Wivenhoe dam

THE OPTIMISATION STUDY OF THE OPERATION OF WIVENHOE DAM  
A review
G McMahon
Publication Date (Web): 23 November 2017
DOI: https://doi.org/10.21139/wej.2018.001


The Queensland Flood Commission of Inquiry (2011) recommended that a full and proper technical review be conducted of the Wivenhoe Manual for the flood operations of Wivenhoe Dam to include a study of what the 'best' rainfall forecast meant, how the forecasts were to be used, and their reliability. The Optimisations Study (WSDOS) was to be that technical review.

The control of floods through Wivenhoe Dam is representative of the generic challenges posed by gated, combined water supply and flood mitigation dams that are vulnerable to overtopping and dam break. The 2009 Manual operating rules were one of the measures adopted to reduce the risk of Wivenhoe being breached or overtopped by a large flood.

This paper evaluates this Optimisation Study, and maps the differences between the operations framework advocated by WSDOS, versus the 2009 Manual (Revision 7). WSDOS favoured Seqwater's 2013 Manual (Revision 11).

The primary focus of this evaluation is a comparison of the rules in Revisions 7 and 11 pertaining to the flood control regime. The comparison includes the operating rules for entering the flood mitigation regime, and those pertaining to entering the Save-the-Dam regime. The evaluation thus is focused upon the threat of a rising flood, whether that rise is the first or second or even third peak in that flood.

This mapping, however, identifies that, again, the principal issue is whether or not the best available rainfall forecast should guide the decisions on dam releases. The merits of the framework described by Revision 7 versus the framework advocated by the Optimisation Study (Revision 11) on the forecast rainfall factor are discussed. Practices overseas, like the Uncertainty Imperative (US National Weather Service), Forecast Based Operations and Adaptive Management (USACE) are cited for comparison, among others.

A critical test for deciding this issue was to conduct a model run of what would have happened if the Manual (Revision 7) had been complied with during the 2011 flood at Wivenhoe by using the best available rainfall forecast in making dam release decisions. This was not attempted by the Optimisation Study, which, like Queensland Floods Commission of Inquiry, passed on the forecast issues to later research.

There appears to be a fundamental divergence in philosophy. A key to explaining the differences in approach between Revisions 7 versus 11 may be the differences in the disciplines from which each revision of the Manual appears to have been derived. Revision 11 may be dominated by the deterministic hydrology paradigm, while Revision 7 may be led by the risk management discipline.

Current revisions of the Manual appear to have lost important aspects of the Risk Management approach to dealing with the uncertainties in rainfall. The importance of positioning the storage for the probable errors in forecasting may not have been appreciated by WSDOS. WSDOS may have considered that basing release decisions on actual rainfalls only was not making a forecast.

The paper recommends that further work is required in order to return, to the Manual, the principles of risk management, vital to dealing with uncertainty during large floods. 

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