The Hierarchy of Drainage – National Planning Practice Guidance
The National Planning Practice Guidance sets out The Hierarchy of Drainage to promote the use of Sustainable Drainage Systems, by aligning modern drainage systems with natural water processes. The aim of Hierarchy of Drainage is to drain surface water run-off as sustainable, as reasonably practicable.
The increase in infrastructure and the use of traditional drainage networks (pipes and culverts) are resulting in downstream flooding and a deterioration in water quality of controlled waters, due to foul sewer overflow. Therefore, sustainable drainage systems aim to alleviate these problems by storing or re-using surface water at the source. This decreases the flow rates to watercourses and improves water quality.
SuDS designs control surface water run-off (rainfall) by closely resembling that of natural drainage. SuDS features include the use of soakaways, filter strips and swales, filter drains, permeable surfaces, ponds, etc.
The Hierarchy of Drainage
As stated in the National Planning Practice Guidance, the aim should be to discharge surface water run-off as high up the drainage hierarchy, as reasonably practicable:
- into the ground (infiltration);
- to a surface water body*;
- to a surface water sewer, highway drain, or another drainage system*;
- to a combined sewer*.
*Local Authorities and water boards generally require proof (BRE Digest 365 Soakaway Tests) that the ground is not suitable infiltration into the ground, before connecting to sewers, drains and other drainage systems.
Infiltration into the Ground – BRE Digest 365 Soakaway Tests
The most Sustainable Drainage Systems is infiltration of surface water run-off into the ground. To determine whether the ground is suitable for infiltrating the surface water into the ground, BRE Digest 365 Soakaway Tests are required.
BRE Digest 365 Soakaway Tests should be carried out by a competent person (Sub Surface). A soakaway test consists of a machine excavated trial pit being dug, logged and filled with water. The rate at which the water drains into the ground is recorded to calculate the Soil Infiltration Rate.
If the water drains away at a sufficient soil infiltration rate, the ground may be suitable for a sustainable infiltration system. For example, soakaways, filter drains, etc. In this case, requiring a site specific Surface Water Drainage Strategy.
Do you have to use Sustainable Drainage System?
The proposed development and site location determines whether a SuDS requires consideration. For example, where there may be concerns about flooding and/or water quality. Sustainable drainage systems may also not be practicable on some sites. For example, where there is a risk of chalk solution features. Therefore, it is down to the discretion of the local planning authorities to determine whether sustainable drainage systems are not appropriate, and where steps can be bypassed.