Assessment of baseflow seasonality and application to design flood events in southwest Western Australia

Abstract

Design flood events are strongly influenced by seasonality in southwest Western Australia (WA). This region is also known to be dominated by potentially high baseflow contributions during large floods. The proportion of baseflow to use in design has limited understanding, especially in the large to extreme flood range. Australian Rainfall and Runoff Update Project 7 used an adapted version of the Lyne & Hollick (1979) baseflow separation method to develop prediction equations that estimate baseflow contributions for annual design floods. This study found that the Lyne & Hollick separation technique was not applicable to all sites in southwest WA. Instead a manual graphical separation technique was adopted which provided more representative baseflow separations. Catchment specific and regional seasonal trends were derived and upper envelope curves were adopted for regional design estimates to ensure baseflow contribution was not underestimated. In the Darling Scarp region the recommended baseflow proportion to the total flood peak for rare to extreme design floods is 4% in summer and 11% in winter. The highly variable nature of baseflow response suggests that, where possible, at-site gauged data should be analysed to assess design baseflow contribution.

Keywords:

• Baseflow
• Seasonality
• southwest Western Australia
• Australian Rainfall and Runoff Revision Project 7
• large to extreme design floods

Additional information

Notes on contributors

K Kinkela

Kai Kinkela is an environmental engineer with 3 years’ experience at the Water Corporation of Western Australia. He graduated from the University of Western Australia in 2009 with a Bachelor of Environmental Engineering (First Class Honours). Since graduating he has primarily been involved in large to extreme flood studies and dam safety inspections. He is currently undertaking the revision for the Ord Dam large to extreme flood study looking at a continuous simulation approach.

L J Pearce

Leanne Pearce has more than 25 years’ hydrology experience primarily focused on dams. She has been responsible for both yield and flood assessments of almost every small and large dam associated with the Water Corporation in Western Australia. Her experience extends to other states in Australia and southeast Asia, including small to large hydroelectricity schemes. Leanne’s specialised field is extreme flood hydrology.