When it comes to specifying drainage channels, there are vital factors that enable a system to meet requirements of a housing development project. Rob Butcher of ACO Water Management looks at the standards that must be upheld from the earliest stages of the project.

Many areas of the building design process have guidelines in place to help housebuilders and developers avoid confusion. There is, however, still some misunderstanding around drainage. Coming in January 2024, Schedule 3 of the Flood Act will see changes to the legal requirements around drainage systems across England, making sustainability mandatory across new schemes. Because of this, it is vital that all involved in property building and development understand drainage channel specification, though the reality is that many are unaware of some crucial points.

In fact, the results of an ACO survey of 100 highways and infrastructure engineers strongly pointed towards a lack of clarity around the key factors that need to be taken into account when designing drainage schemes. Only 1% of survey respondents said that they do not face barriers in SuDS schemes, and fewer than half claim to fully understand load class specification requirements. So, what is the need-to-know on load class specification?

LOAD CLASS

An often-overlooked aspect is drainage channel load classes, defined by BS EN1433. In cases where the wrong load class is specified, the risk of flooding is increased. This is because the channel may sustain significant damage if it is unable to bear the load from traffic passing over it. When drainage channels fail, costly repairs are incurred as well as devastating losses to businesses and homes. In some cases, such as roads where people are travelling at high speeds, the collapse of a drainage channel can result in fatal accidents.

It is the anticipated use that determines load class, ranging from A 15 to F 900. This covers pedestrian use (A 15) through to heavy industrial and military installations (F 900).

Sometimes it is wrongly assumed that residential developments only need A 15, however, if vehicles are going to be driven over drainage channels, A 15 will not be sufficient and will break prematurely. To be certain that the correct load class is specified, it is of course always best to consult an expert.

In areas with fast-moving traffic, it may be better to use a monolithic drainage channel. This means the channel and grating comes as a single, one-piece design and there is less risk of the grating coming loose and posing a danger to vehicles and pedestrians.

One point to keep in mind is that the load class is based on the combination of the grating and channel. For example, a channel with a B 125 rating, when used with an A 15 rated grating, automatically becomes an A 15 channel and should only be used in areas where pedestrians and cyclists are likely to be going over it.

The load class specification should also take into account whether vehicles will likely be turning as they pass over the drainage structure. The turning motion can increase the pressure applied and therefore a different load class may be required to ensure safety. It is because of nuances like this that drainage experts should be involved with a project right from the design stage.

CHANNEL CAPACITY

Our survey also revealed that the element deemed least important was channel capacity (30%). This may be surprising given its significance in managing water runoff and mitigating flood risks. A reason for this is perhaps that full channel integration embedded within a drainage network model is frequently overlooked due to being a time-consuming process to factor into design. However, ignoring this can be counterproductive. In the long-run, channels that have the optimum hydraulic capacity will prove to be a more cost-effective approach.

In heavy rainfall events, and when appropriate capacity has not been properly considered, drainage systems can overflow and lead to site flooding. Repairs can subsequently be very expensive due
to the potential retrofitting required. Going forward, developers need to make the most of design consultation during these decisions.

Thankfully there have been several advances in digital design tools such as MicroDrainage or Causeway Flow. These can be used in tandem with Channel Design software to incorporate the capacity provided from linear drainage channels within a whole site drainage model.

EXPERT CONSULTATION

Although steps are being taken to manage surface water effectively, the survey results show that there remain areas that require further attention. Drainage channels should be a foremost consideration with sustainability, performance and maintenance collectively held up as key priorities. To meet the changing demands, the industry as a whole needs to become more collaborative.

At the same time, it is crucial the challenges in achieving these ambitions are addressed, especially with regard to knowledge gaps. One of the first and most important steps to take is to understand the different products available and how to design these effectively. By consulting with local authorities and ecologists as well as drainage manufacturers, property developers can be equipped with effective drainage systems that protect people and buildings, while enhancing biodiversity for decades to come. 

Rob Butcher is design services manager at ACO Water Management