Flash floods should be warning

First published in Letters
Last updated

Sir – The recent flash flooding after a short period of rain is a salutary reminder of a major unresolved issue.

A popular myth is gathering steam that development of land is a solution to flooding problems – actually, it’s a cause.

This convenient propaganda is used by those seeking to increase housing targets, to convince people genuinely concerned about the environment.

It supports Oxfordshire’s Strategic Housing Market Assessment, which concludes we ‘need’ effectively twice as many houses as national targets dictate, which comes hot on the heels of recent serious and widespread flooding.

However, history has shown that often development worsens flooding, despite mitigation. Betterment simply isn’t always possible. Concreting over large areas of land which absorb rainwater, certainly presents a challenge for designers. Hydrology is complex. Mitigation of flooding in one area, can exacerbate flooding elsewhere.

Development of green space undeniably causes increased net run-off. Often drainage schemes are too simplistic. They may be based on investigations carried out when there’s no ground water, although in reality it is present when flooding is at its worst.

It therefore appears as if the residual green space has capacity to absorb the additional run-off generated by development. However, net run-off has not been calculated for the worst case, which is when remaining ground is already at least partially saturated.

Often flood mitigation schemes rely on sustainable drainage systems which are not tried and tested long term and which require significant maintenance. This does not always happen.

Designs sometimes include unattractive tanks or ponds that appear like craters when dry. When full, open water in large ponds is dangerous, so must be fenced off.

What we need is development on brownfield land, while key greenfield sites should have mitigation without development.

Justine Garbutt, Alvescot

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