Rapid Assessment of Physical Habitat Sensitivity to Abstraction (RAPHSA)

Physical habitat is one measure of river ecosystem health that is sensitive to changes in flow or channel geometry. It can be used to assess the impacts on the river ecosystem of changes to the flow regime caused by abstractions, or impoundment or alterations to channel geometry that result from river engineering, such as flood management works. An appropriate flow regime is recognised as an essential supporting element to achieving Good Ecological Status in water bodies, as required under the European Water Framework Directive. In England and Wales, the
Environment Agency achieves appropriate flow regimes through the management of abstractions and ensuring sufficient water is released from dams. In addition, the
Environment Agency is required to undertake environmental impact assessments of flood defence schemes and other river engineering works.

Physical habitat comprises the physical conditions in a river that determine its suitability for different species and communities, including water depth and velocity. The
relationship between physical habitat variables and discharge define the degree to which the physical environment changes as flow varies. How these relationships
change with channel geometry provides a measure of the sensitivity of the river to flow change. Physical habitat–flow relationships can be defined for any river reach, but
require repeat measurements of hydraulic properties at a range of flows, followed by hydraulic modelling and habitat use data by target species (if the curves are to be
related to particular organisms).

The Environment Agency thus requires a set of operational tools that assess physical habitat in a consistent manner regardless of the amount of data available. This paper
reports on a study to define a risk-based toolkit for physical habitat assessment. The risk-based approach is a trade-off between avoiding unnecessary work, and the costs
of achieving an acceptable level of certainty such that decisions can be made with reasonable confidence. This approach involves starting with simple tools and adopting
more complex techniques if necessary; that is, use the simplest approach that gives an acceptable level of confidence, moving to a higher level if the degree of uncertainty is too high. The toolkit was developed through analysis of 66 physical habitat modelling
studies across the UK. Each tool requires different input data, thus entailing various levels of investment in field data collection. It is recognised that the results from all
tools are uncertain. However, in broad terms, the more data available and the more complex a tool used, the better the understanding, although employing a complex tool does not guarantee less uncertainty.