Working out your Biodiversity Net Gain? You must include any watercourse within 10m of your site boundary.
Key points:
- Any watercourse with a bank top lying within 10 metres (or 5 metres for ditches) of your site boundary must be included in the biodiversity metric to calculate your baseline and post-development biodiversity values.
- Plainly, the watercourse can be outside of your site boundary, but you will still have to ensure a 10% net gain in Watercourse Units.
- Access to appropriately accredited expertise allows you as a developer or landowner to make informed decisions on site layout, assessment requirements, and potential costs.
What is the 10-metre watercourse rule in Biodiversity Net Gain (BNG) legislation?
Under Biodiversity Net Gain (BNG) legislation in the UK, where the site boundary crosses into or includes the riparian zone of a watercourse (10 m from bank top for rivers/streams/canals, and 5 m for ditches), that length of watercourse must be included in the metric. This means that the baseline assessment must cover the watercourse, and the 10% biodiversity net gain requirement applies to it as well as to the terrestrial habitats on the site – even where the main channel lies just outside the site boundary.
The riparian zone is defined as a set area measured from the bank top of a watercourse, with widths varying by habitat type. Rivers, streams and canals all have a 10-metre riparian zone from each bank top, while ditches have a 5-metre riparian zone. Culverts do not have a riparian zone, although culverts still carry Watercourse Units and, if present within the site, a net gain must still be achieved.
Watercourses can be challenging to enhance, and without expert hydrological input, any alterations can result in unintended negative consequences. As such, the ancing their condition can be somewhat limited, unlike examples such as grassland or woodland. This can make it more difficult to achieve a 10% net gain.
Where on-site improvements cannot achieve the required uplift, the alternative is to secure off-site units through off-site unit providers or statutory biodiversity credits. This can become a significant cost depending on the importance of the habitats in the baseline, which can impact the business sense of the development. When planning your development, keep the 10-metre rule in mind early on, as adjusting your site boundary, where feasible, may be simpler than achieving a 10% net gain on a watercourse.
In some cases, the simplest way to manage this is through site design. Where a site boundary can be drawn more than 10 metres from the watercourse bank top, it removes the need to include the river in the metric. However, where this is not possible, a formal river condition assessment will be required.
What is a River Condition Assessment (RCA)?
River Condition Assessments (RCAs) are necessary for Biodiversity Net Gain where a development site contains, or lies within 10 metres of, a watercourse bank top.
An RCA categorises the river type and evaluates the condition of a watercourse by analysing parameters such as naturalness, the flow dynamics, water quality, bank structure, and vegetation. For example, a river which has maintained its natural course through the landscape without widening or straightening may have a higher condition score.
RCAs must be carried out by trained surveyors accredited in MoRPh (Modular River Physical) survey methodology. MoRPh surveys inform river condition for input into the BNG metric and can help identify any enhancement opportunities required to achieve a net gain. Only accredited surveyors are permitted to undertake these surveys, as the data needs to meet recognised standards for inclusion in the metric.
Having a Biodiversity Net Gain team with MoRPh-accredited surveyors ensures that data collection is compliant, robust, and suitable for regulatory purposes. It also provides clarity on what opportunities for condition improvement may exist, and where off-site credits may be needed if on-site gains cannot be achieved.
What is considered a watercourse?
A watercourse is any natural or artificial channel through which water flows, either permanently or intermittently. This includes:
- Culverts: recorded within the site boundary but do not require a condition assessment.
- Ditches: recorded if bank top within, or within 5 metres of, the site boundary. A condition assessment is required using the Statutory Biodiversity Metric condition sheets.
- All other watercourses (such as rivers, streams and canals): RCA is required if bank top is within 10 metres of the site boundary.
Early awareness and access to appropriately accredited expertise allows developers and landowners to make informed decisions on site layout, assessment requirements, and potential costs. To learn more about Biodiversity Net Gain, visit our Knowledge Hub.
How Cura Terrae supports your development
We can support our clients with options appraisals and feasibility, for example, where you are scoping out a number of sites for development. Determining the status of each site in respect of watercourses can help you manage financial and programme risks. Get in contact with our experts to hear more.
