Examples of the Fluvial Audit

The following case studies provide examples of the how the FA methodology has been deployed, in a variety of catchments. The FA method has provided a framework for land owners, private consultants, government bodies, and agencies to identify areas of sensitivity and any potentially destabilising phenomena. Using a holistic, catchment-based approach, geomorphological issues can be identified and remediated, in order to improve the ecological status of channel reaches, prior to, and following intervention. The examples demonstrate the application of the FA approach by river trusts, agencies, and through academic investigation.

Example 1 – By Brook catchment – Bristol Avon Rivers Trust

In 2014, a fluvial audit was conducted by Bristol Avon Rivers Trust and the Environment Agency (EA) within the By Brook catchment (Gloucestershire, UK) (Figure 1). The aim of the audit was to identifying and classifying both diffuse and point sources of pollutants (e.g. phosphates and ammonia). Barriers to fish migration within the catchment were also sought, in line with the Water Framework Directive.

The FA allowed for the classification of the watercourse ecological status, relative to the WFD. Many of the reaches were classified as failing with the remainder classified as having moderate or poor ecological status.

45km of the channel were surveyed using a field survey, through which 486 sources of both point and diffuse sources of pollution were identified. 640 features were also identified, including barriers to fish passage, large woody debris, and depositional features. These geomorphological issues were considered to prevent specific reaches of the catchment from achieving a good ecological status, as the physical habitats these reaches supported were of a heightened sensitivity to changes in both geomorphological and hydrological parameters.

From this FA, four key issues were identified, which have the potential to cause detriment to the catchment. These included: the presence of in-channel structures, the introduction of fine sediments and nutrients, and bank erosion. As a result, potential remediating interventions have been considered and these include: softbank revetment, livestock fencing, bank re-profiling, removal of fish barriers, and public engagement.

Figure 1. The By Brook river (Biggane, 2016)

Example 2 – The Sarre Penn – Kent, UK

In 2014, the South East Water’s Water Resources Management Plan proposed for the development of a reservoir, dam, and water treatment works (Figure 2).

The Sarre Penn, a tributary of the Stour River, is a ephemeral gravel stream, activated during high flows. 2km of the Sarre Penn is encompassed within the original design of the reservoir. Development of this region was described as being potentially detrimental to the condition of the riparian habitats.

A Fluvial Audit of the site was conducted in order to gain a baseline characterisation of the entire catchment, identify areas of sensitivity to change, understand the sediment dynamics of the system, and to determine the channels degree of deviation from a state of naturalness.

Figure 2. Preposed plans for the(South East Water, 2016)

A desk based study was undertaken, using aerial photography, which was then supplemented with a field reconnaissance survey. During this survey, a variety of geomorphological potentially destabilising phenomena were described, these include: confinement of the channel, winnowing of fine sediments with increasing distance downstream, heavy modification of the channels natural form, immobility of the channel bed, and intermittent and unsustainable sediment sources. Without consideration of these geomorphological sensitivities, any potential interventions, management and rehabilitation measures would cause significant degradation to the system.

The results of the FA provided an understanding of the catchment and its dynamics. This information provided a basis to identify the most sustainable methods for ensuring the Sarre Penn retains its WFD ecological status. The most appropriate intervention has been described as the implementation of a diversion channel, of which will ensure continuity of the stream and promote a healthy habitat function. The implementation and encouragement of depositional features was also suggested, in order to mimic the natural channel processes and forms (i.e. promote a sinuous low flow channel).

Example 3 – Fluvial Audit and multi-criteria assessment, Nar River, East England. (Sear et al., 2009)

This example describes how a multi-criteria assessment (MCA) process can be used, in addition to a Fluvial Audit. Filed surveying and desk based analysis of available catchment data are classified to create indices that quantify parameters such as: channel modification, sediment flux (storage and sources of sediment), and the degree of channel naturalness. The MCA is centred upon the use of a conceptual model of the key forms, processes, and subsequent habitats associated with a specific channel and catchment. A catchment wide restoration strategy was sought from the results of this framework. For more information on MCA’s, see here.

