2009 – Burdekin Falls Dam – Testing the Boundary of Hydrology

Rob Ayre, Simone Gillespie

The Burdekin Falls Dam (BFD) is a SunWater-owned dam completed in 1987. BFD is located in North Queensland, approximately 180 kilometres south of Townsville. BFD is the largest dam in Queensland having a storage capacity of 1,860,000 ML and it has the largest spillway capacity in Australia. The Burdekin River basin drains an area of about 114,770 km2 which is nearly twice the size of Tasmania. Runoff in the catchment is very reliable and flows have overtopped the spillway every year, except one, since it was built. The volume of inflow into the dam during a flood event is considerable, and water spills from the dam for an average of three months each year.

SunWater is investigating the raising of BFD, to increase the storage capacity of the dam by two metres or approximately 30% of its current storage capacity to 2,446,000 ML. In addition SunWater are investigating provisions to further stabilise the concrete gravity main dam to improve dam safety performance by ensuring it complies with current guidelines. Design flood estimation has advanced since BFD was constructed, as the techniques for determining extreme rainfall have been progressively refined. To meet current Acceptable Flood Capacity (AFC) guidelines, the flood discharge capacity at BFD must be increased by 35%. However, whilst this estimate was derived in accordance with current relevant guidelines (ARR, Book VI, 2001) the size of the BFD catchment means that this particular catchment lies on the fringe of the applicability of these guidelines.

Of particular concern is the assignment of the Annual Exceedance Probability (AEP) of the Probable Maximum Precipitation (PMP), which is based upon catchment area. The adoption of the AEP of the PMP for BFD at 1 in 9,000 has implications for the application of risked based approaches for the design.

This paper discusses the existing methodology of design flood hydrology used in Australia and identifies areas of concern for the application of such techniques for large catchments. It also discusses the methodology SunWater utilised in an attempt to meet existing guidelines within these limitations.

Keywords: Burdekin Falls Dam, Flood Hydrology, Probable Maximum Precipitation, Annual Exceedence, Probability