Guidance on Environmental Flow
Releases from Impoundments to Implement the Water Framework Directive
Project Extension 2
– Validation of Managed Flow Standards
Background to research
Under the Water Framework Directive (WFD), member states of the
European Union should aim to (a) achieve the objective of at least
"Good Status", comprising Good Chemical Status and Good Ecological
Status, in all bodies of surface water and groundwater, and (b) to
prevent deterioration in the status of those water bodies.
SNIFFER research project WFD82 was completed in May 2007, and produced
guidance on environmental flow releases from impoundments for the Water
Framework Directive. It recommended thresholds of alteration (condition
limits) from the natural flow regime which determined the risk of
failure of Good Ecological Status. The condition limits determine the
deviation, expressed as a percentage of the natural condition, of a set
of flow regime descriptors. The condition limits recommended in WFD82
were chosen using expert judgement. WFD82 identified a clear need to
improve the scientific basis of these condition limits using
hydrological and ecological data. This study has used river flow and
macroinvertebrate biology data to try to validate these proposed
Objectives of research
- Agree an approach for the project,
specifically the target biological group and the data that would be
available to indicate generic ecological impacts of managed flow
- Collate from national agencies
(Environment Agency – England and Wales, SEPA - Scotland, DOE
NI – Northern Ireland) a matched hydrology-biology dataset
suitable for analysis to answer the project aims and process the
dataset to a form suitable for subsequent analysis. This includes
- hydrological data,
including gauged flows and long term natural flow statistics estimated
using Low Flows 2000 for downstream gauging stations and biology sites.
- Biology data as raw
taxonomic data and indices, plus expected values for indices (Average
Score per Taxon – ASPT and Lotic Invertebrate Indicator for
Flow Evaluation) calculated using RIVPACS.
- Determine the degree to which the
impact of a dam on a watercourse can be identified through differences
between indices calculated from downstream and reference biology samples
- Determine the
the magnitude of any impact to the biotic indices, and the magnitude of
deviation from estimated natural values for the hydrological parameters
identified as part of the WFD82 project
- Validate the WFD 82 flow
against biology status classes for at least one quality element
(overall project aim)
- Assess the suitability of
- The project assembled a dataset from
national monitoring sources. For selected sites downstream of
impoundments, It paired observed (impacted) and expected (natural)
hydrological data with observed biology data and expected biotic
scores. Where possible downstream sites were also paired with reference
sites upstream of the impoundment or on an unregulated tributary. The
analysis was split into two parts; firstly considering upstream /
downstream differences, secondly considering change in hydrological
impact as one moves downstream.
- From the initial analysis of
Environment Agency data it appeared that there was a consistent
difference in O/E (observed/expected) biotic scores between the
potentially impacted biology sites downstream of dams and associated
reference sites. Although statistically significant, the reference /
downstream difference may not be of practical significance given other
sources of variation in the biology scores. The LIFE O/E score was a
more precise indicator than APST O/E. For status class intervals based
on ASPT, out of 16 impoundments considered, there were only three cases
where the downstream site(s) failed to achieve good status but the
reference sites did achieve good status.
- For the SEPA data alone, there was no
evidence of a consistent upstream / downstream effect on O/E ASPT
scores, although it should be noted that on average, the SEPA sites
were less hydrologically impacted than those of the Environment Agency.
- For both SEPA and Environment Agency
data there was clear evidence for significantly reduced variability in
biotic scores downstream of impoundments. There could well be
consistent trends in the raw biological data which lead to this reduced
- Considering degree of hydrological
alteration which varies between impoundments and reduces as one moves
downstream, there were relatively few downstream biology sites with a
greater than 40% alteration to the flow regime. There was no overall
clear pattern between degree of hydrological alteration and the biotic
scores. For the situations with multiple sampling sites downstream of a
single impoundment there were some weak patterns, but there were not
sufficient examples to make further conclusions. Hence overall, it was
not possible to validate the WFD82 hydrological standards.
- There were relatively few sites with
elevated flows. There did not seem to be a continuum of response of the
biota from reduced flows, through minimally altered flows to elevated
flows. However there is some weak evidence that sites with elevated
flows did show lower variability in biotic scores.
Key words: water framework directive, compensation flow, dam,
impoundment, flow regime, flow release, hydrological regime,
validation, standards, rivers, biotic scores, monitoring, APST, LIFE.
- Further progress to define
hydrological standards for impoundments will need an alteration of the
current monitoring network, or additional monitoring sites.
- Additional biological data are
required to improve representation of downstream attenuation of
hydrological impact, in order to characterise reference sites and to
improve spatial representation.
- Further analysis would also benefit
from more complete characterisation of the releases from the
impoundments; this would also enable more detailed temporal analysis of
the biological data and better characterisation of changes in flow
variability that may be driving the observed changes in the variability
of the biological indices.
- This project has focused on using
biotic indices as the response variable, and in particular ASPT. An
alternative tool would be multivariate analysis. If this technique is
applied to spatial and temporal biological data, it can be hampered by
the natural differences in community composition between catchments.
However the community response needs to be addressed and further work
is definitely needed to define indicator taxa.
- Further work is needed to assess the
suitability of the RIVPACS approach to defining expected values for
indices. This is primarily because the at-site physical variables used
in RIVPACS may themselves be impacted by impoundment. Because of the
complexities in impacts of dam regulation, accompanying
hydromorphological data are particularly valuable for assessing
- Collation and analysis of data for
algae, macrophytes and fish will undoubtedly improve our understanding
of the impacts of impoundments.
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.
The report is available for download from the SNIFFER Website