Anti-flood drainage: Building (or not building) in 'inappropriate' locations

In the light of Sir Nicholas Stern's review 'The Economics of Climate Change', the government has issued guidance on better flood defences. Stern predicts that annual flood losses could cost 0.2 - 0.4% of GDP in 100 years. In the interim, fitting an anti-flood device for a hundred quid or so seems like a pragmatic solution.

In this exclusive extract from Shortcuts: Book 1 by Austin Williams, we look at the risk to domestic drainage systems. 

Domestic drainage systems

Many domestic drainage systems are laid with reasonably shallow falls in compliance with the recommendations in Approved Document H (AD H) . This suggests that 75 mm and 100 mm rainwater drains should be laid at not less than 1:100 falls, and 150 mm drains and sewers should be laid at gradients not less than 1:150. The falls on foul drainage depend on the number of connections that affect the flow rate so that a 100 mm diameter pipe with a flow of more than 1 L/ sec must be laid at a gradient of not less than 1:80.

In general then, an average small housing estate constructed with minimum drainage falls is likely to have just half a metre of rise from the invert of the main sewer connection to the invert of the shallowest manhole; and possibly only a further 450 mm between that manhole invert and a ground floor wc trap. If the main combined drain should become blocked, theoretically there is only an arm's length preventing your toilet spilling over.

BS 1717: 2001 provides detailed examples of airlocks and vents to prevent potable water becoming contaminated by backflow. Category 1 water (potable) can be polluted by infiltration of Category 2 (stagnant water, say, from a water tank) through to Category 5 (sewer waste or swimming pool water). Essentially, if a potable water network (including a 'water draining device') connects with a drain, there must be either a physical separation of the supply and the drain 'connections', or a connecting device that has a vent/ grille sufficient to ensure that any potential contaminating backflow will overspill rather than back up. AD H states that this is the basic line of defence against contamination in relatively low-risk areas.

High-risk conditions

In severe floods, a storm sewer has the potential to surcharge, i.e. the water level in the manhole rises above the top of the pipe, and at some critical point, hydraulic pressure will cause drains below the rising water level to overflow (see diagram). The potential for - and fear of - drainage backflow and consequent flooding has been greatly increased of late due to:

• WCs situated in basements
• Houses built in low lying/ flood-risk areas
• Contemporary fears about increased rainfall detrimentally affecting existing combined sewers.

In high-risk areas, AD H recommends anti-flooding valves that can be fitted to the main drains to resist any backflow pressure. These valves comprise a 'gate', like a float valve, that allows drainage water to flow in the designed direction but will automatically cut off if water starts to flow - above a certain volume - in the opposite direction. AD H states that these devices should be of the double valve type, be suitable for foul water, have a manual fail-safe and comply with pr EN 13564.

The AD H description is equivalent to pr EN 13564-1's Type 2 category of anti-flooding device, although at the time of writing, this is deceptive. Only ACO manufactures an anti-flooding device that is designated to deal with foul water flow and this is a higher specification (Type 3). This is an automated device rather than an 'automatic' one. ACO's Type 3 device has a high-torque electric motor with battery backup ensuring that the valve can close even in the presence of solid waste. Such faecal matter would undoubtedly clog up the 'passive' float or gate valve of a Type 2 anti-flooding systems. Even though pr EN 13564-1 states that passive Type 2 anti-flooding devices are used in foul drainage systems in Austria, there are no official UK regulatory guidelines on this matter. Therefore, only Type 3 (and possibly a higher specification) is suitable for foul water use. However, note that while Type 2 anti-flood devices cost around £100, Type 3 devices cost around ten times that amount, and therefore careful consideration should be given to where Type 3 devices should be fitted.

Shortcuts provide an eclectic mix of at-a-glance guides to the minefield of regulations, new materials, and technologies that confront building designers today. Comprising hand-drawn sketches, technical drawings, and punchy articles, each Shortcut takes you right to the heart of a key topic in architecture and construction, presenting a wealth of invaluable information in an accessible and lively way.

Shortcuts: Structure and Fabric by Austin Williams focuses on issues to do with the structure and fabric of buildings. Grouped into four main subject areas, the Shortcuts range from rainscreen cladding to fire protection, from lofts to lifts, and from LEDs to SUDS.

Copyright RIBA Publishing March 2009.

Additional publications and services

Available to buy now from RIBA Bookshops:

Shortcuts: Sustainability and Practice

NBS Domestic Specification (2nd Edition)

Also available:

NBS Building Regulations

Related NBS information:

Articles:

• SUDS (Sustainable Urban Drainage Systems)

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