Hydrodynamic Variability in the Physical Biotopes of a Trout Stream

River research and management activities often focus on mesoscale habitat (mesohabitat) classifications such as physical biotopes (e.g. pools, glides, runs, riffles). The size and surface flow characteristics of these features are such that they can easily be mapped from the bank and incorporated into rapid field reconnaissance surveys (e.g. UK River Habitat Survey). Information from such surveys can then be used in physical habitat assessment, habitat modelling and river rehabilitation applications. The results of recent research suggest, however, that only broad associations exist between hydraulics and mesohabitats. So called ‘standard’ hydraulic variables (depth, mean velocity, Froude number) do not provide adequate discrimination between physical biotopes and may not constitute the most mechanistic descriptors of habitat for plants, benthic invertebrates and fish that have strong associations with high shear zones. Instead, the concept of ‘within-biotope hydrodynamic heterogeneity’ - defined as spatial and discharge-related variation in turbulent flow properties describing the intensity, periodicity, orientation and scale of turbulence - may hold promise as the foundation for a more effective and ecologically relevant classification. Given the dearth of information on the hydrodynamic characteristics of physical biotopes in UK lowland streams, the first step in testing this conceptual framework was to collect high frequency velocity data from a range of habitats on the Leigh Brook, a third-order stream in Central England. A series of 90 second velocity records was taken from each of four types of physical biotope commonly found on lowland rivers using a 25 Hz Acoustic Doppler Velocimeter (Nortek NDV). Measurements were taken at 60 mm from the bed, in order to characterise nose velocities of trout, and at 0.6 of the flow depth. In discriminating between physical biotopes, turbulent flow properties were at least as important as standard hydraulic variables. Levels of spatial and discharge-related variability in turbulent flow characteristics differed between habitats, providing support for a classificatory framework based on within-biotope hydrodynamic heterogeneity.