Difference between revisions of "Introduction to characterisation: Multi-scale Hierarchical Framework"

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== Hydromorphological characterisation ==
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== River Characterisation: background ==
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Effective river restoration calls for an understanding of how rivers work. A key step for this is hydromorphological characterization, looking at rivers from a perspective that discloses the relevant processes and forms. Hydromorphology is a matter of water and sediment, but also of vegetation interacting with water and sediment. This makes both geomorphological and ecological processes relevant.
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River management often focuses on individual reaches of river networks. However, the character and dynamism of river reaches depends not only upon natural processes and human interventions within the reach, but also within the catchment, particularly upstream of the reach (see Figure 1). The character of river reaches often responds to processes and interventions across the catchment in a delayed way as changes in processes propagate from their source areas through the river network to individual reaches.
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River characterisation is a process to determine the hydromorphological condition of a river. It investigates the river's current form and behaviour, and how these have changed over time.  
  
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A river is a connected ecosystem generated by hydrological, geomorphological and ecological processes that interact at many temporal and spatial scales. Hydromorphological characterization (1) aims at capturing and explaining this complexity. This is the key step in developing a fuller understanding of how a river functions physically, as a foundation for evaluating river conditions and developing a programme of restoration measures.
+
Rivers naturally adjust over time, but human interventions in the channel, floodplain and wider landscape have dramatically altered their form and behaviour. Rivers have been straightened and deepened for navigation, dammed for milling and water supplies, and embanked for flood protection; floodplain have been converted to agriculture and are now home to millions of people; and in the wider landscape forests have been cleared, marshes drained and sediment sources blocked. These transitions have been ongoing for hundreds to thousands of years in Europe, and consequently few, if any, pristine rivers remain.
  
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The spatial organization (2) of rivers and the way how such spatial organization evolves through time create the variety of forms and processes we observe in nature. This builds the theoretical framework (3) for characterizing relevant spatial and temporal scales at which key fluvial processes occur.
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Interventions that have occurred upstream, downstream or within the reach can influence its hydromorphological form and behaviour. For example, siltation in a lowland river reach can be caused by upstream changes in land use that increase the delivery of sediment to the channel (e.g. intensification of arable agiculture), impoundments downstream that produce a backwater effect that induces sediment deposition (e.g. dams and weirs), or modifications to the channel form or dimensions that reduce its capacity to transport sediment (e.g. channel deeping and widening). In addition, hydromophology responds in a delayed way to processes and interventions within the catchment. As a result, understanding hydromorphology at the reach scale requires an understanding of current and past processes and interventions at larger spatial scales. Without such a multi-scale understanding, management strategies are not fully informed and may not provide sustainable solutions.
  
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== How are river systems organized in space? ==
 
  
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When thinking about a river, we usually imagine a reach of a few kilometres in length. This is the key spatial scale within a framework of spatial components of the river landscape (see Figure 1). For characterizing the full complexity of rivers, however, it is useful to consider components larger and smaller than a reach too. A hierarchical framework for this helps in adopting the relevant spatial scales to describe specific river system characteristics.
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=== Link to ecology and river restoration ===
  
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[[File:RiverCharacterisation_Figure1_last.png|center|thumb|600px| Figure 1. The spatial units and scales of the REFORM hierachical framework]]
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A multi-scale approach to investigating hydromorphology inevitably focuses on geomorphological characteristics and the hydrological and geomorphological processes that influence those characteristics across time and space, but these are crucial for river ecology. Hydromorphological processes drive longitudinal and lateral connectivity within river networks and corridors, the assemblage and turnover of habitats, and the sedimentary and vegetation structures associated with those habitats. All of these processes and structures are relevant to the provision of habitats to support the entire life cycle of organisms including refugia, feeding, spawning etc. As a result, a process-based, multi-scale understanding of hydromorphology is essential for identifying degraded segments and reaches of river and for developing sustainable restoration approaches that are in sympathy with hydromorphological functioning from catchment to reach scales.
  
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The hierarchical framework proposes eight different sizes of spatial unit (4) to investigate different river features. Seven of them are represented in Figure 1. The catchment unit encloses the land that is drained by a river and its tributaries, for which the area is typically 10^2-10^5 km^2. Landscape units are portions of the catchment that show different forms of physical landscape that can be summarised by properties such as land surface elevation, land steepness, and valley density. They have a typical area of 10^2-10^3 km2. The segment unit is a section of the river system that is located within a valley of sufficiently uniform gradient and width, so that the river is confined by its valley to a similar degree and has similar energy through the segment. River segments are typically 10^1-10^2 km long.
 
