The Field Survey
The Fluvial Audit deploys a multifaceted analytical approach, drawing upon archival, secondary, and primary information sources. This is undertaken to gain a broad and detailed understanding of the wider catchment sediment dynamics. This information is usually obtained through a Catchment Baseline Survey (CBS). Investigation and interpretation of the historical and contemporary evolution of the catchment and its sediment storage and transfers, is also required.
This information is compiled within a CBS analysis, and further supplemented using a field survey. These components of the FA provide an understanding of how the catchment, channel, and floodplain have changed through time and given rise to the contemporary processes and form of the catchment (Sear and Newson, 2003). These changes may have been induced by Potentially destabilising phenomena (PDP’s) (see here for more information).
The channel can then be classified into reaches of varying degrees of sensitivity to disturbance. This classification observes the existing sediment dynamics, any in channel structures that impede the passage of sediment, as well as the divergence of the channel from its natural form. The FA also provides a structure to assess the degree of ‘naturalness’ of a channel; that is its deviation from a conceptual baseline form and processes and function. This is achieved per reach length.
The FA also seeks to accurately classify each reaches’ vertical stability; that is whether it is stable, agrading or incising. This is achieved by identifying forms and processes, within the field, that are indicative of channel stability. Table 1 describes indicators of channels stability that can be identified in the field, during a field reconnaissance survey (as described in Sear, Newson, and Thorne, 2010).
Table 1. indicators of channels stability (as described in Sear, Newson, and Thorne, 2010).
| Category | Upland (Source) | Middle (transfer) | Lower (sink) | ||||||
| Evidence of Incision | bank failures (both banks) | ||||||||
| thick gravel exposure in the banks overlain by fines | |||||||||
| undermined structures | |||||||||
| old channels in floodplain | |||||||||
| armoured/ compacted bed | |||||||||
| exposed tree roots | |||||||||
| tree collapse | |||||||||
| old slope failures | downed trees in channel | narrow/deep channel exposed tree roots | |||||||
| narrow/deep channel | tree leaning towards channel | tree leaning towards channel | |||||||
| Perched boulder berms | Terraces | compacted bed sediments | |||||||
| Evidence of Aggradation | Buried structures | ||||||||
| buried soils | |||||||||
| eroding banks at shallows | |||||||||
| contracting bridge openings | |||||||||
| deep fine sediment overlying coarse particles in bed/banks | |||||||||
| many compacted ‘overloose’ bars | large, uncompacted bars | many compacted ‘overloose’ bars | |||||||
| many unvegetated bars | many unvegetated bars | ||||||||
| Evidence of Stability | Vegetated bars and banks | ||||||||
| Compacted, weed covered beds | |||||||||
| bank erosion is rare | |||||||||
| old structures in position | |||||||||
| no evidence of change from old maps | |||||||||
| well established trees on banks | |||||||||
| Little large woody debris | |||||||||
River reconnaissance surveys have historically been use to provide a framework to assess channel processes and forms within the field and to present a standardised format to collect both quantitative and qualitative data (e.g. (Thorne, 1998, Natural England’s “Proposed Guidance for the restoration of physical and geomorphological favourable condition on river SSSIs in England, The River Habitat Survey (RHS) (Environment Agency, 2003). River surveying sheets have long been used compile data for the purpose of a CBS which then can be used to supplement an FA (Thorne 1998).
Ultimately, this data provides the basis of geomorphological classification which can be used to support sustainable riverine engineering and management (Thorne, 1998).
Using a field reconnaissance surveying form, a detailed account of the channels fluvial and sedimentological form and processes can be measured and assessed. This standardised method provides a semi-quantitative approach in documenting, specifically, the channel and its floodplain.
Repeat surveying also allows for any observations, made during the desk based analysis, to be corroborated and assessed through field surveying.
Whilst many of these surveys focus on the assemblages of habitats located within the reach, fewer provide a framework for the in depth understanding of sediment fluxes and storage. This dynamism is a fundamental concept within the FA.
The following section outlines the specifics of the proposed field surveying methodology.
Field Reconnaissance Sheet
A customised Field Reconnaissance Logging sheet has been adapted from Thorne (1998) and The Environment Agency to provide a template to document the observed hydrogeomorphological and sedimentological forms and processes operating within the Cuckmere river and floodplain.
For each geomorphically distinct section of the river (a reach), a separate form should be completed. Unlike the RHS, a reach is not defined by length, instead by the characteristics of the channel (e.g. its form or the dominant processes that can be observed).
The survey should be conducted during the UK summer period (June- August) to assess low flow channel form and processes, and again during the high flow period (December-February), within the water year. This will ensure the majority of typical processes and forms are documented to gain a better understanding of fluvial dynamics. Further, in the tidally influenced reaches, the channel should be surveyed following both spring high and low tides. This ensures the channel is surveyed during the two most extreme fluvial cycles, which can provide insight into how different flow patterns and stages can influence the channel geomorphology. See the Surveying plan sheet and draft risk assessment for further information.
Once compiled, reconciled and digitised, the information gained from these surveys can be imported and analysed within a GIS, to supplement the desk based analysis.
References
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., Newson, M. and Thorne, C., (2010). Guidebook of Applied Fluvial Geomorphology. London: Thomas Telford, Pp.22-134.
Thorne, C., (1998). Geomorphological stream reconnaissance handbook. New York: John Wiley, pp.28-107.