Assessing the Fluvial System

Ultimately, the geomorphology of the fluvial system is dictated by the abundance, availability, and transportation of both sediment and water (Sear, Newson and Thorne, 2010). Downstream channel form is dependent on the upstream adjustment – this relationship between  channel form and the upstream storage of sediment, acts independently to the wider catchment (Sear, Newson and Thorne, 2010).

This concept can be extended, in that these reach-scale processes are contextualised within their catchments, of which have inherited their past geomorphological process. Hence, the form of the channel is partially determined by the boundary conditions of the channel itself (e.g. geologies, climatological, slope) (Sear, Newson and Thorne, 2010).

Localised channel controls induce changes within a catchment, a proportion of which cannot always be anticipated (Sear, Newson and Thorne, 2010). This induced change in known to differ depending on not only the natural of the local control measure, but the channel type (e.g. high energy channels, lowland rivers, confined channels with cohesive sediment etc.). To understand the induced change, a baseline understanding of the channel’s propensity, susceptibility and resilience to changes, in the availability of sediment and discharge, should be understood (Sear, Newson and Thorne, 2010). A multitude of frameworks have been developed to aid in the identification and understandings of factors that contribute to the propensity and resilience, to strategically assess catchment status and to provide the most appropriate treatment and maintenance (Environment Agency, 2005).

The supply, transportation, and storage of sediment interacts at both localised (reach) and broad (catchment wide) scales. Hence, the entirety of the catchment, must be considered when any intervention is considered. The catchment equilibrates the fluxes in sediment through changes at the reach scale. As a result, many frameworks survey and monitor changes at the reach scale.

Let’s try a New Approach!

In the UK, river maintenance projects have previously relied on the experience and knowledge of practicing engineers, with many projects treating only the symptoms of fluvial issues rather than their root causes (Sear, Newson, and Brookes, 1995). The inability of these practitioners to recognise the catchment interactions relating to sediment supply and their threshold, has undermined past projects, resulting in misapplications and failure (Sear, Newson and Thorne, 2010).

Traditionally, restoration schemes were dominated by solutions centred around isolated sections of the drainage network and controlling the river system as a result of politicisation and industrial purposes (Eyquem, 2007). These methods have been described as relying upon pre-supposed knowledge of fluvial processes and spatially limited empiricism within arbitrary timescales (Eyquem, 2007). These approaches negated to account for the uniqueness inherent in each fluvial system (Sear, Newson, and Brookes, 1995).

These schemes were usually based on two contrasting objectives; land drainage and flood defence, whereby the former objective attempts to remove water from sections of the channel as quickly as possible, whereas the latter favours the widening of the channel to accommodate flow. This failure to consider wider catchment systems  so has ultimately led to failure in design, through sedimentation (Eyquem, 2007; Sear, 1994).

An alternative, yet complementary approach, is based on the science of fluvial geomorphology (figure 1). This perspective offers the potential for interpreting the catchment dynamics and relating these to changes in channel form and process (Sear,  Newson, and Brookes, 1995). This holistic, multi-disciplinary understanding of the catchment is now favoured in order to manage rivers sustainably and harmoniously with natural fluvial processes (Ollero et al., 2011).

Figure 1.The complementary relationship between fluvial geomorphology and river engineering (Sear et al., 1995).

From the hydrogeomorphological perspective, localised issues are to be considered within the context of a wider basin with regard to their up- and down-stream impacts (Belletti, et al., 2015; Eyquem, 2007). This is achieved by identifying and understanding the site-specific boundary conditions and their interrelationships with the channel’s morphological behaviour (Sear, Newson, and Brookes, 1995).

The hydrogeomorphological perspective encompasses: how modified the channel is, the extent of the impact of channel intervention has on sediment transport, biota and water flow, the degree of modification of the channel morphology (Sear et al., 2008).

Utilising knowledge of the connectivity between a channel network and its catchment, aids in the understating of riverine processes, as the catchment wide processes govern the movement of water and sediment through the channel. These in turn create the associated physical habitat of the channel. Therefore, restoration projects must understand the river basin holistically as a series of interconnected system to provide the most appropriate channel intervention (Sear, 1994; Sear, and Arnell,  2006; Newson and Clark, 2008; Sear et al., 2009).

