Evaluation of bankfull stage from plotted channel geometries

Abstract

Study region: North-eastern Tasmania, Australia. Study focus: Two quantitative methods for determining the bankfull stage of alluvial rivers from plotted channel cross-sections were assessed. Bankfull stage estimates determined using the minimum width-to-depth ratio and the first maximum of the bench-index were compared, and then evaluated against qualitative estimates of bankfull stage, using data from 89 cross-sectional surveys along nine alluvial river reaches in north-eastern Tasmania. New hydrological insights for the region: The minimum width-to-depth ratio was found to provide lower estimates of bankfull stage than the bench-index, with an overall mean ratio between the two methods of 0.84. Qualitative estimates of bankfull stage generally fell between those provided by the two models, with the minimum width-to-depth ratio performing better when assessed against qualitative estimates. These results show that while neither method provides a suitable stand-alone means for estimating bankfull stage, in combination they provide an effective means to approximate the range of bankfull stage. The high variability in bankfull stage found along a reach in this study indicates that where possible bankfull stage is best determined at the reach rather than single cross-sectional scale. Bankfull stage should be presented as a range of values with associated confidence intervals rather than as a finite result, with definition of bankfull stage estimates and sample number also reported.

Keywords

• River channel
• Cross-section
• River reach
• Bankfull stage
• Tasmania
• Australia