Information about the distribution and characteristics of weirs is important in modelling fish movement and connectivity. One weir characteristic that is rarely available in literature is the weir drown-out threshold. This threshold describes the stream discharge where tailwater levels exceed the height of the weir, providing a theoretical opportunity for fish to move over the weir. It therefore provides a means of relating stream discharge to opportunities for fish movement. In this project, we have used an Excel spreadsheet devised by Keller, Peterken & Berghuis (2012) to calculate weir drown-out thresholds for four gauging weirs in the Upper Condamine River catchment, and validated the results using depth loggers and gauge data.
Each gauging weir site was surveyed to populate the spreadsheet with the weir dimensions and the slope, roughness, cross-section and rating table of the downstream channel. Loggers were installed upstream and downstream of the weir to record depth over 12-18 months, thus detecting drown-out events. The discharge at drown-out was determined by cross-referencing the time at which drown-out commenced with discharge recorded by the gauge. The frequency of flows exceeding the calculated and measured drown-out thresholds (ML/day) were compared using the Wilcoxon Rank Sum Test. The results showed that the thresholds were not significantly different for three of the four sites.
The Keller, Peterken & Berghuis (2012) spreadsheet provides a method for calculating a drown-out threshold for critical barriers to fish movement. This can be achieved quickly, with minimal cost and at an accuracy suitable for modelling fish movement. The drown-out threshold provides water managers with tool to determine whether the existing or proposed infrastructure provides opportunities for fish passage, and at what frequency. At the same time, it produces values that can be used to attribute passability scores to barriers when developing connectivity models.