A Deep Dive into Watercourses and BNG

No, it should not be considered exempt. The fact that a stream now takes in surface runoff does not change its classification as a watercourse. Most streams and rivers across the country carry surface runoff, and this is a normal hydrological function. Historic mapping is a valid and recommended method for identifying watercourses, even if they are now culverted. Therefore, arguments suggesting exemption based solely on surface runoff are not valid and should be challenged with appropriate evidence.

There is currently no formal expiry date for RCA surveys. However, a resurvey may be necessary if there has been a material change in the environment or site conditions. 

If drainage infrastructure is required for the development, and the works fall under the criteria for inclusion within the red line boundary, then these areas must be included within the RLB. This applies even where the infrastructure lies outside the main development footprint.

Any works within the riparian use zone, typically 10 m from the bank top of rivers, streams and canals, and 5 m from ditches, are considered part of the development’s ecological impact and therefore must be included in the BNG metric. Outfalls, headwalls and associated pipework can affect the condition of watercourses and their terrestrial habitats, so excluding them from the RLB risks under-recording impacts and failing to meet BNG requirements.

Attempts to exclude drainage infrastructure that is functionally required for the development are likely to cause issues later, as land cannot be retrospectively added to the metric once permission is granted. Current guidance is that the red line should encompass all land to which the development relates, including underground infrastructure, to ensure that all ecological impacts are properly identified and assessed.

In cases where the red line boundary is too short to accommodate the full MoRPh5 survey subreach, the standard approach for BNG is to allow a degree of duplication to meet the minimum requirement of five modules for calculating a preliminary score. RCA assessors should refer to Modular River Survey and (other) BNG watercourse FAQ guidance on how to apply this in their calculations. For very short lengths, a MoRPh field survey may not be feasible. However, knowledge of the RCA method and professional judgement are essential and must be applied to enable a defensible estimate of river condition for the geomorphic river type being assessed. 

Yes, if a watercourse intersects with the red line boundary, even if no physical changes are proposed to it, it does trigger the need for a watercourse assessment under the statutory biodiversity metric. In such cases, the main metric must be used, not the small site metric. The small site metric only applies to canals, culverts, or ditches, not to other rivers and streams. Bridges crossing streams do not qualify as culverts unless they meet the specific definition (typically a pipe-like structure). Therefore, the applicant would need to:

• Correct the red line boundary to include the full access route.
• Use the main BNG metric.
• Engage a qualified ecologist to carry out a River Condition Assessment (RCA) and MoRPh survey.
• Purchase watercourse units if no enhancements are possible.

The need for the full metric reflects the ecological importance of short sections of watercourse.

BNG assessments can include enhancements on IDB-managed watercourses, but the periodic disturbance, such as bank cutting and silt spreading every few years must be factored into the condition scoring. For example, adding a grass margin may improve biodiversity, but the condition score should reflect the likelihood of disturbance, meaning only lower condition categories (e.g., modified or poor condition grassland) may be appropriate.

While tree planting may not be feasible due to IDB operational needs, sensitive management practices such as ditching from the centre without scraping the banks can significantly improve ecological value. Some IDBs are actively reviewing their management to support BNG outcomes, including reducing encroachment and ceasing maintenance in selected areas.

However, quantifying these improvements within the watercourse module can be challenging, especially when aligning them with specific condition criteria. Collaboration with IDBs and careful documentation of management practices can help ensure realistic and defensible scoring. 

If access to the watercourse is denied, the surveyor should still attempt to collect as much reasonable information as possible from the accessible bank, using tools like binoculars, aerial imagery, and historic river corridor or habitat surveys. The assessment should clearly note the limitations, including which bank was surveyed and any visibility constraints. While this may restrict the detail and accuracy of the RCA, it is acceptable to proceed with the best available data. In such cases, the precautionary principle may apply, meaning the watercourse could be scored more conservatively, which may impact the applicant’s BNG obligations, such as needing to purchase watercourse units.

You can download the reviewer checklist here: 

If you wish to have an accessible version of this document please contact [email protected]

These short videos have been created to provide quick and accessible tips for reviewers, when it comes to working with watercourses. There are currently five short, recorded bites, which can be accessed below; 

• This fact file provides guidance on when the watercourse module of the statutory biodiversity metric tool should be applied to local planning applications. 

• This fact file clarifies the fundamentals of the riparian zone, why it is essential to consider in the context of biodiversity net gain and highlights important considerations for local planning applications. 

• This fact file helps the reviewer understand habitat distinctiveness in relation to watercourses, showcasing essential resources to support in the identification of the different watercourse types. 

• This fact file provides top tips in identifying watercourse enhancement opportunities, taking into account historic use, considerations for constraints, potential gains from the removal of riparian encroachment and much more. 

• This fact file explains how to avoid missing opportunities when it comes to delivering net gain in relation to waterbodies. This short provides further insight into the importance of waterbodies, key considerations for climate change and best practice for identifying solutions earlier in the process. 

Please note this session references an outdated version of the reviewer checklist training (slide 23). 

In certain habitats, particularly fenland and other historically modified landscapes, sustained human intervention has resulted in a network of large man-made watercourses.