Law and Litigation 

The advent of European-wide policies, including theEU Water Framework Directive (WFD 2000/60/EC), Habitats Directive and their precursors, acted as major political drivers in achieving sustainable management of the fluvial environment, across Europe. These policies placed a legislative requirement on political leaders and project managers to monitor the physical state of the catchment and the changes within them and prevent their degradation and promote a ‘good ecological status’ (Sear and Arnell ,2006 ; Sear, Newson and Thorne, 2003). These frameworks sought to improve understanding of the ecological function, value and structure of fluvial environments, and how these parameters subsequently affected biological communities (Raven et al., 2010).

The quality of channel ecology is sensitive to changes in hydrogeomorphology, hence the status of channel hydrogeomorphology is a key metric to when trying to understand, preserve or improve a riverine environment and find appropriate restorative measures (Rinaldi, et al., 2017).

Moreover, the WFD appreciates that rivers are ecosystems comprised of their chemical biological and hydrological quality -all of which define a ‘good ecological status’ (Logan, 2001, Wallerstein et al., 2006). Whilst the Habitats Directive specifically relates to the conservation of wildlife and habitats, the WFD focuses on the incorporation of water management within the catchment (Sear et al., 2009; Eyquem, 2007). However, at each spatial scale, there are various limitations on the applicability of these international frameworks. These barriers to implementation are usually born out of either a lack of understanding of the fluvial process or from inefficient management practices , that are insufficient in both spatial and temporal scales (Figure 2) (Newson and Large, 2006).

figure 2. Gaps, problems and uncertainties within the knowledge used to guide the implementation of the WFD (Newson and Large, 2006).

 

The WFD also introduced the term ‘hydromorphology’; which places the onus on member states to consider the impact of interventions to flow regime, sediment transport, river morphology, and lateral channel mobility  (Rinaldi et al., 2013).

A channels ecology is partially determined by its hydrogeomorphology, hence improved knowledge and assessment of hydrogeomorphological features, is key to understanding how and where to improve a channels ecology (England, and Gurnell  2016), in line with European legislation.

These new approaches in fluvial geomorphology are studied at wider geomorphic scales which are then ‘broken down’ into geomorphologically homogeneous reaches and analysed to ‘build up’ an understanding of the dynamics of the catchment, including its channel morphology, ecology, and fluvial and sediment regimes (Eyquem, 2007 ; Environment agency, 2005).

Following the introduction of these international directives, and the identification of the shortcomings of previous approaches, there has been a rapid development in numerous comprehensive stream assessment methodologies (Belletti et al., 2015). These aim to assess and monitor the separate components, or interrelated processes, of fluvial water bodies, based on sound understanding of their  interconnected processes, to a high level of accuracy (Rinaldi et al., 2017).

A need for broader assessment of catchment condition are required, of which go beyond inventorying physical habitats, or mapping these features in isolation, with a greater concentration on the processes that control the hydrogeomorphological response and riverine processes (Belletti et al., 2015). Inventories alone cannot provide sufficient information required for a comprehensive and sustainable restoration intervention (Sear, Hill and Down, 2008). Hence, this variety of  methodologies reflect the nuanced conceptual approaches and disciplines (e.g. hydrological, geomorphology, biology) of each assessment method (Belletti et al., 2015).

The Different Approaches

The abundance of methodologies highlights a paradigm shift away from introducing fragmentary controls within the channel and its catchment – a more process-based restoration methodology is now sought (Beechie et al., 2010). This new approach includes a suite of techniques, databases and standardised metrics born out of academic understanding of catchment processes, alongside practical engineering knowledge (Beechie et al., 2010; Sear and Newson, 2003).

These assessments utilise a broad evaluation on the physical processes within the catchment, measuring both the pressure and response variables,  (the hydromorphology of the channel and the reaction of the biological indicators) in order to observe the cause-effect relationships that regulate observed changes in system conditions (Belletti, et al., 2015). Such assessments are considered supporting tools in implementing the WFD at a localised scale.

