Geomorphological Assessment Procedure (GAP)

In the UK, agencies working on behalf of government bodies, river trusts, and academics, have provided the most comprehensive and applied set of procedures in the application of fluvial geomorphology.  The Geomorphological Assessment Procedure (GAP) provides an overarching framework that draws together the different procedures, expected outputs and comprehensive geomorphological evaluations of geomorphic surveys.

Depending on the scale, hydrogeomorphological focus, and level of detail required for a specific investigation, different elements of the GAP framework can be used, ranging from more simplistic methods  (i.e. Catchment Baseline Study or Fluvial Assessments) to a full Geomorphological assessment, design and post-project appraisal (Sear, Newson and Thorne, 2010). CBS’s and FA’s provide information on the sensitivity and value of a channel system to disturbance and sediment dynamics.

Within the planning stage, the CBS and FA offer geomorphological assessments of the catchment, while the GAP framework encourages the use of Geomorphological Dynamic Assessments to provide quantitative guidance on the interaction of catchment principles (e.g. stream power, sediment transport and bank stability processes) to understand these reach dynamics and how they include morphology directly. This is achieved using GIS to aid in visualisation, analysis and to provide a relationship database (Sear et al., 2009).

Geomorphological dynamic designs and appraisals can then be used to design the most appropriate form of intervention and assess the degree of compliance between the design and the expected outcomes of the projects, respectively (Sear, Newson and Thorne, 2010). The use of GIS allows for visualisation of the effects of change, prior to implantation, from a variety of stakeholder viewpoints and between competing conceptual models of restoration (Figure 1).

Figure 1. The GAP assessment workflow (Sear et al., 2009). 

A key component of GAP is the use of Multi-Criteria Assessment (MCA), based on key channel processes forms, and habitats associated with a given channel type. The MCA is used to classify the retrieved data into reach-based indices of river modification, geomorphological function and degree of naturalness (Sear et al., 2009). However, naturalness can be difficult to define in the absence of accurate historic data or may be unachievable given the radical changes in land use and river utility, which may conflict with the aims of the project.

The MCA utilises combinations of spatial data, which are scored and weighted, per reach, to determine their importance when designing the most appropriate action. Wider issues influencing the degree of naturalness of the channel are analysed, prior to those that influence the less natural or more modified reaches. This is because the more natural reaches are considered to have a greater degree of sensitivity to change and have the greatest conservation value (Sear et al., 2009).

River – Morphological Impact Assessment System (MImAS)

MImAS (SEPA, 2012) is a qualitative tool used by the Scottish Environment Protection Agency (SEPA) (in conjunction with the centre for Ecosystem Science) to aid decision makers in determining the potential impacts of hydromorphological alterations (UKTAG, 2008).

This tool allows regulators to identify impacts, such as those that are non-compliant with wider EU and UK regulations and directives (i.e. ‘good ecological status’ WFD). MImAS also identifies interventions that are likely to result in environmental deterioration, or to ascertain whether intervention is appropriate and if so, on what scale. The MImAS technique aims to define the reach-sale hydrogeomorphological characteristics of a river, by incorporating the Fluvial Audit Methods, to aid in the implementation of the WFD (Sear, Newson and Thorne, 2010).

The River-MImAS includes a variety of ‘modules’ which cumulatively provide an assessment of the risk of impacts upon the rivers geomorphological conditions, under different combinations of interventions. These modules are centred around the morphological quality elements described in the WFD).

This tool assumes that there is a predictable relationship between the degree of morphological perturbation and the impact of the change upon the ecological status. The MImAS tool uses the concept of System Capacity which describes a system whereby if capacity is consumed by human activities, it follows that there will be an increased risk that morphological and ecological conditions will degrade (Williams and Cooper, 2012; Centre for River Ecosystem Science, n.d.). System capacity varies between catchments and their sub-scales, and is dependent upon the type of alteration and its spatial scale, as well as the systems sensitivity to change (Sear, Newson and Thorne, 2010). This concept is informative for decision makers as to where more detailed assessment must be undertaken. This concept assumes that the system can accommodate change or has the ability to change at all (Figure 2).

Figure 2. Summary of the MimAS system workflow (SEPA, 2012).

These concepts of degree of channel resistance (the ability of a channel features to remain unchanged under pressure) and resilience (the ability of a channel to recover following disturbance) are closely linked to channel type and are explored further in River-MImAS (Sniffer, 2006).

