What is a Fluvial Audit?

Hydrogeomorphology is a key parameter when trying to understand, preserve or improve a riverine environment as the quality of channel ecology is sensitive to changes in hydrogeomorphology (Rinaldi, et al., 2017).

Predicating the spatial and temporal scales of adjustment, can be difficult, as a multiple of parameters and variables are able to reposed to a change with varying spatial and temporal scales.  Complex responses can even arise and differ across morphologically similar reaches (Rinaldi, et al., 2017). Hence, being able to identify attributes within a channel or catchment, that are sensitive or induce change, is imperative for channel management. This complex response makes it difficult to determine where and when an adjustment will occur.

The role of the Fluvial Audit (FA) has developed from a problem-solving assessment to encompass a strategic integrated approach at a catchment scale, to support a range of channel management and conservation (Sear et al., 2008).

Figure 1 describes the workflow of undertaking a FA. The desk-based study forms an initial part of the FA and is ideally undertaken prior to the field reconnaissance survey (catchment baseline survey) to provide an understanding of the catchment and any key issues. The DBS also clarifies where further data is needed or where the FA would benefit from gaining a further understanding of the physical, localised channel dynamics.

Figure 1. Fluvial Audit work flow (Sear et al., 2009)

Developed in the late 1980’s by Newson and Bathurst, (1988), The Fluvial audit (FA) presents a geomorphological, qualitative data collection and presentation framework, designed to assess sediment-related issues within a wider catchment context (Eyquem, 2007). The channel is delineated into reaches that act as sources, stores and transportation pathways of sediment (Charlton, 2008; Sear et al., 2009)

Expanded upon and pioneered by the Environment Agency and Sear et al (2003), this widely applied processed-based approach is performed at the catchment level, and aims to determine and address sediment related issues. This is to facilitate a sound geomorphological understanding of the catchment dynamics, required for a sustainable solution (Soar et al., 2017; Sear, et al., 2009; 2006; Thorne et al., 2010). The third iteration of the FA aims to aid in identifying and assisting the restoration of degraded fluvial habitats, in conformity with the European Union Habitats Directive (92/43/EEC) and national legislation regarding the protection of Sites of Special Scientific Interest (SSSI)

The FA provides a semi-quantitative understanding of the wider sediment budget and geomorphological processes that induce instability. This is undertaken by understanding these issues within the wider context of the channel history, contemporary  changes and processes (Sear et al., 2003) but can be implemented at smaller, channel project reach scale. Since its origin, the FA has been developed to support localised, reach based river restoration (Sear, 1996; German and Sear, 2003), bank erosion management (Atkinson et al., 2003) and strategic environmental assessment (German et al., 2000).’ Sear et al., 1999. (Environment Agency, 2005)

This technique offers a flexible approach in determining the sediment-related status of each reach, as the FA can be tailored to the requirements of each river whilst still being standardised between reaches and comparable to other assessments (Sear et al., 2009; 2008).

The process involves breaking down the watercourse into homogenous geomorphological reaches, based on parameters such as: Bank material, Channel modification, Cross-sectional profile, Gradient etc. (Environment Agency, 2005; Sear et al., 2009).

The FA requires detailed field surveys, of which are mapped for analysis (Sear et al., 2008). The magnitude and location of channel instability (areas of incision, aggradation or stability) are identified. This information is pivotal in characterising the sediment regime of the channel (Kondolf and Piégay, 2003; Wallerstein et al., 2006) An important aspect of FA is the contextualisation of the present geomorphological processes within the history of the catchment (including previous intervention and management). Digitized Historical data sets are used in conjunction with the field detailed surveys, meteorological and land use data, to provide an in depth understanding of catchment dynamics, across all process pathways to assess temporal changes in river variables (Wallerstein et al., 2006). The FA has made use of the rapid developments in data acquisition tools and processing, to support catchment scale assessment of point and diffuse sediment sources (Thorne et al., 2010).

Remote sensing methods are used to supplement the desk-based investigation, through the provision of aerial photography, terrestrial lidar, satellite imagery  and bathymetric data, where available. Whilst some data sets are open source, provided by government bodies on a wide variety of catchments and individual water courses (flow rates, stage height, imagery etc.), other data sets are not easily attainable (e.g. sediment calibre, channel geometry). The gaps in knowledge can be filled through the collection of data, first hand in the field, subject to the access of appropriate measuring equipment.

This process identifies a range of ‘Potentially Destabilising Phenomena’ (PDP’s) for sediment-related river management issues (Sear et al., 2009) These are factors that could have significant effect on channel stability as a result of the culmination channel process. PDP’s can also be transient, having acted in the past present or future with varying magnitudes. Table 1 describes some common PDP’s.

Table 1. Common PDP’s  (adapted from Thorne et al., 2010).

Since the advent of international legal frameworks (e.g. Habitats directive 9/43/EEC), on the protection of some riverine habitats, a greater consideration for the relationship between channel hydrogeomorphological and sediment dynamics and these protected sites of special scientific interest (SSSI) and special areas of conservation (SAC). The focus of FA’s now centres on the effect and extent of modifications to the channel, and hence the habitats it supports as well as the best possible management practices of maintain favourable habitat conditions (Thorne et al., 2010).

However, elements of the FA are limited by the availability of accurate, spatial data. As a result, misinformation or inaccuracies can result in inappropriate design. At worst, this can result in negative or unwanted hydrogeomorphological impacts (Soar et al., 2017). This is most pronounced risk in catchments that are poorly understood (e.g. pristine groundwater dominated rivers), perhaps due to their rarity (e.g. low energy, natural clay streams) (Thorne et al., 2010).

