Update on the River Condition Assessment within Biodiversity Metric 3.0

With the release of Biodiversity Metric 3.0, a final stage has been added to the River Condition Assessment for unconfined or partly confined, alluvial rivers. This concerns the degree to which a river is hydrologically/ecologically connected to its flood plain.

Within the new guidance, Biodiversity Metric 3.0 Technical Supplement p.29 states,

‘a final stage is to consider the likely hydrological connectivity among the habitats that are present. If the surveyed channels are identified as being too deep relative to their width to be fully hydrologically connected, the final condition is downgraded from Good to Fairly Good or from Fairly Good to Moderate’ 

This page provides the reasons for introducing this new element of the River Condition Assessment part of the River Metric for BNG and guidance on how it should be applied:

Background:

Many streams and rivers within England, particularly in the lowlands, suffer from a legacy of channel modifications. One important outcome is the existence of channels that are ‘overdeep’ such that they are to some extent hydrologically and thus ecologically disconnected from their riparian margins and floodplains. Overdeep channels may support a diverse range of physical habitats but if these are hydrologically disconnected to some degree it reduces their potential to support biodiversity.

The recognition of an overdeep channel is challenging and is best undertaken by a geomorphologist based on field inspection and investigation of any accompanying data. However, over the last 12 months, we have analysed over 1600 Citizen Science MoRPh surveys to produce a simple means of identifying sites that are highly likely to be affected by overdeepening. The surveys selected for analysis possessed uploaded photographs that clearly illustrated the channel cross section and suggested that the recorded channel dimension measurements were sufficiently reliable for analysis. Using this data set, we have estimated some threshold values of the channel width to depth ratio that are indicative of the channel being overdeep or at least highly likely to be overdeep. We have incorporated the results of this analysis into a final stage of assessing the condition of a surveyed river.

How the new element should be applied:

1. This new final stage in the RCA is only applicable to intermediate to relatively low gradient, unconfined or partly confined, alluvial rivers. The assessment of overdeepening is relevant to single thread river types F, G, H, I, K, L; the main channel of multi-thread types E, J and M; and occasionally single thread type D (only when you consider it to be appropriate). This stage can also be applied to large or navigable rivers if you are confident that your estimates of water depth are sufficiently accurate to give a reliable outcome.

2. The MoRPh5 indicators generated by the information system include two river channel shape indicators: Average width, River shape. These are both calculated from the channel dimensions recorded in your 5 MoRPh Surveys:

Average width = Average MoRPh width

River shape = (Average MoRPh width) / (Average (water depth+lower bank height)).

NOTE that we use the MoRPh width rather than bankfull width as our measure of channel width because this is more likely to give an indication of the currently active channel width in human-modified rivers.

3. River shape is used to assess the likelihood of a surveyed channel being sufficiently overdeep to adversely affect its hydrological/ecological lateral connectivity:

  • If River Shape has a value of < 2 your river is almost certainly overdeep.
  • If River Shape has a value of < 4 your river is highly likely to be overdeep, especially if your average MoRPh width is greater than 10 m.

Whatever the value of the River Shape index, you should first use your professional judgement and consider whether this simple numerical estimate of overdeepening seems reasonable at your site.

4. If the presence of an overdeep channel seems to be a reasonable judgement, then the RCA for your site should be reduced by one class (e.g. from Good to Fairly Good, or from Moderate to Fairly Poor) when it is entered into the BM3.0 spreadsheet. At the same time, this reduction in the Final Condition Score also becomes an important indicator of how the river environment could be ‘improved’.