  
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The reach unit is a section of the river network in which not only the valley and flow energy are relatively uniform, but also other conditions, such as bed sediments, bank properties and riparian vegetation. This uniformity in conditions gives the river reach a consistent appearance through its length as a result of a near-consistent internal set of process-form interactions. A reach unit is typically 10^-1-10^1 km in length, and a river segment can contain one to several reaches.
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== The Framework ==
  
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A geomorphic unit is a portion of a reach that contains a landform created by erosion or deposition of sediment, sometimes in association with vegetation, with a typical length of 10^0-10^2 m. Geomorphic units can be located within the channel (bed and mid-channel features), along the channel edges (marginal and bank features) or on the floodplain. A hydraulic unit is a spatially distinct patch of relatively homogeneous surface flow and substrate character with a typical size of 10^-1-10^1 m. A single geomorphic unit can include one to several hydraulic units. River elements include individuals and patches of sediment particles, plants, and wood (10^-2-10^-1 km). The framework is hierarchical in that each of the spatial units nests within one another so that their boundaries do not overlap.
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The multi-scale hierarchical framework has been developed to be a flexible, open-ended approach to hydromorphological assessment that is in line with the [http://ec.europa.eu/environment/water/water-framework/index_en.html Water Framework Directive]. As a result, the framework is compatible with WFD, but being process-based and multi-scale it allows practitioners to delve into further detail on the underlying drivers of hydromorphology. For more information on the links to WFD see [[Hierarchical Framework and WFD]].  
  
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Each unit represents a particular spatial scale that is suited to investigating specific processes, human pressures and forms that eventually impact on the character and dynamics of river reach units. Figure 2 lists some examples of indicators of processes, human pressures and forms (5) which can be calculated for each spatial unit. At the catchment scale, for example, it is relevant to monitor land cover type because it affects run-off production. At the landscape unit scale, potential sources of sediment (for instance gullies, landslides) can be located to estimate the likely supply of such sediment to smaller units. The segment scale is suitable to investigate physical pressures altering longitudinal connectivity, riparian corridor features, valley features and river flow regimes (6). All of these can be assumed to be fairly homogenous at this scale and thus well-represented by summary indicators. At the reach scale, indicators summarising channel dimensions and type, bank and bed sediments, and riparian properties can be represented by informative indicators (7). Within reach units, geomorphic units such as riffles, bars or islands can be identified and at a finer scale specific hydraulic units and river elements can be surveyed.
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The framework leads practitioners through a series of steps to delineate their river into spatial units; characterise the relevant hydrological, geomorphological and ecological charactersitics at each scale; and assess the current hydromorphological condition of the river and its sensitivity to change. It aims to make best use of currently available surveys (physical habitat, riparian habitat, morphological, hydrological regime and fish continuity, etc) and readily-available (mainly free) Pan-European data sets to guide users on:
  
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[[File:RiverCharacterisation_Figure2_last.png|center|thumb|600px| Figure 1. Examples of relevant indicators of processes, human pressures and forms for each spatial unit]]
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* What information is required at each spatial scale in the context of the data sets that are available and any new data that may be needed.
 +
* How the above information can be collected or generated, and how it can be analysed, in order to:
 +
* Describe and, crucially, explain variation in river character and behaviour within a catchment.
 +
* Provide users with a basis upon which they are able to understand past and present behaviour and predict how a particular reach might react to changes (e.g. removal of local engineering modifications, flow regime naturalisation, reinstatement of longitudinal sediment connectivity).
 +
* Allow users to define potential, site-specific, “reference” conditions against which current hydromorphological and ecological condition could be assessed.
  
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It is at the reach scale that the many features found across floodplains and river channels adjust to the cascade of influences that propagate to the reach from larger spatial units and scales. This is also the scale that is affected by interactions and feedbacks within the reach among fluvial processes, geomorphic units, hydraulic units, and smaller river elements such as sediment particles, logs, and plants.
 