When used together, the Fluvial Audit and CBS, offer a standardised approach for geomorphological assessment, investigation and visualisation Seat et al, (2003), however the detailed field reconnaissance and transcription of this data, should be undertaken by an experienced fluvial geomorphologist. This may add an increased level of cost or make the assessment difficult to deploy in projects with limited experience of geomorphological experience (Skinner and Thorne, 2005; Joyce, 2020).

Indicative features of channel morphology are used to assess the degree and location of channel stability and delineate these reaches for the most sustainable approach to channel management and restoration. Record sheets are used to standardise the process, detailing the cross-section data, evidence of instability and its likely causes, maintenance activities etc. a variety of stakeholders are employed to ensure a broad baseline knowledge is ascertained regarding the history and desired future of the channel morphology.

The FA method provides no measure of the magnitude nor scale of the channel stability between the identified reaches (Wallerstein et al., 2006). Hence an objective, hierarchal framework to tactically address and remediate channel instability cannot be derived from an FA, as so with other broad-scale approaches to assess catchment morphology (e.g. River Styles Framework (Brierley and Fryirs.,2013; Soar et al., 2017; Parker, 2010).

These assessment result in a compilation of spatial materials within a GIS, describing the inter-relationships of channel parameters and their susceptibility (and sensitivity) to change and are commendations for future work or continued assessment. From these outputs, the sources and causes of sediment-related issues can be identified and sustainable solutions sought. This may be presented as slope maps and areas of vulnerable to erosion, and Reports detailing findings, interpretations and recommendations etc. (Skinner and Thorne, 2005).

Ultimately, the objectives of the FA are independent to each project. However, the overarching aim of FA’s is to provide a synthesis of information and reference to aid in the implementation of sustainable and ecologically beneficial management actions. This is ultimately sought to adhere to national and international policies, of which are centred on improving and managing rivers and surface water (Thorne et al., 2010). 

 References

Atkinson, P.M., German, S.E., Sear, D.A. and Clark, M.J., (2003). Exploring the relations between riverbank erosion and geomorphological controls using geographically weighted logistic regression. Geographical Analysis, 35(1), pp.58-82

Brierley, G.J. and Fryirs, K.A., (2013). Geomorphology and river management: applications of the river styles framework. John Wiley & Sons.

Environment Agency, (2005). Method Description (Report A). Fluvial Audit – A Method for Catchment-Scale Geomorphological Assessment. [online] pp.2-19. Available at: <https://www.therrc.co.uk/sites/default/files/files/Designated_Rivers/Axe/fluvial_audit_-_method_description_-_report_a_-_final_a01.pdf> [Accessed 19 March 2021].

Eyquem, J., (2007). Using fluvial geomorphology to inform integrated river basin management. Water and Environment Journal, 21(1), pp.54-60.

German, S E (2000) Bank erosion processes and river bank management on the Afon Dyfi, Wales, Unpublished PhD thesis, Dept of Geography, University of Southampton, 377pp

German, S.E., & Sear, D.A. (2003). Geomorphological Audit of the River Wylye. Conserving Natura 2000 Rivers. Conservation Techniques Series No.9. Peterborough: English Nature.

Joyce, H., (2020). A reach, catchment and multiple catchment scale assessment of the patterns and controls of historic upland river planform adjustments (Doctoral dissertation, Durham University).

Kondolf, M. and Piégay, H., (2003). Tools in Fluvial Geomorphology. 1st ed. Chichester: J. Wiley, pp.643-644.

Newson M D and Bathurst J C, (1988), Sediment movement in gravel-bed rivers. Seminar Paper 59, Department of Geography, University of Newcastle upon Tyne

Parker, C., (2010). Quantifying catchment-scale coarse sediment dynamics in British rivers (Doctoral dissertation, University of Nottingham).

Rinaldi, M., Belletti, B., Bussettini, M., Comiti, F., Golfieri, B., Lastoria, B., Marchese, E., Nardi, L. and Surian, N., (2017). New tools for the hydromorphological assessment and monitoring of European streams. Journal of environmental management, 202, pp.363-378

Sear D A (1996). Sediment transport in pool-riffle sequences. Earth Surface Processes & Landforms, 21, 1996, 147 – 164.

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.

Sear, D.A. and Newson, M.D., (2003). Environmental change in river channels: a neglected element. Towards geomorphological typologies, standards and monitoring. Science of the Total Environment, 310(1-3), pp.17-23.

Sear, D.A., Hill, C.T. and Downes, R.H.E., (2008). Geomorphological assessment of riverine SSSIs for the strategic planning of physical restoration. Report NERR013. Natural England Research.

Sear, D.A., Newson, M., Old, J.C. and Hill, C., (2006). Geomorphological appraisal of the River Wensum special area of conservation. English Nature Research Reports, 685.

Skinner, K. and Thorne, C., (2005). Review of Impact Assessment Tools and Post Project Monitoring Guidance. Nottingham University Consultants.

Soar, P., Wallerstein, N. & Thorne, C. (2017), “Quantifying River Channel Stability at the Basin Scale”, Water (Basel), vol. 9, no. 2, pp. 133.

Thorne, C.R.; Soar, P.J.; Skinner, K.S. Characterising and managing river sediment dynamics. In Guidebook of Applied Fluvial Geomorphology; Sear, D.A., Thorne, C.R., Newson, M.D., Eds.; Thomas Telford: London, UK, 2010; pp. 120–195.

Wallerstein, N.P., Soar, P.J. and Thorne, C.R., (2006). River Energy Auditing Scheme (REAS) for catchment flood management planning. Proceedings of the IAHR River Flow, Lisbon, Portugal, pp.6-8.