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Culvert, screen and outfalls: Importance of good screen design

Dr Amanda Kitchen, Consultant Principal Civil Engineer, Mott MacDonald and co-author of the C786 guidance outlines the importance of  screen design

Heavy rainfall and flooding in parts of Yorkshire and Lancashire went almost unnoticed earlier this month, drowned out by the United States presidential election. As my own town witnessed out-of-bank flows for the fourth time in five years, a slick and well-rehearsed community flood response swung into action, closing property flood gates, moving items upstairs and distributing sandbags to those in need.

Living near the river, my daily dog walk allows me to observe the accumulation and removal of debris at a debris and security screen during all flow conditions. The river carries a heavy debris load, from leaves and twigs to tree trunks and rowing boats. Located at a hydropower offtake near the riverbank, the screen escapes the larger debris conveyed by the main flow, but being perpendicular to the approach flow, it nevertheless builds up a substantial raft of smaller material, sometimes several metres deep. My daily observations gave me cause to reflect on good and not-so-good practice in screen design.

Submergence of the working platform and/or access routes affects safe access for cleaning during flood conditions. Whilst it’s useful to clear a screen before heavy rainfall, considerable debris can build up during flood flows. Setting working platforms and accesses above flood level may facilitate cleaning during an event, particularly useful if blockage could cause flooding of properties or critical infrastructure.

Manual cleaning is hard work, especially when debris is pushed against the screen by water pressure. Although smaller, natural materials can be pushed through the screen, man-made and larger materials may need to be dragged or lifted onto the working platform for removal. Limiting the height and inclination of screens can reduce the manual handling risks.

Measures to prevent falling from the working platform into the water can impair access to the screen for raking. Gates, chains or rails along the top of the screen can be opened or removed to allow cleaning without too much twisting or bending. Harnesses provide personal fall protection for operatives; anchor points or cables can be retrofitted to existing screens.

Finally, the demands on a screen can change over its design life: stronger winds and higher flows resulting from climate change may increase debris load; closure of a maintenance depot may increase operational response time, or development could affect access to a screen. Taking a modular approach to bar spacing - using intervals of 150, 300 or 450mm - allows a debris screen to be converted to a security screen, or vice versa, by adding or removing bars, providing a degree of future proofing.

For further guidance on good practice in screen design, assessing need for screens or avoiding screens by design join CIRIA training and download the CIRIA Culvert, screen and outfall manual C786.