Biodiversity Net Gain (BNG) is a component of any sustainable development, ensuring that new development projects leave the natural environment in a measurably better state than before. The regulations guiding BNG came into effect in 2024 as part of the Environment Act 2021 which aimed to promote nature positive change in the UK. By April 2024, the act had been expanded to include small sites in England. Watercourses, including rivers, streams and ditches, are not left out of these ecological assessments. These water bodies provide critical ecological functions by supporting biodiversity, improving water quality and mitigating flood risks. However, their health is increasingly threatened by pollution, climate change and urbanisation. Understanding how to measure, enhance and calculate biodiversity net gain for watercourses is crucial for meeting regulatory requirements and safeguarding these vital ecosystems.
What is a Watercourse
A watercourse is any natural or artificial channel through which water flows, either permanently or intermittently. This includes:
- Rivers and Streams: Natural flowing bodies of water that provide habitat for diverse aquatic and riparian species.
- Ditches: Artificial or modified channels designed for drainage, but which can also support ecological functions.
- Canals: Man-made navigable waterways that may contain naturalised biodiversity.
- Culverts: Underground channels that direct water flow but can be barriers to habitat connectivity.
England boasts an impressive network of waterways stretching 242,000 kilometres. These watercourses cover rivers, lakes, ponds, estuaries and wetlands that are the foundations of wildlife corridors. The numbers tell a concerning story - only 16% of England's water bodies are in good ecological condition. This shows we need better ways to assess and manage these resources. When it comes to BNG for watercourses, developers, ecologists and local authorities must assess and enhance aquatic habitats using specific metrics and methodologies.
Measuring BNG for Watercourses
The Watercourse Metric (previously called the Rivers and Streams Metric) is a standardised tool for assessing the ecological value of rivers, streams and ditches. It is designed to support the Environment Act's provisions for mandatory biodiversity net gain if a development falls within 10 meters of a watercourse and calculates biodiversity losses and gains for rivers, streams and other kinds of watercourse. Developed as part of the Statutory Biodiversity Metric, this methodology quantifies biodiversity based on key indicators, such as:
- Habitat condition: The health of the watercourse and adjacent riparian zones.
- Connectivity: How well the watercourse links with other ecological networks.
- Species diversity: The variety of aquatic species present.
The statutory biodiversity metric establishes clear scientific classifications for watercourse assessment. Rivers and streams with high ecological value get placed in the 'very high distinctiveness' band. The metric also includes other rivers, streams and canals as separate categories within its framework. Ditches are artificial water channels less than 5 metres wide. They usually hold water for more than 4 months each year. While their main goal is to drain land, they stay connected to river systems either partially or fully. Using the watercourse metric, developers and conservationists can determine baseline biodiversity values of rivers and ditches and measure the potential net gain from proposed enhancements.
Read More: Biodiversity Net Gain
Assessing River and Ditch Condition for BNG
Two critical condition assessments needed for the BNG of watercourses are the River Condition Assessment (RCA) and Ditch Condition Assessment:
River Condition Assessment (RCA)
The river condition assessment (RCA) evaluates the ecological quality of rivers by analysing parameters such as:
- Flow dynamics – The presence of natural flow regimes that support biodiversity.
- Water quality – Levels of pollutants, nutrients and sedimentation.
- Bank structure – The stability and naturalness of riverbanks and riparian vegetation.
- Species richness – The presence of indicator species reflecting a healthy ecosystem.
Conducting an RCA helps ecologists to determine degrees of degradation and create goals for enhancement of biodiversity.
Ditch Condition Assessment
Ditches, although often overlooked, are significant for biodiversity. The ditch condition assessment method measures:
- Water quality indicators, including turbidity levels
- Presence of diverse plant types (emergent, submerged, and floating)
- Coverage of filamentous algae and duckweed (less than 10%)
- Marginal vegetation extent (minimum 75% coverage)
- Physical damage assessment (less than 5% damage threshold)
- Water level maintenance (50cm minimum in minor drains, 1m in main drains)
- Shading effects (less than 10% heavy shade)
- Absence of non-native species
For complex sites containing multiple habitat types, developers must achieve the mandatory 10% biodiversity net gain for each impacted habitat independently. Moreover, enhancement measures must align precisely with the affected habitat type, meaning woodland improvements cannot offset watercourse impacts. Improving ditch conditions can contribute significantly to overall biodiversity net gain watercourse targets, particularly in agricultural and peri-urban landscapes.
Step-by-Step Guide to Calculating BNG for Watercourses
Biodiversity net gain assessment for watercourses demands careful attention to detail and strict protocol adherence. A methodical approach will give accurate measurements and evaluation of biodiversity changes. BNG for watercourses is calculated by comparing the baseline biodiversity value with the projected post-development value. This involves:
Baseline Assessment:
- Conducting river condition assessment RCA and ditch condition assessment.
- Using the watercourse metric to establish existing biodiversity units. During assessment, surveyors must record: Pre-intervention baseline biodiversity units, post-intervention projected units and net changes in biodiversity value.
- Identifying key degradation factors.
Habitat Enhancement Strategies:
- Restoring riparian vegetation – Planting native species to stabilise banks and improve habitat diversity.
- Improving water quality – Implementing pollution control measures such as constructed wetlands and buffer strips.
- Enhancing flow regimes – Removing artificial barriers and restoring natural hydrological patterns.
Post-Development Monitoring:
- Ongoing assessment using the watercourse metric.
- Adaptive management to ensure long-term biodiversity improvements for watercourses in the habitat.
According to UK Government guidelines, a minimum 10% net gain must be demonstrated for all developments impacting biodiversity, including watercourses.
The Bottom Line
Requirements around BNG during developments continue to be improved upon and expanded. Though some developers may not typically factor water bodies into the BNG planning process, biodiversity net gain for watercourse is essential for sustainable development. By using tools like the watercourse metric, RCA, and ditch condition assessment, developers and conservationists can effectively measure and improve biodiversity outcomes. As watercourses face increasing pressures, proactive BNG strategies contribute to healthier ecosystems and compliance with UK environmental regulations. To learn more about BNG for watercourses, visit Tunley Environmental’s Biodiversity Net Gain page.