The Development of a River Habitat Survey Methodology for Tidal River Sections
It is notoriously difficult to characterise and evaluate complex natural, semi-natural and impacted environments. Even when visual contrasts between sites appear obvious, their measured characteristics may be frustratingly similar. Yet many of the priorities of environmental management – including quality assessment, sensitivity ranking and sustainability – demand some form of survey that allows sites to be characterised and their significant changes to be monitored. If we add the practical restrictions on funding for survey and the limits on the breadth of skills of available surveyors (most of whom will have trained in a single environmental discipline), the challenge is compounded. And yet the demands continue to mount: criteria and indicators to be established, accountability and transparency to be demonstrated, and now the Water Framework Directive to be absorbed.
It is against such a background that the development of a Tidal River Habitat Survey (TRHS) must be considered, and through which the viability of proposed survey approaches should be assessed. The twin requirements of good science and robust management have to be balanced within a system that is defensible from both points of view. If this challenge seems daunting, then comfort can be taken from the fact that the RHS (River Habitat Survey) initially faced similar difficulties, but after some five years of development has emerged as an approach with major routine achievements, a clear role within the conservation agencies, and an even greater future potential. Whether or not a TRHS would offer a similarly positive balance of advantage over disadvantage is addressed in this report.
The River Habitat Survey (RHS) offers a semi-objective method of assessing the quality of river habitats. Having been initiated by the National Rivers Authority and Environment Agency of England and Wales, the RHS has subsequently been developed jointly with Scotland and Northern Ireland. The technique and its associated national database and training/accreditation system are now operational. However, the approach was developed and tested within fluvial sections of rivers, without a clear definition of the point downstream at which the morphological characteristics and processes influencing habitat and the ecophysiological conditions are modified by tidal flows. Consequently, Sniffer commissioned the GeoData Institute to evaluate the scope for developing a system of survey, comparable to RHS, for tidal river sections – a Tidal River Habitat Survey (TRHS).
It is clear that considerably less survey and research attention has been given to tidal river sections, and consequently the characteristics, drivers and range of variation is less clearly appreciated. Of necessity it is essential to identify the aims and objectives for developing such an approach and then review the technical requirements for its successful implementation. A series of high level aims and operational objectives identified for RHS (Raven 1998) have counterparts in tidal river sections, although the context within the catchment and the specifics of the operational tasks performed and practical applications of TRHS may be significantly different and the Water Framework Directive may introduce some variation in priorities.
Developing a TRHS technical methodology will need to address similar issues experienced within the evolution of the RHS methodology, namely, the framework of stratified random sampling, standardised reach definition, and spot-check and sweep-up recording, surveyor accreditation and data entry and management procedures. The technical methodology also needs to reflect the specific objectives of the TRHS in determining habitat quality and modification scores. Despite the experience developed within RHS it is anticipated that modification of the procedures will be needed to account for the specific characteristics and range of variation found in tidal river reaches.
Despite considerable overlap between the aims and approaches within THRS and RHS there remain a number of fundamental distinctions of both method and technique that need to be addressed. These include:
The definition of what constitutes a tidal river section is uncertain both at the downstream and upstream limits and may be thought of as a transitional zone over which many of the controlling ecological and physical variables fluctuate, such as salinity, tidal rise and fall and freshwater flow conditions. Thus there is a continuum from fully fluvial conditions through tidal river to estuarine and coastal systems. Various classifications are reviewed and a preliminary assessment of the extent of tidal river sections in Scotland and Northern Ireland estimates there to be about 433 and 83 km respectively, or about 1% of the fluvial extent. However, the full extent of the TRHS survey zone requires further field assessment and may be based more on pragmatic issues than a rigidly and consistently identifiable limit. Tidal influence extension upstream is clearly underestimated by the tidal limit identified on OS maps, given the zone of over-topping tidal influence and the additional tidal influence in river flood flows that will clearly depend on slope. Such blurring of boundaries also extends downstream suggesting that a link with estuary and coastal habitat surveys may be appropriate. Alternative definitions of the extent of the tidal river section, for example, based on salinity also appear problematic to apply. Salinity zones constituting tidal rivers vary seasonally and cannot be identified without averaged salinity recording, which are unavailable in most systems. Thus whilst salinity status may be regarded as a habitat attribute that could be recorded alongside TRHS site survey if time and instrumentation permitted, it cannot serve as a primary definition for survey design purposes. A pragmatic definition of the downstream limits is suggested based on tidal section dimensions, but requires specific testing.
Standardisation to absolute rather than relative dimensions for survey followed a long debate within RHS. The balance of advantage stresses maintenance of compatibility where this can be achieved in a scientifically robust and operationally effective manner. As a starting point it is recommended that spot and sweep-up should be adopted for the TRHS. The default 500m should be adopted with 10 equally spaced cross sections as spot checks. However, within piloting of the approach the operation of the 500m reach length should be examined further.
The narrower definition of the tidal river sections when compared to the fluvial RHS may make it harder to generate a representative sample of sites that accurately reflect the variation and the potential for geographical variation in a statistically valid way. The RHS selection of a sampling framework of 10km2 with a number of sites within each provided a robust measure of spatial variation. It is therefore recommended that every tidal river should be allocated at least one TRHS site. Additionally, at project level the number of RHS sites has been increased. The approach adopted in RHS is recommended for TRHS with a low density national coverage and subsequently enhanced sampling with more detailed survey. In order to produce an acceptable minimum density of sites it is recommended that another sample criterion be adopted. This might select a minimum of one site per 5km length of tidal river.
