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Characterization of microhabitat flow regimes in streams and rivers on the basis of near-bed velocity, shear stress and turbulence intensity measured with an Acoustic Doppler Velocimeter (ADV)

Bouckaert, Frederick (1995) Characterization of microhabitat flow regimes in streams and rivers on the basis of near-bed velocity, shear stress and turbulence intensity measured with an Acoustic Doppler Velocimeter (ADV). Honours thesis, Murdoch University.

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Near-bed flow regimes in streams and rivers remain a poorly researched topic, despite its importance for the distribution of benthic invertebrates. The poor characterization of macroinvertebrate microhabitat severely constrains the further understanding of instream flow requirements to protect the ecological integrity of streams and rivers, and may be responsible for the wide confidence intervals often associated with macroinvertebrate sampling. Traditionally, benthic habitats have been characterized by depth, substratum and mean velocity (measured at 0.4 of the depth from the bed) despite evidence that these parameters are insufficient to describe microflow regimes. Some researchers have introduced more complex hydraulic variables to characterize flow regimes. Yet many of those variables cannot be measured directly, and the inferred values are not sufficiently accurate to describe microflow in ·quantitative terms. The difficulties involved in measuring velocities close to the streambed have now been overcome by the development of an Acoustic Doppler Velocimeter (ADV) which enables the real-time three dimensional measurement of velocities within centimeters of the bed. The resolution of 25 Hz also enables the calculation of turbulence statistics, so that shear stress and turbulence intensity can be obtained at a spatial resolution small enough to quantify flow regimes within 5 mm from the bed.

The objective of this study was to investigate the hypothesis that differences in the distribution of benthic macroinvertebrates in stream and rivers are partially or wholly related to differences in benthic flow regimes.

To this aim, an ADV was used in an artificial stream channel to examine the effects of different (homogeneous) substrata on near-bed flow regimes under conditions of constant depth and mean velocity. Near-bed flow regimes were characterized by velocity, shear stress and turbulence intensity. Macroinvertebrate distributions were examined upstream and downstream of isolated roughness elements in a riffle of a stream section, and related to microflow characteristics obtained with the ADV.

Under conditions of constant depth and velocity, homogeneous substrata of varying roughness resulted in significantly different microflow regimes. In the field, the distribution of benthic invertebrates contrasted significantly between upstream and downstream sampling sites, despite the absence of statistically significant differences in microflow regimes at those locations. It is suggested that the indirect effects of flow may sometimes be more important than the direct 'flow exposure' effects with regard to macroinvertebrate community composition.

The ADV has enabled a quantitative characterization of microflow regimes, including the description of turbulent flow conditions. This will enable further research into tolerance ranges of microflow regimes for benthic invertebrates and may offer the possibility to investigate critical values for shear stress and turbulence intensity. The quantification of microflow regime will enable a more accurate definition of macroinvertebrate (micro )habitats. This knowledge will find applications in many other areas of limnological research and in the management of streams and rivers.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Environmental Science
Supervisor(s): Davis, Jenny
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