Keeping pipework clear and debris-free is essential for sustainable drainage systems, which is why the consideration of headwall grilles and trash screens can no longer be an afterthought in new developments and infrastructure projects says Fabweld Steel Products’ Wayne Carter.
In January this year, the Department for Environment, Food and Rural Affairs (DEFRA) published its sustainable drainage systems (SuDS) review, looking at the benefits and impacts of making measures to reduce the risk of flooding and pollution a legal requirement for new developments in England and Wales. The recommendation of the review was to make sustainable drainage systems mandatory – a step in the right direction towards resolving some of the issues caused by increased urbanisation coupled with increasingly unpredictable weather patterns.
How the recommendations of the review transpire into law will be shaped by the public consultation happening later this year, with those views informing the Government’s final recommendations for Schedule 3 to the Flood and Water Management Act 2010, expected to be implemented sometime in 2024.
While the exact scope and remit of the Act is to be decided, there are some sections of the forthcoming legislation which are likely to be familiar to architects as January’s review cited that the design, implementation and maintenance of sustainable drainage should draw from the existing DEFRA technical standards and SuDS Manual 2015.
In some ways this is good news as it will be less of an adjustment for the industry; in other respects it doesn’t go far enough. The current non-statutory DEFRA guidelines minimise the pivotal role that some aspects of sustainable drainage design play, namely the inclusion of headwall grilles and trash screens, which are often overlooked in installations where they should be included as best practice.
Screen stars
Headwall grilles and trash screens are steel constructions that keep pipework and culverts safe, secure and operating efficiently. They are used in a variety of applications from infrastructure projects to new housing developments.
Headwall grilles restrict unauthorised access to larger pipes and culverts, particularly by children and small mammals. They are compulsory on outfall pipes greater than 350mm in diameter and should be hinged and secured with a padlock, allowing for access by authorised personnel. The grille bars also stop larger debris from entering the pipe.
The specification of headwall grilles is relatively straightforward as these are available to fit over standard pipe sizes, although most come with 100mm play either side to ensure a secure fit on site. Galvanized mild steel is preferable material-wise as it’s guaranteed for up to 25 years.
Although they look similar in design, trash screens serve a very different purpose to headwall grilles. Their job is to stop debris and litter from entering the drainage system, which can reduce the flow of water or stop it altogether. In times of heavy rainfall, blockages can cause flooding. Debris and litter can also contaminate the water.
Trash screens are usually custom designed. The bars are closer together than those on a headwall grille, and graded so they can be closer on applications where smaller debris, like leaves, is problematic. Trash screen bars are pitched at an angle to match the exact requirements of an installation so that any debris and litter is pushed up and over the pipe. This keeps the inlet clear to maintain water flow and makes is easier for service teams to collect and clear the debris during routine maintenance visits.
Forgotten hero of sustainable drainage
Litter and debris removal is an integral part of SuDS maintenance and headwall grilles and trash screens have an important role to play in this, but they are often an afterthought in a system specification. There is a common misconception that a headwall grille and a trash screen are the same thing, or if you have a headwall grille, a trash screen isn’t required.
What you need in practice is dependent on the application. Factors to be considered include the risk of people accessing the drainage pipes, a history of blockages, the location and size of the opening.
In remote locations, for example, the risk of a culvert or headwall being accessed is low and it may have no history of blockages therefore trash screens or grilles aren’t required. A site survey may recommend a screen on the inlet to direct debris away from the pipe to make routine cleaning easier. Should the volume of debris or the water flow change over time, a trash screen or headwall grille can be retrofitted.
Some culverts with large openings may still be classed as low risk as they are big enough for debris to pass straight through without issue. In fact, the addition of a trash screen or grille could potentially restrict the water flow and cause blockages at either end of the culvert.
One place you definitely need to consider screens is on culverts that are longer or have a change of direction or bend. A trash screen may be fitted at the inlet side of the culvert to prevent things entering that could become lodged part way along the pipe, which is awkward to access and difficult to clear.
In terms of design, choose products which conform to the DEFRA and CIRIA design guidance, although these are not mandatory. A specialist company will be familiar with standardised design concepts and can use these as the basis for standard or bespoke products. Trash screens and headwall grilles may look like simple constructions, but they require specialist knowledge and expertise to make them correctly. We have heard anecdotes of jobs where contractors have asked a local fabricator to build them a trash screen and the screen has fallen short of expectations, to say the least.
Your finished article should be reasonably priced without compromising on material quality, design or build – anything else is a false economy. The product should be a good fit over the culvert or drainage, without gaps or openings which could let debris through.
Considering trash screens and headwall grilles from the outset can help architects to design developments which encourage a preventative maintenance ethos to sustainable drainage, helping to protect the built and natural environment from the devastation of flooding now and in the future.