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Dan Borg and others Cooperative Research Centre for Catchment Hydrology, School of Anthropology, Geography and Environmental Studies, The University of Melbourne, Australia, 3010. Abstract: Geomorphologists and engineers contribute to many stream rehabilitation projects by predicting the physical effect of habitat restoration structures. This paper describes one such contribution in a multidisciplinary trial. The project began as an evaluation of the scour associated with introduction of artificial wood structures into small sand-bed streams in NE Victoria, Australia. A main aim of the structures was to produce scour holes in the otherwise uniform, planar sand bed. The field trial reported here, on the Granite Creeks, had control sites, replication, and more frequent sampling than similar trials reported elsewhere. The uniform design of the structures was intended to minimise variables that influence scour, so that variation would be largely due to discharge alone. The results demonstrate that building simple wood structures across sand-bed streams will almost certainly increase scour depth beneath the log. However, scour is highly variable, in space and time, even in these uniform conditions. In general the understanding that comes from the monitoring (every few months) provides a poor basis for predicting the physical effects of future structures. This led us to develop a method of continuous monitoring of scour, using a pressure transducer approach, which we applied to another sand-bed stream, the Snowy River. The continuous monitoring showed a remarkably complex relationship between streamflow and pool scour. These detailed measurements provided even less confidence in a simple relationship between flow and scour. Scour response is variable and intimately related to all aspects of the flow regime. Scour is not only dependent on the size of a given flood, but also the duration of floods, and how frequently they occur. Both the Snowy and the Granite Creek trials have also shown that the pattern of scour is further complicated by build up of debris and larger scale changes in the bed. We argue for a need to rethink the traditional geomorphic and engineering scour approach. Traditionally engineering models of scour around instream structures have been concerned with thresholds of failure, and with bankfull flows. This failure-non failure dichotomy is of limited use for habitat applications. Nevertheless, infilling of scour pools has received little attention. The present notion of geomorphic effectiveness defines the effect of a flow relative to other flows, and emphasises maximum scour related to flows of lower recurrence interval (>1.5 years). Geomorphic effectiveness is not synonymous with biological effectiveness. The key issue for an organism, is not whether there is scour around an object (whether natural or artificial), but whether the scour occurs at a useful time. By considering annually occurring key life stages of organisms (such as migration or spawning), meshed with hydraulics and hydrology, we can gain insight into the effectiveness of structures for biota. |
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