Dealing effectively with surface water is a key consideration for the development of any site. Here, Mark Jones, our SuDS (Sustainable Drainage System) and Flood Risk specialist, explains how the best drainage designs should start with infiltration testing.

Infiltration testing is used to find out how quickly surface water from roofs and paved areas may soak away naturally into the ground. Part H of the Building Regulations sets out a Hierarchy of Drainage that has infiltration as the preferred option above discharge into a water course and surface water sewer, so it is a sensible place to start. It is also often required by Lead Local Flooding Authorities to prove that connections to water courses or sewers are necessary.

Given it is a building block of the design solution, consistent and robust testing is vital. Our team works to BS5930 and BRE 365 to undertake testing, to ensure that a representative infiltration rate is calculated. Trial pits are dug and filled with a clean and consistent sized stone before being filled with water. The infiltration flow rate is then monitored through a fixed pipe. Up to three tests are undertaken at each location, to ascertain a representative long-term flow rate.

Once the results are in, we can then work closely with the developer to design a scheme that not only meets all the regulatory requirements but can help with the implementation of SuDS elements such as infiltration trenches, permeable paving and basins. Not only are these excellent technical solutions backed up by the testing data, but SuDS can have the additional benefits of increasing biodiversity, creating amenity space and reducing flows into public sewers.

So, it really is worth investing a little time up-front to get the infiltration testing just right on your next development. This will ensure the final scheme has the best possible chance of being approved in planning by the Lead Local Flood Authority and minimise any delays.