  
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== How do river systems develop in time? ==
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=== Rules of Engagement ===
 +
Some ‘rules of engagement’ need to be born in mind before embarking on the application of the methodologies proposed in this framework:
 +
# [[Rules of Engagement: 1 | Involve a professional geomorphologist]]
 +
# [[Rules of Engagement: 2 | Carry out a field survey]]
 +
# [[Rules of Engagement: 3 | View the framework as a guide to an approach rather than a recipe book]]
 +
# [[Rules of Engagement: 4 |  Accept that true reference reaches devoid of human influence do not exist, and instead,]]
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# [[Rules of Engagement: 5 |  Focus on hydromorphological processes.]]
  
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The temporal dimension is necessary for a complete characterization of river processes. This adds knowledge about the changes in features and indicators through time within each spatial unit, and so discloses information that helps to explain how changes occur at different spatial scales and dynamically interact between scales. The historical evolution of hydromorphological features and human interventions may be quite different among catchments and may influence river forms and processes in different ways in different catchment environments. Thus, the identification of possible changes in controlling variables (8) forms a basis for the assessment of current and past conditions, as well as for the prediction of future scenarios (9). Changes through time can be investigated in their historical context, for instance over the last century for changes at catchment to reach scales. A resolution of about 10 to 20 years is suited to the reach scale and below (Figure 3). Proper characterization of historical and contemporary changes at the relevant spatial scales is essential for diagnosing river problems and designing sustainable restoration measures (9)(10?).
 
  
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Figure 3 illustrates the changing processes monitored and their temporal scale for each spatial unit in an example catchment. Land cover change over more than 50 years is shown at the catchment scale. Significant land cover and management alterations have occurred over this period throughout Europe, and have affected runoff production and consequently the entire cascade of river processes that drive river hydromorphology and features within the catchment. At the landscape unit scale, afforestation of a valley is illustrated. Extension of forest land cover stabilizes hill slopes and decreases runoff production, leading to a reduction in soil erosion and the supply of sediment to river channel. Such changes in the delivery of water and sediment lead to changes in the size and the form of a river. Sometimes even the type of river is changed. The segment scale illustrates an increase over the last century in infrastructure that directly impacts the river and it processes. Dams disconnect the down-river movement of sediment, often leading to a sediment deficit downstream the dam and degradation of the river bed. Bank protection limits lateral movement of the river and its capacity to erode bank sediment.
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=== Hierarchy of spatial units ===
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The hierarchy of spatial units within which relevant properties, forms and processes can be investigated to understand and assess hydromorphology and its potential impact on ecology is presented in Figure 1. The units are arranged in decreasing over of size (indicative spatial scale) and persistence (indicative time scale):[[Region| region]], [[Catchment| catchment]], [[Landscape unit|landscape unit]], [[Segment| segment]], [[Reach | reach]] and [[Units| units]]. The reach is the key spatial scale at which the mosaic of features found within river channels and floodplains (i) responds to the cascade of influences from larger spatial scales and (ii) is influenced by interactions and feedbacks between geomorphic and hydraulic units and smaller elements such as plants, large wood and sediment particles within the reach.
  
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The effects of the installation of river infrastructures that decrease sediment supply and constrain river channel movements are most clearly seen at the reach scale. In the example river reach, a river that was multi-thread braided in the 1950s has been transformed into a single-thread sinuous river accompanied by a severe reduction in the width of the active channel. Changes are also observed in the geomorphic units, with encroachment of vegetation incorporating an island into the floodplain within a period of ten years. At the hydraulic unit scale, the example illustrates the impact of changes in river stage on unit character, even when the river channel form has not changed. At the river element scale, the example shows significant changes in the type and distribution of elements present as the result of a single flood, which has redistributed them within and between river reaches.
 
