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Screen test

Letters

When will designers realise that screens at culvert inlets are easily and quickly obscured by trash- (Why flood defence failed twice, NCE 10 November).

Clear guidance is given in Trash Screens - Design and Operation Manual (Defra/ EA, 2002), and in the Culvert Design Guide (CIRIA, 1997).

The sort of debris referred to in the article will not block a culvert, but can block a screen. If the primary aim of the screen is to prevent children getting into the culvert, then a bar spacing of 150mm would be more appropriate and less prone to blockage.

Even so, any screen carries a risk of blockage, and it is never possible to guarantee that a screen will be clean when a flood occurs.

It is therefore essential to ensure that, in the event of a screen being blocked, the inevitable rise in water level upstream does not cause flooding of the very infrastructure that the scheme was designed to protect. This may require the construction of a flood bypass, which can add considerably to the cost.

The best answer is don't provide a screen unless there are compelling reasons. If you do have to, make sure that your design allows for the inevitable build-up of trash.

Charlie Rickard(F), 4 Millards Lane, Lode, Cambridge CB5 9ES

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