STAGE 1 REPORT: IDENTIFICATION OF HYDRO-MORPHOLOGICAL PARAMETERS TO WHICH THE AQUATIC ECOSYSTEM IS SENSITIVE

DEVELOPMENT OF ENVIRONMENTAL STANDARDS
(WATER RESOURCES)

STAGE 1 REPORT: IDENTIFICATION OF HYDRO-MORPHOLOGICAL PARAMETERS TO WHICH THE AQUATIC ECOSYSTEM IS SENSITIVE

WFD48 Stage 1
March 2005

EXECUTIVE SUMMARY

Background to research

This project arises as part of a coordinated effort by the UK environmental agencies to prepare for implementation of the Water Framework Directive – specifically in relation to water resources regulation. Its overall aim is to develop methods for the establishment of regulatory standards for rivers and lakes. The project excludes Heavily Modified Water Bodies.

Objectives of research

This is a Stage 1 report based on review of the international literature in order to identify all hydro-morphological parameters which affect aquatic ecosystems – either being used by water users/regulators around the world or identified within the research literature. It includes a gap analysis to report on parameters which have not been adopted in reported studies or practices, but which may merit consideration in the UK. It provides a focus for discussion between members of the project steering group and the contractors, and a sense of direction for future stages of the project.

Key findings and recommendations

Most countries have various methods of determining environmental flows, each defined for a different purpose, e.g. strategic analysis, scoping or impact assessment.
Licensing of reservoir releases and abstractions present quite different problems and different methods have been developed to deal with these issues. With reservoir releases, the whole flow regime (apart from very large floods that by-pass the dam) needs to be created. Abstractions, by and large, have no impact on high flows and so the focus is on low flow impacts.
Where data are scarce, expert opinion is used, and increasingly a formal structured approach to getting consensus amongst a group of experts, including academics and practitioners is favoured.
There is wide acceptance that all parts of the flow regime have some ecological importance. As a result, there is a growing move away from single low flow indices.
Many methods determine environmental flows in relation to the natural flow regime of the river. Some methods define flow in terms of site characteristics, such as flow per unit width needed for salmon migration in Lancashire, but it has not been possible to examine the data or the basis of these derivations. Other methods define environmental requirements in terms of more direct hydromorphological elements, such as water depth and velocity.
Small scale studies have shown that flow interacts with morphology to define physical habitat (such as width, depth, velocity and substrate) for specific organisms. These quality elements vary spatially; water is deep in pools and shallow on riffle; velocity is high in riffles and slow in pools. Standards based on these quality elements at the broad water body scale cannot be readily defined. To implement standards at the reach scale, site data are essential.
Implementation of the WFD will require that environmental standards are applied for all bodies regardless of hydrological and ecological data available. Consequently, standards are required that can be applied without having to visit the water body. This means that standards must be related to parameters than can be obtained from maps or digital databases, such as river flow, catchment area or geology. Any resulting standards will have less predictive power at a local scale and cannot be tested using site data.
A hierarchical approach may be needed in which a broad scale approach, perhaps based on flow, is used as a screening tool to assess all water bodies. A more detailed approach, perhaps based on depth or velocity, may be applied to a smaller number of sites identified as requiring close attention.
The flow regime is complex and is characterised by timing, magnitude, duration and frequency; all of which are important for different aspects of the river ecosystem. To produce operational standards, there is a need to identify a small number of parameters that capture its most significant characteristics. For example the number of high flow events greater than three times the median flow has been shown to be related to the structure of macrophyte and macro-invertebrate communities in New Zealand (Clausen, 1997).
The equivalent for lakes is the water level regime. Water level is of direct ecological relevance since it determines the area of littoral zone exposed and, given its variability, the timing and duration of exposure. It is also directly related to water depth; it influences a range of system state variables including effective fetch, wave-base and re-suspension of fine-grained bed sediments; and it is linked to residence time. As for the river flow regime, there is a need to identify the most significant characteristics of the lake water level regime; for example annual or weekly ranges, seasonal maxima or minima, or rates of rise and fall.

The main outcome of Stage 1 was that the regulatory parameter for environmental standards for rivers at a broad scale should be flow, since data on potentially more ecological meaningful parameters such as depth and velocity are not widely monitored and cannot be determined with detailed surveys at all sites. Since flow varies greatly between water bodies, generic flow standards need to be expressed in dimensionless terms, such as proportions of natural flow or unit flow per drainage area or channel width. Nevertheless, UK agencies should develop a hierarchical approach to standards, where broad scales methods based on flow are used for screening, but detailed scale methods based on more directly ecologically meaningful parameters, such as depth and velocity, are used for site level impact assessment and license setting.

For lakes, water level is the key hydromorphological parameter because of its integrative role in relation to the volume and dynamics of flow (including residence time) and its relative simplicity of measurement. The relative ease of measurement however belies the paucity of existing long-term data in relation to natural regimes of lakes across the UK.

Key words: abstraction, flow release, hydrological regime, timing, magnitude, duration, frequency, hydro-morphology, parameters, standards, rivers, lakes.

Copies of this report are available from the Foundation, in electronic format on CDRom at £20.00 + VAT or hard copy at £25.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website