  
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The integration of spatial characteristics and their changes through time allows the investigator to identify which spatial units and temporal scales drive the relevant forms and processes. The hierarchical framework provides an integrated and scientifically sound basis for diagnosing causes and effects of the hydromorphological process cascade. This process-based characterization is suitable to support river restoration from basin-scale planning to the implementation of local projects.
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<gallery>
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File:River characterisation hierarchy of spatial scales.png| Figure 1. Hierarchy of spatial scales for the European Framework, including indicative spatial dimensions and timescales over which these units are likely to persist.
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File:Delineation_diagram_new.png|Figure 2: Delineation of spatial units
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</gallery>
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= Four stages of river characterisation =
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 +
 
 +
== Stage 1: Delineation of spatial units ==
 +
 
 +
=== General Guidance ===
 +
 
 +
For catchment assessment and management purposes, the aim should be to subdivide the entire catchment into a complete set of units at all spatial scales from catchment to reach. In large catchments, this may not be possible, but it is necessary to subdivide the catchment to the scale of its major landscape units, after which representative subcatchments within each landscape unit can be analysed, delineating segments and reaches along the main channel and major tributaries for detailed analysis.
 +
 
 +
For assessment of a particular reach or segment, a minimum assessment needs to focus on the specific reach or segment and larger spatial units that contain and are immediately upstream of the reach or segment under consideration
 +
 
 +
 
 +
=== Delineating the spatial units ===
 +
The following links provide summaries and detailed information on the delineation of units at each spatial scale
 +
 
 +
''Region: '' [[Region| Summary]] - [[Region delineation | Detailed guidance]]
 +
 
 +
''Catchment: '' [[Catchment| Summary]] - [[Catchment delineation| Detailed guidance]]
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 +
''Landscape unit: '' [[Landscape unit|Summary]] - [[Landscape unit delineation| Detailed guidance]]
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 +
''Segment: '' [[Segment| Summary]] - [[Segment delineation| Detailed guidance]]
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 +
''Reach: '' [[Reach | Summary]] - [[Reach delineation| Detailed guidance]]
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 +
''Units: '' Geomorphic untis, hydraulic uinits and river elements do not require ‘delineation’ at this stage of the analysis, but in later analytical stages we emphasise the identification of geomorphic units as key characteristics of river reaches.
 +
 
 +
 
 +
[[Media:Table_4.2_Datasources.pdf | Table 4.2]] provides more information on the pan-European data sources referred to in the preceding links.
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 +
 
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== Stage 2:  Characterisation of current and past condition ==
 +
Characterisation of the spatial units at the different scales attempts to draw together readily-available information, surveys and data sets in ways that will promote understanding of the units and the linkages between them. Recommendations for characterisation take account of WFD requirements and make best use of existing pan-European and National data sets, including the outputs from physical habitat, riparian habitat, morphological, hydrological regime and fish continuity assessments, where they are available. Essential components of the characterisation are stressed.
 +
 
 +
Characterisation is divided into two steps: (1) the assessment of current condition and (2) the assessment of temporal change and quantification of rates of processes
 +
 
 +
=== Characterising current condition===
 +
The links below provide information on the aims of characterisation; data layers and hydromorphologically relevant parameters; potential data sources; and characteristics for each spatial unit.
 +
 
 +
 
 +
[[Region characterisation|Region]]
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 +
[[Catchment characterisation|Catchment]]
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 +
[[Landscape unit characterisation|Landscape unit]]
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 +
[[Segment characterisation|Segment]]
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 +
[[Reach characterisation|Reach]]
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 +
[[Unit characterisation|Unit]]
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 +
 
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=== Characterising past condition and quantifying rates of processes ===
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 +
The charactersiation process is extended back in time to investigate what the channel, floodplain and catchment looked like in the past; how and why they changed; and the rates of key hydromorphological processes. This provides managers with information on the underlying causes of hydromorphological degradation in a reach, the constraints on current and future management and restoration, and the likely evolutionary trajectories for the reach under different management scenarios.
 +
 
 +
 
 +
[[File:TimescalesApproaches.jpg|right]]
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[[Temporal analysis: Introduction | Introduction]]
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[[Temporal analysis: Field survey | Field survey]]
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[[Temporal analysis: Remote sensing | Remote sensing]]
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[[Temporal analysis: Historical | Historical information]]
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[[Temporal analysis: Palaeo | Palaeo techniques]]
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[[Temporal analysis: Integration | Integrating data]]
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 +
 
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Details on recommended data sources and analytical method for charactersitics at each spatial scale are available in chapter 6 of [[media: Deliverable2_1.pdf | REFORM Deliverable 2.1]]
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 +
 
 +
==Stage 3: Indicators==
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The information assembled during the characterisation phases supports a list of indicators of the current and past condition of a catchment and its spatial units. These key indicators provide an overview of current and past functioning of key processes at each spatial scale.
 +
 
