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The influence of cleaning method on screen design

Dr Amanda Kitchen, Technical Director, JBA Consulting and co-author of the CIRIA C786 Manual and CIRIA trainer considers the influence of cleaning method on screen design.

In our September 2020 blog, we looked at the different methods for cleaning screens on watercourses, which accumulate debris regardless of whether they have been designed as a security screen to reduce safety risks or a debris screen to reduce the risk of internal blockage.  Routine and reactive cleaning are needed, often out of hours, during adverse flow conditions or at night.

The choice of cleaning method is an important factor influencing design and should be considered at an early stage.  Influential factors include the quantity, type and size of debris, the potential impacts of blockage, access, resources for cleaning during flooding conditions – and budget.

Manual cleaning involves raking debris up a sloping screen onto the working platform, from where it can be removed, or lifting large items from the watercourse manually.  This is suitable for small, accessible screens with a light debris load that can be removed by manual handling.  Screen panels should be designed with modest height, slope and inclined length (not exceeding two metres) to avoid creating a manual handling risk, whilst screen bars should facilitate raking from bottom to top and over onto the working platform – horizontal bars or grids are best avoided.

Mechanical cleaning uses a davit or lorry-mounted grab to rake or lift larger or heavier debris items from the screen and requires vehicular access and parking adjacent to the screen.  This might be considered for larger screens with a heavier debris load or larger debris items that are too much to clear manually but insufficient to justify automatic cleaning.  Safe vehicular access, timely response and adequate resources are essential.  The screen panels must be within reach of the grab so a compact screen may be preferable to a very long screen, and screen bars must be sufficiently robust to withstand any accidental imposed load as undersized bars are easily bent.

Automatic cleaning uses a back-raked screen, gantry-mounted grab or telescopic grab to lift and transport debris from the screen to a holding area or skip.  This can be expensive but may be justified for large volumes of debris such as weeds involving frequent raking or manual handling, where cleaning is critical to manage risk to the catchment, the screen or equipment behind the screen, or where excessive resources are needed.  It may also be suitable if access during flood conditions is impaired by flooding, flood storage or remote location.  The screen must have an electricity supply, sufficient space for debris storage and controls, and vehicular access for debris removal.  Screen panels may be steeper than for manual or mechanical cleaning, but screen bars must still be sufficiently robust to withstand accidental imposed loads.  Inspection and maintenance may be more onerous due to regulations such as LOLER.

The choice of cleaning method and its influence on screen design is an emerging area and the CIRIA Culvert, screen and outfall design training takes a step towards combining industry experience and disparate guidance.  Nevertheless, in our experience, further work is needed to ensure owners and designers can make an informed choice and design safer, more robust and durable screens.