Oxford Times letter submitted 25 June 2019

Flood Scheme: Thinking `smooth'

Building on last week’s letters by Caroline Roaf and Jonathan Madden (Oxford Times, July 20), there is `good flood news' for Oxford. Mrs Roaf’s upstream storage will always be needed, but regrettably it will not in itself resolve the man-made blockage at Old Abingdon Road, where the Environment Agency has used commendable tenacity to negotiate a new opening.

In local terms, this leaves the question of what feeds into this opening? Dr Madden lists three alternatives, each of which reduces the impact on landscape, ecology and traffic disruption as compared with the Agency’s proposed channel. The newest of the three, the `enclosed storage area‘, would additionally give the Agency what it is seeking by way of compensatory flood storage. It is based on the Agency’s recently-completed flood scheme at Salford, and could avoid transport disruption at South Hinksey and the A34 because Salford was entirely `site won’ in construction materials.

Storage is not enough of course, and the Agency’s proposed channel is designed to keep the flood moving. Such flow is affected by the `roughness‘ of the channel floor, which in a numerical model is defined by Manning’s coefficient. We have therefore asked the Agency for the values they built into their model, i.e. how ’rough' does it assume Oxford’s natural valley floor to be?

A preference between the three alternatives will to some extent depend on how the roughness has been modelled. Collectively however the range of options should ease the CPO process, paving the way for an imaginatively-conditioned planning permission and a smooth start on improving Oxford’s flood resilience.

Brian Durham

South Oxford

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