 +
[[File:Indicators.png|400px|center]]
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The most important indicator is channel type. This reach-scale indicator summarises many of the important characteristics of the river’s hydromorphology, including river confinement, planform style and river bed sediment size. An extended river typology with 22 river types was developed, which is consistent with WFD’s high-level typology. Channel type is a core component of the final stage of the hierarchical assessment framework, in which current channel type is compared to the typologies for floodplains and groundwater:surface water interactions and the changes that have occurred over time at wider spatial scales to assess current reach condition, sensitivity and trajectories of change
 +
 
 +
[[File:Typology.png|600px|center]]
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 +
 
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More details on indicators of past and present condition can be found in Chapter 8 of the [[media: Deliverable2_1.pdf | REFORM Deliverable 2.1 (Part 1)]].
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 +
Details of the typology can be found in Chapter 7 of the [[media: Deliverable2_1.pdf | REFORM Deliverable 2.1 (Part 1)]], along with typologies for floodplains and groundwater:surface water interactions.
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 +
Examples of the evaluation of indicators are provided in [[media: Deliverable2_1_P3.pdf | REFORM Deliverable 2.1 (Part 3)]] describing catchment case study applications .
 +
 
 +
==Stage 4: Interpreting condition, trajectories of change, and sensitivity==
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===Condition===
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The term ‘condition’ is used to capture the degree to which observed hydrogeomorphological properties conform to what would be expected in a naturally-functioning situation, and thus how far the properties have deviated from that naturally-functioning state. However, the degree to which such a deviation is seen to be small or large depends upon the biogeographical, socio-economic, and cultural context. What might be considered to be a small deviation from natural function that is of no concern in some contexts, might be seen to be a considerable and notable deviation in other contexts.
 +
 
 +
Whilst the reach scale is often the main focus of interest, indicators representative of other spatial scales, particularly of the segment and landscape units in which the reaches are situated, provide important contextual information for interpreting reach scale indicators. Multi-scale indicators can provide much management-relevant information including:
 +
# Assessing current reach condition and degree of alteration
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# Understanding associations between landscape unit, segment and reach properties. In other words, what types of naturally functioning reach are sustainable and feasible within particular segment and landscape unit conditions and how do degraded conditions at the reach scale reflect processes or factors operating at the segment or landscape unit scales?
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# Assessing potential reach condition in the context of its segment and landscape unit setting. In other words, to what extent and in what ways is the reach altered from the naturally-functioning reach types that are feasible in the segment and landscape unit setting, and to what extent does the condition of a reach conform to or differ from the condition of the segment in which it is situated?
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# Establishing the spatial structure and condition of the river network. In other words, analyse the distribution of reaches of different style and condition throughout the network to assess (a) the presence and spacing of reaches that are in good condition, and (b) the degree of alteration of intervening reaches.
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 +
 
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These types of information can feed into:
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# Identification of the best condition reaches so that they may be protected.
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# Selection of the most effective locations for restoration and the balance of expenditure on better condition reaches and linking reaches (according to both hydromorphological and ecological criteria).
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# Selection of appropriate styles of restoration for the segment and landscape unit context of reaches.
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 +
===Trajectories of change===
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The identification of “channel change”, “channel adjustment” or “trajectories of change” and their causes is fundamental to understanding the current condition and status of a reach and its response (adjustment and trajectories of change) to particular pressures.
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 +
Identification of reach adjustments is based on a combination of contemporary and historical evidence and is initially investigated at the reach scale. However, such reach scale evidence more often than not reflects processes operating beyond the reach, and under such circumstances, it is crucial to consider what the processes and pressures might be that are inducing current condition and associated adjustments.
 +
 
 +
Channel adjustments are most frequently induced by a distinct change in the discharge regime or the quantity of sediment supplied to the reach. Such changes may result in (i) too much sediment being supplied for river flows to move the sediment on through the reach, leading to the accumulation of sediment within the reach; or (ii) insufficient sediment being supplied to satisfy the sediment transport ability of the river flows, resulting in erosion of the bed or banks within the reach.
 +
 
 +
A good tool to visualise changes is a chronology that illustrates how the channel characteristics and the factors that may have influenced them have changed over time. For example, a chronology the documents incision in a gravel bed river would record changes in bed level over time as well as the timing and magnitude of river engineering works, torrent controls, gravel mining from the channel, etc.
 +
 