This approach was applied to the River Nar; a groundwater dominated channel, situated in eastern England (Figure 3). The Nar, a tributary of the Great Ouse, is a classified Site of Special Scientific Interest, as a result of its chalk-stream geology, giving rise to habitats of low-gradient river fauna and flora. The channel is characterised with a low relief and considered a low energy river sediment system.

Figure 3. The River Nar (Norfolk Rivers Trust, 2021)

Using a desk based assessment, field survey, and the MCA approach, a conceptual model for the River Nar was developed in order to explain how the channel form had developed over time and to identify the factors responsible for these changes (Stewardson and Rutherfurd, 2008).

This model found that wide scale, quaternary geological processes and land use governed the evolution of the channel. Adjustments in channel gradient, slope and aspect also governed the geomorphology of the channel and its resultant form. The dominant hydrological and geomorphological processes were determined using this model.

In terms of naturalness, only 8 of the 76 reaches along the channel were considered to be unmodified from the conceptual natural state. Sources, storages, and sinks of sediment were also identified, with the headwaters producing a high quantity of fine sediments as a result of bank erosion and agricultural intensification, with a lack of mobile gravel sediment. There is also sedimentological evidence of channel confinement, preventing the lateral movement of the channel, hence its dependence on groundwater sources.

This study also describes the road network as being fundamental within the Nar catchment and must be considered when undertaking restoration and management of the channel.

This approach focuses on the rehabilitation of the channel towards a more conceptually natural state, of which is determined using a conceptual model. This theoretical natural reference condition was determined using physical habitat and geomorphological data, informed from secondary sources and fieldwork.

MCA was then applied using these criteria, to the Fluvial Audit database, providing a score for each reach according to the extent to which the present conditions matched the definitions of natural reference condition.

Relative to the model reference conditions, it was found that nearly 50% of the length of the channel was degraded, with only 23% considered to be pristine, near natural, recovered, or recovering towards a natural geomorphological state. As a result, a variety of management and restorative techniques were proposed. The majority of these approaches focus on:

The natural removal of fine sediment deposits
Removal of channel vegetation re-cutting the channel
Establishing riparian woodland
Reconnection of the floodplain
Introduction of wood into the channel
Raising the bed elevation

By undertaking these practices, the reach status can be improved, compared to the natural reference model. This results in a well-functioning, chalk stream habitat. A catchment monitoring plan was also devised to ensure that any changes in the status of all near-natural/ natural, and recovering reaches were kept track of, following the implementation of the restoration techniques.

References

Biggane, D., (2016). By Brook walk in Box by Nigel Vile. [Blog] Bath Chronicle, Available at: <https://www.bathchronicle.co.uk/whats-on/whats-on-news/brook-walk-box-nigel-vile-34723> [Accessed 4 February 2021].

Norfolk Rivers Trust, (2021). River Nar. [image] Available at: <https://norfolkriverstrust.org/rivers/river-nar/> [Accessed 13 May 2021].

Roberts, S. and Kemble, K., (2016). Geomorphology Fluvial Audit. Study of Interaction between Broad Oak Reservoir and Richborough Connection Project. [online] Reading: Jacobs, pp.3-22. Available at: <https://infrastructure.planninginspectorate.gov.uk/wp-content/ipc/uploads/projects/EN020017/EN020017-001344-South%20East%20Water%20-%20Appendix%2010(Part%2013)%20%20to%20WR%20-%20Appendix%20C%20of%20Stage%201b%20report.PDF> [Accessed 11 May 2021].

Sear, D., Newson, M., Hill, C., Old, J. and Branson, J., (2009). A method for applying fluvial geomorphology in support of catchment‐scale river restoration planning. Aquatic Conservation: Marine and Freshwater Ecosystems, 19(5), pp.506-519.

South East Water, (2016). Broad Oak Reservoir Development of the Concept Design for WRMP19. [online] p.13. Available at: <https://infrastructure.planninginspectorate.gov.uk/wp-content/ipc/uploads/projects/EN020017/EN020017-001440-South%20East%20Water%20-%20Appendix%2016%20for%20WR%20-%20Slides%20for%20SEW%20meeting%20with%20NE_17th%20May%2016.PDF> [Accessed 13 May 2021].