Engineers Australia 29th Hydrology and Water Resources Symposium 21–23 February 2005, Canberra - 1 - Variability in river bench elevation and the implications for environmental flow studies Geoff Vietz Postgraduate Research Student, Cooperative Research Centre for Catchment Hydrology, The University of Melbourne, Melbourne, Australia E-mail: g.vietz@pgrad.unimelb.edu.au Mike Stewardson, Ian Rutherfurd Cooperative Research Centre for Catchment Hydrology, The University of Melbourne, Melbourne, Australia ABSTRACT Environmental flow studies commonly utilise in-channel morphological features to identify ecologically important flows. Horizontal in-channel features, commonly referred to as benches, are often identified for periodic inundation. While the inundation elevation of benches is assumed by these studies to be consistent throughout the reach, considerable variability is evident, and previous investigations have failed to identify a common elevation for benches along a reach. Much of the variability found in bench elevation can be attributed to the effects of local scale hydraulics on bench formation and destruction. Much of the variability can be taken into consideration by classifying benches, according to location in planform or to a channel irregularity as: concave, point, lateral, marginal, tributary confluence or lee/feature benches. Application of this classification for identifying bench types in the Buckland and Ovens Rivers, northeast Victoria, identifies trends in bench inundation elevation between bench types along a reach, as well as patterns in bench type distribution at a catchment scale. Knowledge of relationships in channel form has implications for the use of benches in identifying environmental flows. INTRODUCTION Many rivers in Australia can be described as compound channels, characterised by essentially horizontal in-channel features such as benches and bars. The fine grained and often vegetated horizontal in-channel features commonly identified by environmental flow studies, are defined in this paper as benches. Benches are commonly targeted by environmental flow studies for periodic inundation. Many ecological benefits are cited for bench inundation including: nutrient dynamics such as oxygenation, decomposition and denitrification (Nanson and Page 1983), species regeneration and composition (Junk, Bayley et al. 1989) the distribution of flora and fauna (Changxing, Petts et al. 1999) and the recruitment of Large Woody Debris (LWD). Flow recommendations for bench inundation are generally based on a single target flow, however, previous investigations have found little or no consistency in the elevation of benches along river reaches (Woodyer 1968; Warner, Sinclair et al. 1975; Erskine and Livingstone 1999; Cohen 2003). It is suggested that this is a result of a lack of consideration of local scale hydraulic conditions, which impact on bench formation and destruction. This paper tests the hypothesis that consistent elevations exist for bench types, classified by considering local scale influences on bench formation. Such a classification has been recently developed (Vietz, Rutherfurd et al. 2004) and is summarised in this paper. The classification is applied in the Buckland and Ovens Rivers, north-east Victoria, to examine variability in bench elevations along reaches and throughout a catchment.