 +
===Sensitivity===
 +
Hydromorphological sensitivity refers to the likelihood that a particular river reach will adjust in response to imposed changes (e.g. in flow or sediment supply). In some cases negligible adjustments to imposed changes may occur, and the reach ‘accommodates’ the changes and so has negligible ‘sensitivity’ to those changes. In other cases, quite small changes in controlling processes may result in major adjustments and thus the reach is deemed ‘highly sensitive’. This is often the case when a reach is close to a threshold condition where it may change from one river type to another. Annex G of the[[media: Deliverable2_1_P2.pdf | REFORM Deliverable 2.1, Part 2]] presents some empirical methods for estimating proximity to threshold conditions.
 +
 
 +
 
 +
More details on interpreting condition and trajectories of change can be found in Chapter 9 of the [[media: Deliverable2_1.pdf | REFORM Deliverable 2.1]].

Latest revision as of 13:58, 18 December 2018

River Characterisation: background

River characterisation is a process to determine the hydromorphological condition of a river. It investigates the river's current form and behaviour, and how these have changed over time.

Rivers naturally adjust over time, but human interventions in the channel, floodplain and wider landscape have dramatically altered their form and behaviour. Rivers have been straightened and deepened for navigation, dammed for milling and water supplies, and embanked for flood protection; floodplain have been converted to agriculture and are now home to millions of people; and in the wider landscape forests have been cleared, marshes drained and sediment sources blocked. These transitions have been ongoing for hundreds to thousands of years in Europe, and consequently few, if any, pristine rivers remain.

Interventions that have occurred upstream, downstream or within the reach can influence its hydromorphological form and behaviour. For example, siltation in a lowland river reach can be caused by upstream changes in land use that increase the delivery of sediment to the channel (e.g. intensification of arable agiculture), impoundments downstream that produce a backwater effect that induces sediment deposition (e.g. dams and weirs), or modifications to the channel form or dimensions that reduce its capacity to transport sediment (e.g. channel deeping and widening). In addition, hydromophology responds in a delayed way to processes and interventions within the catchment. As a result, understanding hydromorphology at the reach scale requires an understanding of current and past processes and interventions at larger spatial scales. Without such a multi-scale understanding, management strategies are not fully informed and may not provide sustainable solutions.


Link to ecology and river restoration

A multi-scale approach to investigating hydromorphology inevitably focuses on geomorphological characteristics and the hydrological and geomorphological processes that influence those characteristics across time and space, but these are crucial for river ecology. Hydromorphological processes drive longitudinal and lateral connectivity within river networks and corridors, the assemblage and turnover of habitats, and the sedimentary and vegetation structures associated with those habitats. All of these processes and structures are relevant to the provision of habitats to support the entire life cycle of organisms including refugia, feeding, spawning etc. As a result, a process-based, multi-scale understanding of hydromorphology is essential for identifying degraded segments and reaches of river and for developing sustainable restoration approaches that are in sympathy with hydromorphological functioning from catchment to reach scales.


The Framework

The multi-scale hierarchical framework has been developed to be a flexible, open-ended approach to hydromorphological assessment that is in line with the Water Framework Directive. As a result, the framework is compatible with WFD, but being process-based and multi-scale it allows practitioners to delve into further detail on the underlying drivers of hydromorphology. For more information on the links to WFD see Hierarchical Framework and WFD.

The framework leads practitioners through a series of steps to delineate their river into spatial units; characterise the relevant hydrological, geomorphological and ecological charactersitics at each scale; and assess the current hydromorphological condition of the river and its sensitivity to change. It aims to make best use of currently available surveys (physical habitat, riparian habitat, morphological, hydrological regime and fish continuity, etc) and readily-available (mainly free) Pan-European data sets to guide users on:

  • What information is required at each spatial scale in the context of the data sets that are available and any new data that may be needed.
  • How the above information can be collected or generated, and how it can be analysed, in order to:
  • Describe and, crucially, explain variation in river character and behaviour within a catchment.
  • Provide users with a basis upon which they are able to understand past and present behaviour and predict how a particular reach might react to changes (e.g. removal of local engineering modifications, flow regime naturalisation, reinstatement of longitudinal sediment connectivity).
  • Allow users to define potential, site-specific, “reference” conditions against which current hydromorphological and ecological condition could be assessed.