A multitude of different assessments are available to gauge and assess the conditions of a variety of catchment parameters. These assessments can be complementary to one another, focusing on specific areas of the channel or focused towards a specific field of research; the aims of a specific project and its geographical location, determine the most appropriate metric to use.

For example, Belletti, et al (2015). found, of 121 different assessments studied, they all fall within four broad categories, (1) physical habitat assessment; (2) riparian habitat assessment; (3) morphological assessment; (4) assessment of hydrological regime alteration. Each category is performed over differing spatial scales.

‘The many existing methods vary widely in terms of their concepts, aims, spatial scales, collected data and therefore their applicability’ (Belletti, et al., 2015) (Figure 3).

figure 3 .How the spatial scales of different assessment methods differ. HRA -Hydrological Regime Alteration Assessment, PH -Physical Habitat assessment, M -Morphological assessment, RH- Riparian Habitat Assessment  (Belletti,et al.,2015) 

Further, the level of required experience or applied knowledge varies according to the approach (Skinner and Thorne, 2005), with some methods requiring a high level of post-graduate or doctoral training  (e.g. Geomorphological Assessment Procedure Sear, Newson and Thorne, 2003) whilst other frameworks may only need a limited understanding or a days training,  to accurately conduct the assessment  (e.g. Index of Stream Condition, (Ladson et al., 1999), River Habitat Survey (Sear, Newson and Thorne, 2003)).

During the 1990’s, Physical Habitat Assessments prevailed, consisting of metrics used for localized interventions for habitat improvement, based on extensive and detailed field surveys, with limited use of other geomorphological approaches (e.g. GIS, remote sensing) (e.g. River Habitat Survey (RHS; Raven et al., 1998). Lack of attention is generally given to the dynamism of channel, especially so for vertical (ground water interaction) component, preventing a sound understanding of pressure–response relationships when implementing a channel intervention (Rinadli et al., 2013).

These assessments provide only one facet to an overall hydromorphological assessment. These are applied at limited spatial scales, of may be insufficient, as morphological alteration can stem from processes occurring at the catchment level (Rinaldi et al., 2013).

Riparian Habitat assessments are one of the most specific and contemporary types of assessment, focusing on the composition and development of riparian zones (Sear, Newson and Thorne, 2010).. Similarly to Physical Habitat Assessments, riparian assessments rarely utilise GIS and remote sensing but are conducted at a smaller reach scale. However, a greater proportion of riparian assessments attempt to relate the riparian habitat to physical processes (Sear, Newson and Thorne, 2010). Whilst these assessments are an important contribution towards the WFD, they are rarely more comprehensive than an inventory of riparian habitats within a specified reach, reducing their utility in understanding the catchment dynamics and process-response relationships, required for a project. See Barquín et al., (2011), Dixon et al., (2005) and González Del Tánago, García De Jalón (2011) for examples of Riparian Habitat Surveys.

Morphological assessments include broad evaluations of river conditions, including the assessment of channels forms, geomorphological alterations and human interactions. These assessments are usually carried out at the reach scale (figure) i.e. a variable length with sufficiently homogeneous morphological characteristics and boundary conditions (Belletti et al., 2015). This type of assessment is broader than a PHS, assessing both pressure and response variables within a reach, to formulate a clear understanding of the cause-effect relationships that determine alternation in the channel conditions (Belletti et al., 2015).

A fundamental concept to the implementation of the WFD, is  consideration of a ‘reference state’ condition and the degree of deviation from this status (Belletti et al., 2015).By identifying a reference condition, target driven management can be achieved. The reference state differs according to the type of river and the overall aim of the project. This is used to create a benchmark to determine the magnitude of change and most appropriate channel intervention (Sear and Arnell, 2006).

Unlike the aforementioned assessments, PHS includes a greater variety of data collection techniques and includes field surveying, GIS, remote sensing and historic data. These assessments consider relatively recent historical changes across reach sections of the channel. These methods are centred around a need for evaluating river conditions for restoration design and can include information on broader, catchment wide processes. The river conditions are evaluated in comparison to a reference condition (Belletti et al., 2015).

Fewer morphological assessments are focused on the surveying of instream habitats. A more holistic overview of smaller channel sections are examined (e.g. the floodplain condition, artificial features, bank morphology and previous interventions) (Belletti et al., 2015).

