The houses along the High Street did not flood – these were built many, many years ago just above the level of the floodplain – but at the height of the floods, the houses were not far above the waters. Many of these houses are timber-framed, old and do not have substantial foundations. They have also been recognised as worthy of preservation – they are in a conservation area and many are listed buildings. Any development that reduces the available area of floodplain will also threaten these houses.

At this time it is not known what plans Hills have to manage flooding. There appear to be two options: either the gravel pits are protected, partially or completely, from flooding to protect equipment and operations; or the gravel pits are allowed to flood. If the first option is taken, this will reduce the area of available floodplain and hence exacerbate flooding as described above. If the second option is taken then the flood risk will not be increased and may in fact be reduced. However, during times of flood, the gravel pits will have to cease operations – in the case of the winter of 2013/2014, operations would have been affected between 2 and 3 months. It is not known how acceptable this would be to the quarry operators as a commercial enterprise. The second option also allows flood waters to overflow into the extraction pits that will contain exposed sediment, some of which will become suspended in the water and be carried with the water downstream. Some of this is likely to be in the flooded areas of Clifton Hampden. Some of the sediment would settle out elsewhere downstream, which may cause other problems.

Dr Steve Grey