Most people will think of a watercourse as a river, stream, canal or even a ditch, but in the world of Biodiversity Net Gain (BNG) the definition of a watercourse includes culverts. In BNG terms a culvert is defined as “a covered channel or pipe designed to prevent the obstruction of a watercourse or drainage path by artificial construction.
If there is a culvert within a planning application red line area then the requirement to create a minimum of 10% watercourse BNG is triggered, even where the proposed development will not impact the culvert. Many rural sites have a culvert running under a yard or building or access tracks that cross over culverts and, in some cases it may not be clear where the culvert is or how long the watercourse is. However, unlike the other types of watercourse, culverts do not have a riparian zone, and culverts that are not within the red line area of a planning application do not fall to be considered.
The only way of enhancing a culvert for BNG purposes is to remove them to allow the water to flow in an open channel. However, this is not always practical or achievable and if this enhancement can not be achieved, then enhancing the watercourse ‘off-site’ – i.e. outside the red line of the planning application – or purchasing watercourse BNG units on the private market may be the only available options to achieve the mandatory statutory BNG. This can lead to higher costs and the need for additional planning applications for the same development, which are issues that we discussed in our April 2025 article.