Fixed attributes in the sense applied by the RHS methodology are derived from maps. These are largely catchment-based attributes (selected because they “drive” the river system) and may be less relevant to the tidal sections. Many of the attributes (altitude, distance from source) are surrogates for power (the ability of the system to do work). This measure is less applicable to the tidal river section where the ability to measure “power” indicators may rely on a mix of fluvial and tidal attributes. The selection of component characteristics is a trade-off between what is ideal and what is practical within the constraints of time and survey conditions. This trade-off is inherent in the sampling design, the selection of features to represent characteristics and act as indicators and the extent to which spot-checks and sweep-up developed for RHS have counterparts within TRHS. The actual attributes that can be observed also represent a trade-off between scientific rigour and practical feasibility. The selection of “natural features” has the same inherent problems as experienced within RHS in the lack of truly natural examples, with many upper estuary/tidal sections controlled by weirs and modification of flow, channel bank protection and dredging activities. The selection of semi-natural sites may be the best compromise, as was accepted for the RHS.
Selecting the variables and attributes to be recorded (within the field and desk-based elements) within TRHS and the scales at which the data are recorded becomes a trade-off between data quality and data acquisition costs. Within TRHS there are a number of additional attributes (over those collected for RHS) that are necessary to reflect the diversity and specific character within tidal sections. The attributes and the additional variables are reviewed. It is recommended that the attributes be developed into a prototype field survey form to be tested within selected tidal river sections. Calculating Habitat Quality and Habitat Modification Scores may also adopt similar procedures, but will require further verification of attributes and categories and validation to develop meaningful indices.
Channel forms and interfaces between the river and the estuary or coastal sections represent a variety in detail. However, the variables may conform to a relatively few dominant characteristics and broad classes. The rationale for classifying tidal river habitats allows the assessment of management response that may be appropriate to sites with similar characteristics. Classification will also closely relate to the requirements for mapping and data presentation. Following RHS it will be necessary to develop statistically valid approaches, although there may still be a place for more descriptive classification. Three procedural stages are identified in classification, typology, taxonomy and allocation. The classification system for rivers (both within RHS and previous research) focused on physical characteristics and geomorphological rather than biological variables. It is likely that these will again dominate within tidal systems, although little attention has been given to this aspect within tidal river sections. Broad morphological classifications have been applied to estuarine systems, but these may have limited application within the tidal river sections. The TRHS population is likely to be significantly smaller than for RHS and thus sets limits on the statistical robustness of classification or ordination. The quality of Scottish and Northern Ireland tidal sections is likely to be significantly higher than within other parts of the UK and thus may provide a good range of “semi-natural” sites, although the degree of anthropogenic modification may devalue the use of a semi-natural designation. Given their value within RHS, particular attention should be given to indicators of power within the classification and ordination of tidal river classes.
The value of a TRHS will be significantly enhanced by a data-handling infrastructure built around the incorporation of high quality data. The approaches adopted within RHS have been refined and are very relevant to the application of TRHS, and although compatible data structures (with RHS) are not essential the same principles of an available, co-ordinated inventory will need to apply to fully capitalise on TRHS investment. As importantly, standardisation and accreditation within national survey of tidal rivers will allow the collation within a central database and allow the development of Habitat Quality Index and Modification Scores. Ideally the model and procedures would be extensible to all British (and potentially overseas) tidal river sections. Such extension has particular benefits in extending the sample size. Key tasks will be the development of a database and procedures that validate data entry, although the level of integration with the RHS may be more limited. Spatial (GIS) data handling may benefit TRHS especially where the ability to derive attributes of the system can be enhanced and where it has positive benefits in sample selection procedures.
Once again, RHS procedures for dissemination and availability of the database for research and educational use has increased the profile and range of applications to which RHS data is applied. The same approaches may be adopted for TRHS and match the organisational data policies and EU Directives for wider access to environmental information. Clearer definition of the data storage and data preservation may be necessary within the survey and data management procedures to ensure the longer-term value of the information for monitoring and indicator applications.
A number of potential constraints are likely to apply to the operation of TRHS. Many of these issues have also been faced by RHS implementation and the procedures adopted for health and safety, accreditation and quality control on data entry and processing may apply equally to TRHS. Major issues include:
This project has confirmed that the development of TRHS has great potential to yield similar benefits to those developed within the RHS initiative. There is a clear trade-off between high compatibility with RHS and a survey developed specifically for tidal river sections. Compatibility has benefits in permitting comparison of tidal and non-tidal sections and in capitalising on the operational methodology adopted for RHS. However, the methodologies may only partially reflect the tidal environment and omit important variables, particularly in terms of the energy drivers. The uncertainties of definition of tidal rivers suggests that full compatibility will not be achieved in tidal reaches, and the tantalising prospect of an estuarine or coastal habitat survey would require still further deliberation. Nevertheless, it is suggested on the basis of RHS experience that detailed refinement is best undertaken during the developmental and operational phases, rather than in waiting for the emergence of an ideal specification before committing to the project. TRHS has the potential to provide both a standard survey protocol and a database. If statistically valid, the database offers a site comparator and a monitoring framework, together with the basis for constructing indicators and indices. Whilst not all aspects of the sampling and attributes can be defined at this stage the potential benefits realised through RHS suggests that there is every prospect that similar roles may be achieved by TRHS, even if the methodology evolves. A process of TRHS prototyping through operational application is now required, and the Water Framework Directive offers the ideal incentive and context for this.
Tidal River, Estuary, River Habitat Survey, Classification, Scotland, Northern Ireland.
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