Water Journal : Water Journal March 2011
opinion regular features 14 MARCH 2011 water Much Ado About Dams To dam or not to dam, that is the question... The recent major flood events around Australia resulted in intense debate regarding the application of dams as combined water supply and flood mitigation infrastructure. In light of the tragic aftermath of the floods, the debate suffered from excessive rhetoric and sentiment, missing the opportunity to address the subject with a balanced discussion. Gregory Priest, AWA's Project Manager -- Sustainability, asked two experienced and highly regarded water professionals from both sides of the fence for their views on the subject. We also asked an expert from the Australian Bureau of Meteorology to comment on the severe weather conditions Australia has been experiencing. Keith Murley, Retired Engineer & Consultant Keith Murley completed a Civil Engineering Degree at Melbourne University in 1949 and joined the Victorian State Rivers & Water Supply Commission (later called the Rural Water Commission) in 1950, retiring in 1988 as Chief Designing Engineer. After a year of retirement, he was co-opted onto the NSW Dams Safety Committee as an IEAust rep, and also did some consulting work on dam safety. He also worked with the Australian National Committee on Dams, involved with the preparation of guidelines on floods and dams, and as an Australian expert on the International Committee on Dams. Keith finally retired in 2000. Email: firstname.lastname@example.org More Dams Needed For Long-term Demands "When you are up against nature there is not a lot you can do" This newspaper headline concerning the catastrophic Queensland floods highlights that no measures can ever give full protection against extreme natural events. Such events cannot be accurately predicted in size or occurrence, particularly with our limited data of some 100 to 200 years. There have always been droughts and floods, but the major difference now is the big increase in consequences, with the significant growth of population and buildings in the flood plains. Dams have been built from ancient times to help balance the vagaries of nature, initially to meet the essential life requirement of water when rainfall and river run cannot be relied upon. Australia, the driest continent, has a bigger problem with widely varied climate and unreliable rainfall, subject to natural disasters from irregular cycles of long-term droughts, bushfire and floods. Australia would not have been able to develop its cities, industries and food production without dams to store surplus river flows as available, both to regulate for water supply and, now, to assist in power production and in mitigating the effect of floods. The first settlement at Sydney soon found river runs were not reliable and suffered the 1828 drought. The first dam for Sydney was built in 1856, Melbourne 1857, Hobart 1861 and Brisbane 1866. Towns and farms would not have survived the recent long-term drought without the regulated supplies from dams. Public Misperceptions Many people in the community, whose memories seem short, tend to be misled by statistical information and terms such as "protection for the 100-year flood", believing this to mean that a flood won't happen again for another 100 years. What it really means, however, is that there is a one in 100 chance in any year of such an event -- or bigger -- occurring, even in successive years. This is a relatively common event. Dams are designed for extreme long-term droughts and extreme flood events to a nominal one in a million chance. Similarly, weather is reported related to statistical "averages". Averages, however, are a mathematical concept. Nature has never provided guaranteed or "average" flows. When assessing environmental needs the question must be asked: what happened in the past during droughts and floods, before dams were constructed? Pre the Hume Dam, the Murray River often ran dry for long periods, as shown in photographs taken in 1914 showing locals having a picnic in the river bed, and in 1923 with a future Chairman of the Victorian Water Commission standing in the river bed in a trickle of water. Dam Safety Strategies The Australian National Committee on Large Dams (ANCOLD), in association with the International Committee of Large Dams (ICOLD) has produced Technical Guidelines to world's best standards on all aspects of dam design, construction, operation, risk management and emergency measures to guide dam owners. The safety of the community, considering the range of economic, environmental and social issues, is the major factor. No major dam has failed in Australia. Risk Management, which is a combination of occurrence and consequences/damages, assists in determining the critical issues; however, if a lower-cost remedial measure is adopted courts can be critical if failure occurs and lives are lost. The solutions must involve a balance between community and environmental needs. Large dams are generally designed to safely cope with floods that are far larger than those previously experienced, but can only lessen the effects of flooding by reducing the peak flow. It is not practicable to build a storage to hold back the massive volumes of water that flow into a reservoir over a short period of time; but after absorbing the peak, it must spill water to protect the dam once the flood capacity of the reservoir is reached. ICOLD works through Technical Committees, including Australian representatives, to improve knowledge and promote advances in dam engineering and management and environmental effects. Technical Bulletin 125 in 2003 on "Dams and Floods" reported on a study of: extreme floods throughout the world; the role of dams in reduction of flood damages; safety of dams under flood conditions; and other associated measures, including catchment management, with flood basins and soil conservation, and construction of levees; plus non-structural measures such as zoning, land use controls, insurance, building regulations in flood plains and emergency action plans.
Water Journal April 2011