The majority of morphological assessments are qualitative, with few measurements of channel geometry. Similarly, to PHS, these surveys do not adequately describe the vertical component of channel interaction with groundwater. Further, a lack of attention to the inventory of geomorphic unit and ecological assemblages, useful for ecosystem characterisation. This illustrates a potential limitation of undertaking specific survey on their own, without considering the cross-disciplinary and interrelated processes that interact to allow the channel to function (Belletti et al., 2015).

Examples of morphological assessments include the Morphological Quality Index (MQI), the SYRAH (Système Relationnel d’Audit de l’Hydromorphologie des Cours d’Eau; Chandesris et al., 2008, The River Styles Framework (Brierley and Fryirs, 2005), and IHG (Indice Hydrogeomorfologico; Ollero et al., 2007, 2011).

Finally, methods that are centred upon hydrological regime alteration, assess the deviation of a channel from a hydrological reference condition (Belletti et al., 2015).These metrics involve the processing of hydrologic data through numerical modelling. Remote sensing and historical maps are used to support the evaluation of change induced by remediate works. This framework is deployed within a catchment scale. Multiple indicators are used to predict and model the likely effects of channel intervention and restorative measures. A variety of channel parameters are factored in, providing the most holistic, broad scale approach in understanding catchment processes and likely effects of intervention (Belletti et al., 2015).However, this method requires large scale data of entire catchments and multiple parameters, over prolonged periods time, of which may not be available. High data demand also calls for high availability of computation. Examples of this method include: RVA Richter et al. (1996), HAI Kleynhans et al. (2005), HS_RCI Healey et al. (2012).

Implementation of these methods, at the EU scale.

A number of barriers exist which prevent the implementation of hydromorphological assessment methods within the EU. Critically, the considerations of physical  processes and their omission from many hydromorphological assessment methods, as a result of poor understanding of the interrelationships between different channel parameters (Rinaldi et al., 2017). Moreover, many countries fail to integrate other channel parameters into their assessment, resulting in gaps in knowledge, which may proliferate within a project, to lead to unnecessary and costly failure (Belletti et al., 2015).

Within the EU context, there has been some development in the application of more morphological assessments of which encompass a variety of channel parameters. European based projects, that aim to develop a set of comprehensive hydromorphological tools,  can be used to aid in the understanding of catchment level dynamics, across a variety of channel typologies and projects. However, the requirement of trained and skilled labour, to a post-graduate or doctoral level  limits the ability for assessments to be carried out frequently and across a variety of fluvial typologies (Rinaldi et al., 2017).

A set of multi-scale, process-based, hierarchical, hydromorphological frameworks were developed and expanded under the REFORM (REstoring rivers FOR effective catchment Management) project. These have been developed and deployed within a range of riverine management schemes, across the EU, with varying aims, scales and data requirements (Gurnell et al., 2016). The REFORM framework assists in improving understanding of local hydromorphological function,  to guide restoration and rehabilitation. This understanding can then be applied to heavily modified or impacted reaches. This framework is intentionally flexible, circumventing a prescriptive approach whilst still ensuring the use of localised data sets and knowledge (Gurnell et al., 2016).

The Morphological Quality Index was initially developed and applied by Rinaldi et al (2013) in a variety of Italian rivers, and was expanded to be applied to a variety of European environments, under the REFORM project.

The Geomorphic Units survey and classification System (GUS) (Rinaldi et al., 2015) was also developed;  these tools have great applied advantages, when integrated with existing assessments of channel hydrogeomorphology (England and Gurnell, 2016).

Geomorphic assessments have also been developed across a range of rivers, outside of the EU, in countries such as Australia River Styles Geomorphic classification, (Brierley and Fryris, 2000), Sustainable Rivers Audit (Thomas et al., 2012) and in the USA (Watershed Assessment (Montgomery and Buffington 1998), the Classification of Water Courses developed by the US Forest Service (Rosgen 1996)). However, these methods may not be translatable outside of the physiographic environments of which they were developed in (Sear, Newson and Thorne, 2010).