Rules of Engagement

Some ‘rules of engagement’ need to be born in mind before embarking on the application of the methodologies proposed in this framework:

  1. Involve a professional geomorphologist
  2. Carry out a field survey
  3. View the framework as a guide to an approach rather than a recipe book
  4. Accept that true reference reaches devoid of human influence do not exist, and instead,
  5. Focus on hydromorphological processes.


Hierarchy of spatial units

The hierarchy of spatial units within which relevant properties, forms and processes can be investigated to understand and assess hydromorphology and its potential impact on ecology is presented in Figure 1. The units are arranged in decreasing over of size (indicative spatial scale) and persistence (indicative time scale): region, catchment, landscape unit, segment, reach and units. The reach is the key spatial scale at which the mosaic of features found within river channels and floodplains (i) responds to the cascade of influences from larger spatial scales and (ii) is influenced by interactions and feedbacks between geomorphic and hydraulic units and smaller elements such as plants, large wood and sediment particles within the reach.


  • Figure 1. Hierarchy of spatial scales for the European Framework, including indicative spatial dimensions and timescales over which these units are likely to persist.

  • Figure 2: Delineation of spatial units

Four stages of river characterisation

Stage 1: Delineation of spatial units

General Guidance

For catchment assessment and management purposes, the aim should be to subdivide the entire catchment into a complete set of units at all spatial scales from catchment to reach. In large catchments, this may not be possible, but it is necessary to subdivide the catchment to the scale of its major landscape units, after which representative subcatchments within each landscape unit can be analysed, delineating segments and reaches along the main channel and major tributaries for detailed analysis.

For assessment of a particular reach or segment, a minimum assessment needs to focus on the specific reach or segment and larger spatial units that contain and are immediately upstream of the reach or segment under consideration


Delineating the spatial units

The following links provide summaries and detailed information on the delineation of units at each spatial scale

Region: Summary - Detailed guidance

Catchment: Summary - Detailed guidance

Landscape unit: Summary - Detailed guidance

Segment: Summary - Detailed guidance

Reach: Summary - Detailed guidance

Units: Geomorphic untis, hydraulic uinits and river elements do not require ‘delineation’ at this stage of the analysis, but in later analytical stages we emphasise the identification of geomorphic units as key characteristics of river reaches.


Table 4.2 provides more information on the pan-European data sources referred to in the preceding links.


Stage 2: Characterisation of current and past condition

Characterisation of the spatial units at the different scales attempts to draw together readily-available information, surveys and data sets in ways that will promote understanding of the units and the linkages between them. Recommendations for characterisation take account of WFD requirements and make best use of existing pan-European and National data sets, including the outputs from physical habitat, riparian habitat, morphological, hydrological regime and fish continuity assessments, where they are available. Essential components of the characterisation are stressed.

Characterisation is divided into two steps: (1) the assessment of current condition and (2) the assessment of temporal change and quantification of rates of processes

Characterising current condition

The links below provide information on the aims of characterisation; data layers and hydromorphologically relevant parameters; potential data sources; and characteristics for each spatial unit.


Region

Catchment

Landscape unit

Segment

Reach

Unit


Characterising past condition and quantifying rates of processes

The charactersiation process is extended back in time to investigate what the channel, floodplain and catchment looked like in the past; how and why they changed; and the rates of key hydromorphological processes. This provides managers with information on the underlying causes of hydromorphological degradation in a reach, the constraints on current and future management and restoration, and the likely evolutionary trajectories for the reach under different management scenarios.


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Introduction

Field survey

Remote sensing

Historical information

Palaeo techniques

Integrating data


Details on recommended data sources and analytical method for charactersitics at each spatial scale are available in chapter 6 of REFORM Deliverable 2.1


Stage 3: Indicators

The information assembled during the characterisation phases supports a list of indicators of the current and past condition of a catchment and its spatial units. These key indicators provide an overview of current and past functioning of key processes at each spatial scale.

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The most important indicator is channel type. This reach-scale indicator summarises many of the important characteristics of the river’s hydromorphology, including river confinement, planform style and river bed sediment size. An extended river typology with 22 river types was developed, which is consistent with WFD’s high-level typology. Channel type is a core component of the final stage of the hierarchical assessment framework, in which current channel type is compared to the typologies for floodplains and groundwater:surface water interactions and the changes that have occurred over time at wider spatial scales to assess current reach condition, sensitivity and trajectories of change

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More details on indicators of past and present condition can be found in Chapter 8 of the REFORM Deliverable 2.1 (Part 1).