The MImAS tool is underpinned by a set of assumptions (as described in Williams and Cooper (2012)):

• The system has some capacity to accommodate geomorphological change without detriment to its ecological status
• Morphological alteration and impact on ecological change are related
• There is predictable morphological response to change when the channel is modified (pressure applied)
• There is predictable ecological response to change, depending on the channel sensitivity.

In order to quantify this risk (where intervention could result in a negative impact upon achievement of the objectives described in the WFD), a set of ‘Morphological Condition Limits’ (Williams and Cooper, 2012) have been formulated. Exceedance of these limits suggest a risk to the ecological status of the water environment.

As described by UKTAG (2008) 5 interrelated modules are used collectively, to provide an assessment of the risk of impacts upon the geomorphological conditions, in terms of impact to the system capacity. These five modules include:

1. The Attribute Model – channel attributes such as substrate condition, structure of the riparian environment and bedforms, are used to assess morphological and ecological function. This is because the form of the channel and its catchment indirectly supports the ecological communities.
2. The Typology Module – The channel typology is also a key factor in its system capacity. Within this module, the channel is delineated into 6 categories according to their form (e.g. bedrock, pool-riffle, plane bed, wandering, low gradient, meandering etc.) (see UKTAG, (2008) table 36 for further detail).
3. The Sensitive Module – This module comprises two parts – the ecological and morphological sensitivities. This model considered the ability of the channel to both absorb change and recover from it (its sensitivity). The ecological aspect assesses the degradation of the catchment species community as a result of disturbance. The ecological assessment considered the WFD quality elements.
4. The Pressure Model – The pressure of the alteration; that is the likelihood that morphological change will have impact on the water environment and the spatial extent of this pressure. 15 pressures are considered and include factors such as: the presence of engineering, dredging, culverting or their management.
5. The Scoring System Module- This module collates the aforementioned information and calculates a specific numerical ‘impact rating’ for each channel type. This impact rating is determined for each attribute, averaged within the different channel sections  (i.e. channel, bank and riparian zones), then multiplied by the zone of impact, resulting an overall impact rating per morphological intervention. These can then be compared across different alteration proposals.

Morphological Condition Limits are then defined for each channel section to determine which sections of channel are likely to be impacted by the alteration (UKTAG, 2008).

However, this assessment is not appropriate on channels less than 5km long, given that the capacity of a water body to accommodate change depends on the morphological condition of the wider catchment area, and should not be considered in isolation. Further, this framework does not act as an engineering design tool nor does it define remediation options.

The River-MImAS tool has also been elaborated upon to include assessment of the effects of morphological alterations within transitional and coastal (TraC) waters (TraC-MImAS). These additional components are specific whilst still sharing the ultimate aim of aiding fluvial environments in achieving good ecological status and to reduce the risk of deterioration in ecological status, as a result of intervention (Williams and Cooper, 2012).

A multitude of assessments are available to provide a holistic approach in the management of fluvial environments. These assessments go above and beyond the collection and analysis of spatial data, to provide methods and roadmaps for the implementation of well considered management and restoration.

 

References

Centre for River Ecosystem Science, (n.d). Services. [online] Cress.stir.ac.uk. Available at: <http://www.cress.stir.ac.uk/what/services.html> [Accessed 18 May 2021].

Greig S.M., Richardson R. and Gibson J. (2006). A new impact assessment tool to support river engineering regulatory decisions: SNIFFER Technical Report. Project No. WFD49.

Sear, D., Newson, M. and Thorne, C., (2010). Guidebook of Applied Fluvial Geomorphology. London: Thomas Telford, pp.32-134.

SEPA, (2012). Supporting Guidance (WAT-SG-21) Environmental Standards for River Morphology. [online] sepa.org.uk. Available at: <https://www.sepa.org.uk/media/152194/wat_sg_21.pdf> [Accessed 28 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

UKTAG, (2008). UK ENVIRONMENTAL STANDARDS AND CONDITIONS (PHASE 1) Final report. UK Technical Advisory Group on the Water Framework Directive. [online] Available at: <https://www.wfduk.org/sites/default/files/Media/Environmental%20standards/Environmental%20standards%20phase%201_Finalv2_010408.pdf> [Accessed 18 May 2021].

Williams, M. and Cooper, N., (2012). TraC-MImAS Technical report. Development and Review of a TraC Hydromorphology Decision Support Tool for (a) screening proposed new or altered activities / structures for compliance with WFD water body status and (b) classifying TraC waters under the WFD. [online] pp.4-7. Available at: <https://www.wfduk.org/sites/default/files/Media/TRaC-MImAS%20Technical%20Report%20201212_rev1.PDF> [Accessed 18 May 2021].