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CIRIA C697 or BRE 365

Southwest Environmental Limited can undertake surface water drainage strategies using either of the above methods.

CIRIA C697 & BRE 365 both deal with surface water management, they both rely on rainfall data for a given event falling on a user defined area. Where they differ is that whilst BRE 365 uses field data to derive an infiltration rate and works out a soak away volume based on this rate, CIRIA C697 is based more on limiting flows from site, without considering where they go. . . .

BRE 365 Soak-Away Tests

In order to carry out a BRE 365 Soakaway Test you will need to excavate some trial holes, and fill then with water and watch how quickly in soaks away. In clay soils this can take a long time, so you may need to watch the hole for a few hours, in a gravel soils it could be all over in 10 minutes.

The hole size needs to be "representative" of the soak away you intend to use. Depth is the most important factor to consider in most cases.

Once you have your infiltration data, the calculations are carried out in accordance with BRE 365. Various factors are considered such as region (southeast drier than southwest), an allowance for climate change, size of test hole, and finally your recorded observations. These are then used to establish whether you input area (say 500m2) will create say 5000 liters of water during a 1:10 year rainfall event.

You will then get a results showing the size of the trench you need to dig, in order to provide enough infiltrations drainage.

soak_away_bre_365BRE 365 Soakaway testing can be carried out anywhere in the UK. Around 4 tests can be carried out simultaneously over the course of a day on site. A water supply is not required as we can normally arrange this even at short notice.

BRE 365 soakaway testing cannot be carried out on contaminated sites. This is because the EA do not accept soak-away drainage on contaminated site, and carrying out the tests themselves may wash contaminate off site on in to aquifers.

On very steep sites it can be difficult to get water to the holes, and it may need to be pumped from the nearest available flat area. Each test typically uses around 200 liters of water, it must be added to the hole quickly so as to simulate a natural rapid rainfall event.

Modern software for calculation of soak-away sizes according to BRE 365, require perhaps 5 to 10 readings over the course of the test which should be run until 50% of the water from the hole has been soaked away.

Please note that soak-away for foul drainage is carried out according to a different method. A smaller pit is typically used, which is hand dug, sometimes in the base of a larger machine dug pit.


With CIRIA C697 as with BRE 365 the site area is input in to the calculations, but in this instance you can select areas of differing permeability, for example roofs and or grass areas. The more grass (proportionately) the smaller you attenuation feature will be. 

The other factor to be considered is your out flow, which normally goes to either:

- Infiltration

-Surface Water

-Surface Water Sewer

-Combined Sewer

For the last 2 options in the above list you will need to consult your local sewage undertaker, to find out whether they will allow you to connect at say 2l/s/ha.

The calculations will now provide you with an attenuation volume. Which will provide storage of water so as to smooth the impact of a a heavy rainfall event on the surround surface water system whether that be natural or manmade.

Example Projects

BRE 365 Soak-Away Testing and Design Report - Gloucestershire