Details of the typology can be found in Chapter 7 of the REFORM Deliverable 2.1 (Part 1), along with typologies for floodplains and groundwater:surface water interactions.

Examples of the evaluation of indicators are provided in REFORM Deliverable 2.1 (Part 3) describing catchment case study applications .

Stage 4: Interpreting condition, trajectories of change, and sensitivity

Condition

The term ‘condition’ is used to capture the degree to which observed hydrogeomorphological properties conform to what would be expected in a naturally-functioning situation, and thus how far the properties have deviated from that naturally-functioning state. However, the degree to which such a deviation is seen to be small or large depends upon the biogeographical, socio-economic, and cultural context. What might be considered to be a small deviation from natural function that is of no concern in some contexts, might be seen to be a considerable and notable deviation in other contexts.

Whilst the reach scale is often the main focus of interest, indicators representative of other spatial scales, particularly of the segment and landscape units in which the reaches are situated, provide important contextual information for interpreting reach scale indicators. Multi-scale indicators can provide much management-relevant information including:

  1. Assessing current reach condition and degree of alteration
  2. Understanding associations between landscape unit, segment and reach properties. In other words, what types of naturally functioning reach are sustainable and feasible within particular segment and landscape unit conditions and how do degraded conditions at the reach scale reflect processes or factors operating at the segment or landscape unit scales?
  3. Assessing potential reach condition in the context of its segment and landscape unit setting. In other words, to what extent and in what ways is the reach altered from the naturally-functioning reach types that are feasible in the segment and landscape unit setting, and to what extent does the condition of a reach conform to or differ from the condition of the segment in which it is situated?
  4. Establishing the spatial structure and condition of the river network. In other words, analyse the distribution of reaches of different style and condition throughout the network to assess (a) the presence and spacing of reaches that are in good condition, and (b) the degree of alteration of intervening reaches.


These types of information can feed into:

  1. Identification of the best condition reaches so that they may be protected.
  2. Selection of the most effective locations for restoration and the balance of expenditure on better condition reaches and linking reaches (according to both hydromorphological and ecological criteria).
  3. Selection of appropriate styles of restoration for the segment and landscape unit context of reaches.

Trajectories of change

The identification of “channel change”, “channel adjustment” or “trajectories of change” and their causes is fundamental to understanding the current condition and status of a reach and its response (adjustment and trajectories of change) to particular pressures.

Identification of reach adjustments is based on a combination of contemporary and historical evidence and is initially investigated at the reach scale. However, such reach scale evidence more often than not reflects processes operating beyond the reach, and under such circumstances, it is crucial to consider what the processes and pressures might be that are inducing current condition and associated adjustments.

Channel adjustments are most frequently induced by a distinct change in the discharge regime or the quantity of sediment supplied to the reach. Such changes may result in (i) too much sediment being supplied for river flows to move the sediment on through the reach, leading to the accumulation of sediment within the reach; or (ii) insufficient sediment being supplied to satisfy the sediment transport ability of the river flows, resulting in erosion of the bed or banks within the reach.

A good tool to visualise changes is a chronology that illustrates how the channel characteristics and the factors that may have influenced them have changed over time. For example, a chronology the documents incision in a gravel bed river would record changes in bed level over time as well as the timing and magnitude of river engineering works, torrent controls, gravel mining from the channel, etc.

Sensitivity

Hydromorphological sensitivity refers to the likelihood that a particular river reach will adjust in response to imposed changes (e.g. in flow or sediment supply). In some cases negligible adjustments to imposed changes may occur, and the reach ‘accommodates’ the changes and so has negligible ‘sensitivity’ to those changes. In other cases, quite small changes in controlling processes may result in major adjustments and thus the reach is deemed ‘highly sensitive’. This is often the case when a reach is close to a threshold condition where it may change from one river type to another. Annex G of the REFORM Deliverable 2.1, Part 2 presents some empirical methods for estimating proximity to threshold conditions.


More details on interpreting condition and trajectories of change can be found in Chapter 9 of the REFORM Deliverable 2.1.

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