Standards and good practice

Site drainage can provide a primary pathway, and therefore an accurate understanding of the drainage condition and connectivity is fundamental to site environmental planning and security.

There are a number of relevant standards and good practice that apply to drainage. These include:

• CIRIA C736 Containment systems for the prevention of pollution
• GPP21 Pollution incident response planning
• WRc MSCC5 Manual of sewer condition classification
• Netregs – GPP/PPG series reference importance of accurate drainage information
• BS EN 858-2:2003 Separator systems for light liquids
• CIRIA Suds manual

In CIRIA C736 – Section 5.4.4 it states that:

A survey should be carried out if there is any uncertainty about the layout, condition, or capacity of drainage systems for trade waste. Potential hazards in terms of the systems ability to cope with major incidents should be identified. Typical hazards might include:

• combined surface and trade waste systems, especially where provided with storm overflows on or off-site
• direct runoff to surface and groundwaters (i.e. rivers and soakaways)
• infiltration of contaminants through defective drainage
• unprotected clean water gullies and other devices for collecting stormwater from areas likely to become contaminated
• inadequate or poorly maintained valves and other devices used for flow control
• defective pipes or joints
• blocked gullies, pipes and drains

If comprehensive records are unavailable or have not been maintained, the site drainage system (including surface water, foul and trade effluent systems) should be surveyed. This is normally completed by a CCTV survey that will also provide evidence of physical defects with the pipework such as broken pipes, misaligned joints, root intrusion etc.

GPP 21 Pollution Incident Response Planning section 2.2.1 states that you should have an up-to-date drainage plan including:

• Surface water drainage
• The foul sewer
• Any connections where trade effluent drains into the foul sewer
• Inspection points to detect pollution
• Oil separators/interceptors
• Pollution control devices (shut off valves/penstocks)

You should provide a description on how all facilities operate and make sure they’re clearly labelled above ground as well as noting when an inspection was last carried out on pollution control devices.

When you are drawing up your drainage plan, it is a good opportunity to check for any misconnections from your site, e.g. a foul sewer that has been linked into a watercourse.  This is illegal and may lead you to respond incorrectly during a spillage.

The drainage plan should include the following key drainage features:

• Drain type – foul, storm, trade effluent, combined
• Directional flow
• Final site outfalls
• Catch pit
• Sump
• Soakaway
• Attenuation tank
• Septic tank
• Slot drainage
• Aco drains

Gully pot As well as key layout features:

• Operational activities
• Storage areas
• Fuelling areas
• Wash facilities
• Site topography
• Car park