Learning from experience?19 January 2006
Hubert Chanson and D. Patrick James discuss the long-standing problem of rapid reservoir sedimentation in Australia
SINCE the European settlement of Australia, the development of the country has been linked with the availability of water resources. Australia is a dry continent with an average rainfall of 420mm per year that is characterised by high spatial and temporal variability, including long droughts of 3-10 years and extreme rainfall events, for example 515mm in six hours at Dapto New South Wales (NSW) in 1984. The first inland settlements were significantly affected by the lack of a regular water supply. At the end of the 19th century and through the 20th century, numerous dams were built to provide reliable water supplies.
Between 1890 and 1960, more than forty reservoirs, excluding farm dams, became fully-silted. Some were town water supply reservoirs: e.g., Moore Creek dam, Quipolly dam, Umberumberka dam. Others were railway dams (built to supply water to the steam engines for the railway), such as Cunningham Creek dam and Illalong Creek dam. There were also mining reservoirs, like Junction Reefs dam.
Rapid reservoir siltation in Australia
Umberumberka dam, Broken Hill, NSW
Built between 1913 and 1915, the 41m high Umberumberka concrete arched dam – still in use today – was completed in 1915 to supply water to the town of Broken Hill, NSW, 900km west-north-west of Sydney. The region is very dry and water resources are scarce; the average rainfall over the catchment was about 220mm for the period 1939 to 1972. The original volume of the reservoir was 13.2Mm3 and the catchment area is 420km2 (Wasson and Galloway, 1986).
The dam is located on the Umberumberka creek, at a scarp immediately downstream of an alluvial fan. The dam traps almost all the incoming sediment material, and estimates of volume of sediment trapped in the reservoir have been made regularly since dam construction. Table 1 summarises the experience from the Umberumberka dam and two other dams in the Broken Hill region – the volume of trapped sediment is summarised in column 3. Note that most siltation occurred between 1915 and 1941 and the rate of sedimentation has since decreased drastically (Wasson and Galloway, 1986).
Korrumbyn Creek dam, Murwillumbah, NSW
The Korrumbyn Creek dam was built between 1917 and 1918, and completed in late 1918 to supply water to the town of Murwillumbah, NSW, 150km South of Brisbane. The 14.1m high dam was designed with notable structural features (Chanson and James, 2002), including a single arch wall (1.1m thick at crest, 5.2m thick at base, 61m radius in plan), made of Portland cement concrete. The cross-section of the arch wall has a vertical upstream wall and a battered downstream face. The dam was equipped with two bottom outlets (one pipe outlet valve and one scour valve) and with an overfall spillway. The volume of the reservoir was 27,300m3 with a catchment area of only 3km2 (Chanson and James, 1998).
The reservoir was quickly abandoned because a log jammed the scour pipe entrance during a flood and could not be removed, meaning the dam could no longer be scoured to remove the sediments. Even prior to this incident, the water level used to drop during dry periods and the water would turn green and foul as it warmed up, making it unfit for use. The dam, however, still stands today.
Furthermore, the catchment is very steep (bed slope >7.6º) and the channel bed contains a wide range of sediment materials. As a result, the reservoir silted up very rapidly by bed-load. For these reasons the life of the reservoir was very short (less than 20 years).
Cunningham Creek dam, Harden, NSW
The Cunningham Creek dam, located near Harden, NSW, was completed in 1912 to supply water to the railway. The 13m high dam is a curved arched dam and the catchment area is 850km2. The reservoir was over 1.3km long and about 110m wide, and the reservoir area at dam crest level is 150,000m2. The dam was equipped with a bottom outlet (pipe outlet), and a crane was installed at the dam crest to facilitate the outlet operation and to assist in clearing the outlet (Hellström, 1941).
The reservoir silted up very rapidly (Table 2) and this was well documented (Hellström, 1941). Attempts to flush sediments were not successful. During the first ten years of operation, the silt was deposited at a nearly constant rate of 49,000m3 per annum (60m3/km2/yr). The dam was nearly full (approximately 90%) by 1929. The reservoir was filled primarily by suspended-load. Another railway dam (Illalong Creek dam), located less than 30km from Cunningham Creek dam, suffered a similar sedimentation problem (Table 2) (Chanson, 1998).
Extreme reservoir siltation
Reservoir sedimentation results from soil erosion in the catchment (wind, rainfall and manmade erosion), surface runoff, sediment transport in the creeks, streams and rivers, and sediment trapping in the reservoir. Altogether, reservoir sedimentation is a very complex process. Practically the primary consequence is the reduction of the reservoir capacity and of its economical and strategic impact.
Extreme reservoir sedimentation has been observed all around the world (Table 3). Usually the sedimentation rate is defined as: sediment inflow - sediment flushing and dredging/year/km2 of catchment. Prototype observations are presented in Figure 3 where all reported data may be regarded as exceptional events. In some cases, extreme siltation rates might lead to the reservoir siltation and its disuse. Comparative analysis with overseas experience suggests that reservoir sedimentation rates in Australia were significant (Table 3, Figure 3). An extreme event was the sedimentation of Quipolly reservoir (NSW) between 1941-1943 (Chanson and James 1998, Chanson 1998).
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, society expects the useful life of a reservoir to be 50 years or more. 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. A total catchment management policy must be considered from the early stages of reservoir design.
Interestingly, Australian records indicate that the most extreme siltation periods took place during extreme floods following an El-Niño event. Such extreme siltation events were experienced at the Junction Reefs reservoir (1902 floods after the Great Drought of 1900-1902), the Moore Creek reservoir (flood of February 1908), the Gap weir (floods of 1919), the Quipolly reservoir (floods of 1942-43).
Many Australian reservoirs have been inadequately equipped with scouring devices. Most reservoirs were equipped with a single 0.3m diameter scour outlet that is inadequate to desilt a reservoir. Only few dams were equipped with two or more flushing systems, one example is the Illalong Creek dam, completed in 1914 and fully-silted. In comparison, the ancient Nabataeans (habitants from an ancient kingdom [BC312 to AD106] to the east and southeast of Palestine that included the Neguev desert who built a large number of soil-and-retention dams, some of which are still in use today), Romans and Spaniards equipped their reservoirs with large sediment flushing systems.
For example, Roman engineers equipped the Monte-Novo dam (AD300, Portugal) with two outlets of 1.2 and 1.4m2 cross-section area each. Such an expertise in sediment flushing, gained over the past 22 centuries, was obviously unknown to the early Australian engineers. Altogether this experience suggests that dam and reservoir management in the form of sediment scouring and flushing should be systematically applied and that soil conservation practices in the catchment must be reinforced particularly during drought periods.
Lastly, the effects of reservoir siltation must also be analysed in terms of dam safety. In several case studies, the sediment load associated with major flood events would create high loads on the dam wall, especially in the cases of thin concrete structures and older structures. In such situations the tested concrete strength could be less than twice the load, raising safety questions.
Reservoir sedimentation has been and is still a serious problem in Australia, and Australian engineers should draw upon local and overseas experience, and particularly from past failures. 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. Each fully-silted reservoir failure should be a valuable teaching and pedagogic tool to heighten awareness - society must learn from its mistakes and not repeat them again.
Hubert Chanson, Reader in Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia. Email: [email protected]
D. Patrick James, Patrick James & Associates, 29 Tabrett Street, Banksia, NSW 2216, Australia.