Large uniform land drainage channels in level landscapes perform a significant hydraulic function by facilitating water conveyance, alongside important anthropogenic land drainage functions. However, their biodiversity value is not altered by their role in water conveyance. Their habitat distinctiveness remains the same as other ditches or canals (i.e Medium Distinctiveness), being assigned by ecological qualities as opposed to hydrological function or hydraulic role.

Classifying whether a watercourse is a river or a ditch is not always straightforward, considering the following criteria could support in classifying ditches; please refer to Table 11- (Additional information to inform habitat classification) of the SBMUG. 

Springs are generally regarded as headwaters, although their precise position may shift in response to groundwater levels. Where no defined watercourse feature such as a bed or channel is present, springs should be mapped as wetland features. Where defined watercourse features are present the habitat should be recorded in the watercourse module. This will require careful professional judgement. 

Where a culverted watercourse lies within a site boundary, a 10% gain in watercourse units is required unless the development qualifies for exemption under the de-minimis exemption. Exemption is available only in specific circumstances: when the watercourse is not designated as a Priority River Habitat, when less than 25 sqm of terrestrial habitat is affected, and when the watercourse is not directly impacted by the development, for instance where there is no alteration in condition class or encroachment of less than 5m. In practice, this means that a site containing a culverted watercourse beneath an existing car park with no baseline habitat may be exempt from the requirement. However, BNG offers significant opportunity to daylight culverts for the benefit of biodiversity uplift and can deliver significant watercourse net gain.

The use of the Cartographer.io platform is not mandatory. Ecologists may produce their own RCA outputs by applying the published open access RCA scores. Further methodological detail can be found in this paper; Assessing river condition: A multiscale approach designed for operational application in the context of biodiversity net gain.

It should not be assumed that all rivers are priority habitats. To qualify, a river must demonstrate very high levels of naturalness based on the hydrological regime, water quality, ph, very high water quality, intact natural processes, and no impact from invasive species. Very few waterbodies in England meet this definition. A link to Natural England’s Priority River Habitats is included within the statutory biodiversity metric user guide,  and directly via the Natural England mapping pages:  Priority River Habitat - Rivers - data.gov.uk and Priority River Habitat - Headwater Areas - data.gov.uk

The PRH maps also contain a second, specific map layer of headwaters. The mapping indicates ‘areas’ rather than individual streams. The areas highlight that a PRH could be present, as headwaters are often overlooked as ditches. However, the data is based on crude land cover mapping and the streams need to be assessed and agreed as being highly natural, most usefully using the FBA hosted Naturalness Survey.  In this sense, the PRH headwaters layer acts as an indication that Priority headwater habitat may be present and further investigation is needed.

Watercourse biodiversity units currently command higher market values than terrestrial units. This is largely due to the greater design and engineering costs. Some biodiversity gain sites have expressed concern about the financial implications. In response, organisations such as Nature North are investigating blended finance approaches, where BNG is combined with other forms of investment to help manage these higher costs.

In locations where statutory obligations require the management of water levels, such as those overseen by Internal Drainage Boards, there may be little or no scope to alter river or ditch morphology. This constrains opportunities for delivering BNG on site. One potential approach is to focus on the riparian zone, where adjusting encroachment bands may provide some uplift. In certain areas, reduced management intensity has led to improved condition scores, although this may not be applicable in every drainage district. The Environment Agency and Natural England are actively exploring further options for circumstances where water control obligations apply.

Encroachment within riparian areas should be assessed according to the degree of impact, typically described as major or minor. This classification is primarily based on the percentage extent of encroachment. Land use change is a key factor in determining these categories; for example, the conversion of arable land to modified grassland would normally be treated as a reduction in encroachment.

If the red line boundary of a development lies within 10m of a river, the watercourse module applies even where the site watercourse baseline is otherwise zero, such as in the case of hardstanding. BNG applies unless the development is fully exempt. Developers unable to deliver uplift within the riparian zone will need to consider off-site units, and if there is no available off-site provision, then as a last resort option, statutory credits.

Where the river lies outside the red line boundary and physical access is unavailable, surveyors are expected to make reasonable efforts to collect information. This may include the use of binoculars, aerial imagery, or reference to existing data such as River Habitat Surveys or River Corridor Surveys, noting the date and relevance of such information. Any access restrictions must be clearly recorded in the assessor’s notes, and no biodiversity gain should be assumed for areas where assessment is incomplete.

Private gardens adjacent to riparian zones present a particular challenge. While gardens may contribute positively to biodiversity, the metric tool requires them to be recorded simply as vegetated or unvegetated areas. Habitat creation within private gardens cannot be relied upon because long-term management cannot be secured through planning obligations. For this reason, private gardens are generally considered a site constraint in the context of River Condition Assessment and BNG.

Where long-term management of riparian or adjacent habitats cannot be guaranteed, a precautionary principle should be applied. Planning authorities may reasonably assume that some degree of encroachment into the waterbody will occur during the 30-year management and monitoring period. Applicants may therefore be encouraged to account for a higher level of encroachment at the outset to provide a robust assessment and to avoid underestimating future impacts.