Hydrology of the Australian continent
Australia is a large continent (7,690 E+3 km2) of low relief. Its most prominent topographic feature is the Great Dividing Range, a chain of low mountains and tablelands extending over 2,500 km along the Eastern and South-Eastern coastlines. Although the average annual rainfall is about 420 mm, the spatial and temporal variability is high. The rainfall may vary from zero for several years during droughts to extreme hydrological events (e.g. 515 mm in 6 hours at Dapto NSW, in 1984). The average runoff is only 13% of the rainfall, varying from 0 mm in most Western Australia to over 700 mm in some regions of Eastern Australia and Tasmania. Indeed evaporation is high. Average annual standard pan evaporation exceeds 1,000 mm in nearly all parts of the continent, with extreme evaporation above 3,000 mm in Central Australia. High evaporation coupled with the variability of surface runoff make conservation and development of surface water resources more expensive, less effective and more political than in many countries.

Comparison between Australian and overseas siltation rates
The analysis of (well-documented) extreme siltation events in Australia suggests that sedimentation rates in Australia were high (CHANSON and JAMES 1999, CHANSON 1998). The siltation of Quipolly reservoir No. 1 between 1941-1943 was an extraordinary event : i.e., the sedimentation rate was in excess of 1,140 m3/km2/year. The Korrumbyn Creek dam sedimentation  was very rapid : i.e., less than 7 years ! The site selection was improper (CHANSON and JAMES 1998a,c). Overall sedimentation problems were experienced mostly with small to medium size reservoirs : i.e., catchment area less than 100 km2 typically. In contrast large reservoirs have not been sedimenting rapidly at the exception of Melton, Eppalock and Eildon reservoirs, all in Victoria. Heavy siltation at Eildon was experienced in 1940 during torrential rainfalls, following bushfires which destroyed more than 50% of the catchment forest. At Eppalock, the catchment was affected by extensive gold mining, tree clearing and hydraulic mining during the period 1851-1890, and rabbit infestation was another problem.
Since the 1950s, lower siltation rates have been experienced on the Australia continent. The decrease in reservoir sedimentation is related to the introduction of new farming techniques, new land conservation practices and an awareness of soil erosion problems. The trend is characteristic in New South Wales, Victoria, South Australia and Queensland. Note that most extreme (recorded) siltation rates were observed during short duration studies (1 to 10 years) and there is no information on long-term siltation (over 70 years).
Fully-silted reservoirs are potential hazards. During a large flood event (e.g. PMF), the sediment weight adds to he wall compression stress and the safety factor (ratio of concrete strength to load) could become less than two (CHANSON and JAMES 1999). With concrete structures, the properties of the wall must be tested in-situ and a proper analysis of the reservoir catchment and dam wall should be conducted for each individual structure.

Summary
The main findings of the study are : [1] reservoir sedimentation has been a serious problem in Australia, [2] Australian engineers could draw upon local and overseas experience, and from past failures, [3] reservoir siltation affects the dam safety, creating high loads on the dam wall. Several reservoirs became fully-silted because the designers did not take into account correctly the soil erosion and sediment transport processes, and no soil conservation practice was introduced.Today the society expects the useful life of a reservoir to be more over 30 to 50 years. One lesson from past experience is the need to consider the dam, the reservoir and the catchment as a complete system which cannot be dissociated. Soil erosion and water runoff lead into the streams some sediment material that is trapped downstream by the dam wall. A total catchment management policy must be considered from the early stage of a reservoir design.
Fully-silted reservoirs stand as a source of embarrassment for the scientists and the public. Each reservoir failure must be a valuable teaching and pedagogic tool to heighten the awareness of students, professionals and local authorities and of the public. Society must learn from its mistakes, not to repeat them again!
 

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Hubert CHANSON is a Professor in Civil Engineering, Hydraulic Engineering and Environmental Fluid Mechanics, at the University of Queeensland, Australia. His research interests include design of hydraulic structures, experimental investigations of two-phase flows, applied hydrodynamics, hydraulic engineering, water quality modelling, environmental fluid mechanics, estuarine processes and natural resources. He has been an active consultant for both governmental agencies and private organisations. His publication record includes over 600 international refereed papers and his work was cited over 3,500 times since 1990. Hubert Chanson is the author of several books : "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways" (Pergamon, 1995), "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows" (Academic Press, 1997), "The Hydraulics of Open Channel Flow : An Introduction" (Butterworth-Heinemann, 1st edition 1999, 2nd editon 2004), "The Hydraulics of Stepped Chutes and Spillways" (Balkema, 2001), "Environmental Hydraulics of Open Channel Flows" (Butterworth-Heinemann, 2004), "Applied Hydrodynamics: an Introduction of Ideal and Real Fluid Flows" (CRC Press, 2009), and "Tidal Bores, Aegir, Eagre, Mascaret, Pororoca: Theory And Observations" (World Scientific, 2011). He co-authored two further books "Fluid Mechanics for Ecologists" (IPC Press, 2002) and "Fluid Mechanics for Ecologists. Student Edition" (IPC, 2006). His textbook "The Hydraulics of Open Channel Flows : An Introduction" has already been translated into Spanish (McGraw-Hill Interamericana) and Chinese (Hydrology Bureau of Yellow River Conservancy Committee), and the second edition was published in 2004. In 2003, the IAHR presented him with the 13th Arthur Ippen Award for outstanding achievements in hydraulic engineering. The American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI) presented him with the 2004 award for the Best Practice paper in the Journal of Irrigation and Drainage Engineering ("Energy Dissipation and Air Entrainment in Stepped Storm Waterway" by Chanson and Toombes 2002). Hubert Chanson edited further several books : "Fluvial, Environmental and Coastal Developments in Hydraulic Engineering" (Mossa, Yasuda & Chanson 2004, Balkema), "Hydraulics. The Next Wave" (Chanson & Macintosh 2004, Engineers Australia), "Hydraulic Structures: a Challenge to Engineers and Researchers" (Matos & Chanson 2006, The University of Queensland), "Experiences and Challenges in Sewers: Measurements and Hydrodynamics" (Larrate & Chanson 2008, The University of Queensland), "Hydraulic Structures: Useful Water Harvesting Systems or Relics?" (Janssen & Chanson 2010, The University of Queensland), "Balance and Uncertainty: Water in a Changing World" (Valentine et al. 2011, Engineers Australia). He chaired the Organisation of the 34th IAHR World Congress held in Brisbane, Australia between 26 June and 1 July 2011.
 His Internet home page is http://www.uq.edu.au/~e2hchans. He also developed a gallery of photographs website {http://www.uq.edu.au/~e2hchans/photo.html} that received more than 2,000 hits per month since inception.

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