Is it ok to design screens for manual cleaning?

Dr Amanda Kitchen, Consultant Principal Civil Engineer, Mott MacDonald and co-author of the CIRIA C786 Culvert, Screen and Outfall Manual

 
All screens on watercourses accumulate debris, whether installed to reduce safety risks (security screen) or internal blockage risk (debris screen).  Routine cleaning is essential and emergency response may be needed during high flows, outside working hours or at night.

Debris can be large and heavy, eg large wood, pallets, mattresses.  Manual cleaning by raking debris up onto a working platform can be dangerous in high flow conditions, with a risk of falling into deep, fast-flowing water and entrapment against the screen.  As a result, there is now a move away from manual cleaning to alternative approaches.

Reducing sediment or debris load at source can reduce the need for manual cleaning; examine the watercourse, its tributaries and floodplains upstream to identify possible debris sources.
Mechanical raking eliminates most manual handling risks but can damage a screen unless suitably designed.  A lorry-mounted grab can remove and transport debris from site for disposal at screens with vehicular access and sufficient space; otherwise permanent mechanical raking or grabs can be used.

Automated mechanical raking is often seen at pumping stations where debris builds up steadily and more quickly than can be cleared manually.  Potential issues include power failure, skips filling without warning, disposal of debris, health and safety hazards associated with unauthorised access, inability to cope with unusual debris and reliability in adverse weather conditions.

One emerging method is 'bend and beach' where debris migrates to the outside of a meander upstream of a screen and accumulates on a shallow beach from where it can be removed.  This needs plenty of space and good access.
The best approach is to avoid screening by designing a culvert to reduce safety or blockage risks, but if a screen cannot be avoided, consider alternatives to manual cleaning first.  Manual cleaning may be acceptable if these are unsuitable, but it’s important to assess the risks and adopt ‘safe by design’ principles - for further guidance, see the new CIRIA Culvert, screen and outfall manual C786.

In our next blog, we look at whether it’s possible to future-proof debris and security screens.