Southwest Environmental Limited

CIRIA C753 or BRE 365

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

CIRIA C753 & 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 C753 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.

BRE 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.

For those wanting a little more information please find the below AI generated content. Everything above this line was written by a human.

When developing a surface water strategy, two primary technical methodologies are utilized to manage runoff: BRE 365 and the CIRIA SuDS Manual (formerly C697, now C753). While both deal with the management of rainfall events on a defined development area, they serve different functions within the drainage hierarchy. Choosing the correct method depends on your site's geology, soil permeability, and the available outfall options. At SWEL, we provide technical assessments using both frameworks to ensure your development remains compliant with National Planning Policy Framework (NPPF) requirements.

BRE 365: Infiltration-Based Soakaway Design

The BRE 365 methodology relies on empirical field data to derive an infiltration rate. This involves excavating trial pits that are representative of the proposed soakaway depth and recording the time taken for water to permeate into the soil. These observations are processed through modern software to calculate the specific dimensions of a soakaway or infiltration trench. Factors such as regional rainfall variations, seasonal soil moisture, and mandatory climate change allowances are integrated into the final design. BRE 365 is the preferred method for sites with permeable geology, but it is generally restricted on contaminated land where infiltration could mobilize pollutants into sensitive aquifers. Further details on our field services can be found on our Soakaway Testing (BRE 365) resource page.

CIRIA C753: Attenuation and Flow Control

Where soil permeability is low, such as in heavy clay regions, or where infiltration is not feasible, the CIRIA C753 (The SuDS Manual) framework is applied. This methodology focuses on calculating attenuation volumes required to smooth the impact of a heavy rainfall event on the surrounding environment or sewer network. By modeling different surface types—such as impermeable roofs versus permeable green space—we determine the storage capacity needed to limit outfall rates to greenfield runoff levels. This approach is essential for sites connecting to a surface water sewer or a combined sewer, where discharge rates are strictly limited by the water undertaker. Understanding these requirements is a core part of a comprehensive Surface and Foul Water Drainage strategy.

Integrating Drainage into your Planning Application

Both BRE 365 and CIRIA C753 calculations must be integrated into a site-specific Flood Risk Assessment (FRA). The Lead Local Flood Authority (LLFA) will scrutinize these results to ensure the proposed system can handle a 1-in-100-year storm event plus the relevant climate change allowance. In many cases, a hybrid approach is required, utilizing infiltration where possible and attenuation for residual volumes. This technical integration is often detailed within a wider Sustainable Drainage Systems (SuDS) Strategy to provide a robust technical defense for your planning application.

Professional Technical Advocacy and Results

Providing verifiable, professional drainage calculations is essential for securing planning consent and satisfying building control. Our reports deliver the scientific foundation needed for a successful Planning Statement, ensuring that your water management strategy is both cost-effective and regulator-ready. With over 900 projects delivered across the UK, SWEL brings the technical authority and professional accreditation (IES/CIWEM) required to solve complex drainage challenges in any geotechnical environment.