MICROSCALE HYDRODYNAMICS AND COHERENT FLOW STRUCTURES IN RIVERS: IMPLICATIONS FOR THE CHARACTERIZATION OF PHYSICAL HABITAT

Techniques for the rapid appraisal and documentation of water resources and habitat are increasingly utilized in a variety of legislative compliance situations and environmental management/desion projects. Within river survey, the 'physical biotope' (i.e. riffles, runs, pools, glides) has been advocated as the basic unit of river habitat and an appropriate focus for stream reconnaissance and habitat assessments at the mesoscale of river systems. To date, however, much research into the hydraulic characterization of physical biotopes has focused on spatially- and temporally averaged velocity and depth measures, overlooking more detailed hydraulic parameters and variation at microscales. which may have more direct influences on the survival of individual organisms. This paper outlines the methodology, analytical techniques and initial results of a microscale exploration of the hydraulics of physical biotopes through the analysis of a range of turbulence properties. Results of the analyses offer both detailed descriptions of hydraulic habitat provided by the biotopes studied (related to various mechanisms of turbulence generation), as well as an indication of levels of 'within-biotope' heterogeneity which gives rise to a conceptual classification. A continuum of increasing habitat complexity is observed from glide, to riffle, to pool biotopes, associated with the generation of turbulence and organization of flow structures and with the level of 'internal' spatial and temporal hydraulic heterogeneity. Physical biotope characterization approached in this way may offer a more robust and transferable classification compared to mesoscale methods, particularly if levels of within-biotope heterogeneity are treated as an additional biotope characteristic. Copyright (C) 2008 John Wiley & Sons. Ltd.