The UK Perspective

In the UK, a variety of methods are used to investigate and characterising the geomorphic condition of a channel, and hence understand its sediment dynamics. In line with EU policy, projects that aim to restore, rehabilitate or induce hydromorphological change within a catchment, must have a thorough understanding of the stability and sedimentological properties  of the catchment. In order to be successfully implements and provide the best possible outcomes, approaches  need to be transport, repeatable and auditable (Sear, Newson and Thorne, 2010).

In the UK, the frameworks used to assess geomorphic change are integrated within the engineering, environment and hydrological aspects of the project. A variety of standardised approaches have been developed, in order to accurately document, investigate and report-out the processes that occur within a catchment and the likely effect this will have on a project. Examples include the GeoRHS (Branson et al., 2005), River Reconnaissance Survey (Thorne., 1998). These frameworks were developed and researched within a UK context, (see Table 1 for examples)  and hence are best suited to catchments with similar values of slope, sediment regime and hydrogeomorphology. This is because channel systems and processes differ according to geographic location and the wider processes that form the catchment (Belletti et al., 2015).

 

Table. 1 geomorphic survey methodologies that have been deployed within the UK.

Catchment Baseline Study (CBS) 

A catchment baseline study (CBS) provides an overview of the state of the catchment’s geomorphology. This approach aims to provide classification of the morphological conservation value of the catchment, by developing an understanding of its hydromorphological components (Sear, Newson and Thorne, 2003). Ultimately, holistic solutions for morphological and sediment regime related issues are sought (Sear, Newson and Thorne, 2003).

Using a CBS, the channel network is divided into geomorphic reaches, defined by their geomorphic conservation status (that is the degree to which the natural morphology, and sediment and fluvial processes, are preserved) Sear, Newson and Thorne, 2003)Other factors such as whether the reach supplies, transports or stores sediment (i.e. its stability), is considered. CBS’s produce reports detailing the geomorphology and its conservation values, as well as its sediment dynamics. This is applied at the reach scale, so that actual and potential issues are emphasised to allow for project managers to undertake strategic and considered intervention (Sear, Newson and Thorne, 2003).

This approach also identified reaches that have been adversely affected by previous restorative measures. The WFD requires the hydrogeomorphological value of the system to be conserved and aims to protect the most vulnerable reach by ensuring that all measures  consider their wider catchment impact (Sear, Newson and Thorne, 2010). The evidence base used to conduct these assessments is sufficiently broad, to allow stakeholders to make well informed decisions regarding channel intervention. Areas of degradation and those of high susceptibility to degradation are identified  using this method, highlighted for future protection, given their high geomorphological conservation status (Sear, Newson and Thorne, 2010).

The CBS comprises a desk-based survey and a field work element. By compiling historical information on soil composition, historical interventions, fluvial and pluvial input and drainage as well as other baseline information, the study catchment can be best understood. Some of this information may be derived from other approaches, such as the River Habitats Surveys, which are only taken over 500m reaches of channel, but provide an in-depth understanding of channel dynamics at the unit level.

Stream reconnaissance sheets can be used to assess the channel during fieldwork investigations and can be adapted according to the channel typology and project aims. The physical form of the channel is recorded and some measurements are taken, as well as identification of in-channel structures. Using this information, the channel is delineated into geomorphic reaches as per their geomorphological conservation value. Once this information is compiled in a GIS, catchment maps are produced of the morphological conservation values, reach functions, and other key features relevant to the project (i..e. in channel structures). Consistent scoring of reaches is achieved through quality controls and assessment by individuals with specialist knowledge of riverine process. From these maps, the degree of naturalness and the reach status, can be determined (Sear, Newson and Thorne, 2003).

CBS and other reconnaissance surveys alone are insufficient to provide a level of detail of channel form and process for channel management and restoration measures to be based upon (Sear, Newson and Thorne, 2010). More advanced methods are required to place these smaller, reach scale, findings with a greater catchment context. The resultant reports from these surveys identify critical reaches with the system and the current and potential issues that may arise from future intervention, in relation to the channel geomorphology. The information obtained from the CBS informs the Fluvial Audit framework, when used in tandem with a desk-based study.

Click here to read more about the Fluvial Audit method and its utility in assessing sediment-related issues in riverine management and restoration.

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