Dams pause Chris Davis
Australia has a proud dam building heritage: a quick overview of global statistics shows just where we fit. For each head of population, Australia has more than 4300 cubic metres (or kilolitres) of dam storage capacity in its 561 large dams. By contrast, South Africa has roughly 400 cubic metres, and Ethiopia some 43 cubic metres, per person. At the other end of the spectrum, the United States has around 7000 cubic metres of storage per person. The need for dams is threefold: to even out the extremely variable rainfall patterns by storing water; to protect downstream communities from flooding; and, wherever practicable, to generate hydroelectricity. While a storage dam needs to hold as much water as possible, a flood mitigation dam needs as much airspace as possible to catch a potential flood. That tension became very apparent in 2011 when Queenslandâ€™s Wivenhoe Dam, designed to provide flood protection, overtopped and raised questions about the relative allocation of volume to the conflicting purposes. An ideal dam site enables a relatively small wall across a geologically stable gorge to hold back a large volume of water. The shape of the impoundment should result in a water body that is deep but has a small surface area, thus minimising evaporation losses. Most important, of course, is the need for the dammed river catchment to have a flow profile, which will result in a strong net yield each year. Traditionally, hydrologists based their plans for dams on the historic record of rainfall and runoff in each catchment. However, Australian records are not very long, since collection of data began just over 100 years ago. A further challenge is posed by the fact that dam building peaked during an era when rainfall was probably higher than normal, so expectations were raised. Now, of course, strong evidence of human-induced climate change means that historic records have little to say about the future. Apart from the technical conundrums to be resolved, dams have other significant features: they impinge on the natural environment and the social situation. In the early days of civil
engineering, environmental impacts werenâ€™t understood or even considered, but they can be quite profound. Apart from the most obvious effect, that is, cutting off much of the water flow downstream, a dam interrupts normal patterns of fish migration up and down a watercourse, as well as physically collecting sediment. Moreover, when water is released from a dam, it is usually drawn from its lower levels, where the water is colder. That cold flow can have a negative impact on river ecology up to 200 kilometres downstream. On the social front, dam impacts are mixed: they can underpin agricultural irrigation enterprises, boost tourism and recreation, and earn revenue from power generation, but they also inundate valuable riverside farmlands and displace communities. Because there are winners and losers, dam projects are now almost invariably contentious. Historically, dams were a part of social engineering to provide jobs for returned soldiers, so they were typically not analysed in detail for economic viability. As dam building around Australia progressed the good sites were snapped up, so those currently on the drawing board are generally less than ideal. Applying rigorous economic analysis to a dam project can be fraught, and the decision to proceed becomes doubly difficult when attempts are made to balance economic, social and environmental factors. In the current political climate, it seems unlikely that any major city will build more supply dams in the short term. On a cost-benefit basis, flood mitigation may not be on the agenda either. Hydroelectricity, on the other hand, may play a growing role, since pumped storage systems (the iconic example, of course, being the Snowy Mountains Scheme) can play a stabilising role in the national grid, especially complementing the variable supplies from wind and solar sources. Pragmatically, irrigation dams seem seldom capable of paying their way, so other agendas will be in play for future schemes. A neat resolution to at least some of the common dam issues lies in the use of off-stream dams (those on
Published on Jul 27, 2011
Published on Jul 27, 2011
Securing water supplies has never been easy in Australia. Managing the nation’s rivers and water resources has been a learning experience, r...