Cascade Failure in River Systems with Multiple Dams

It is time once again to speak of dams and things. It is not that I'm becoming paranoid about dams. At least I don't think I am. It is simply that the more I see and read and hear the more I believe dams, and their other attendant water control/management infrastructure, to be perhaps the greatest infrastructure risk for society during the long, painful implosion of the global economy, and our individual national economies, that will follow peak oil. It is not the greatest overall risk, of course.

The greatest risk to our bloated human population will be the collapse of our industrialized agriculture system and our inability to produce and distribute sufficient food for our global numbers, especially with the collapse of the global distribution system with the steady decline of oil and natural gas availability, on which modern agriculture and food processing are critically dependant. Death by starvation is a slow, tortuous process, taking the young, the old and the ill first. But the collapse of a large dam, or a series of dams of various sizes in a common watershed in a cascade failure, represents a sudden and inescapable catastrophe for all of those in harm's way downstream from the collapse.

There are over 45,000 large dams (defined as having a height of more than 15 metres (48.75 feet), or above 5 metres holding a reservoir volume of more than 3 million cubic metres (87.75 million cubic feet)) around the world[6]. The majority of these are, you may be surprised to learn, in developing or underdeveloped nations. Although new dam starts have slowed in the past decade, according to the report 17 Large Dams Under Construction by Basin - Watersheds of the World, "As of 1998, there were 349 dams over 60 meters high under construction (IJHD 1998). The countries with the largest number of dams under construction were Turkey, China, Japan, Iraq, Iran, Greece, Romania, and Spain, as well as the Paraná basin in South America. The river basins with the most, large dams under construction were the Yangtze in China, with 38 dams under construction, the Tigris and Euphrates with 19, and the Danube with 11."[7]

Virtually every large river system in the world has numerous dams on both the main course and the various tributaries flowing into it. Even the mighty Amazon, viewed by most as one of the world's last, great unspoiled rivers, will soon have dozens of dams throughout it's watershed. The Brazilian government plans to build 31 new dams in the Amazon region by 2010. The largest of Brazil's planned hydro projects will "convert the Tocantins River into a series of lakes and hydro-electric dams, stretching for 1,200 miles and consisting of eight large dams and 19 smaller ones."[8]

The greatest risk is not simply that these large rivers have dams. It is the fact that they have multiple dams, most numbering in the dozens. There is great risk of a catastrophic cascade failure initiated by the collapse of a single upstream dam. Like a chain, a multi-dam water management system is as strong as its weakest link. And when that weakest dam is far upstream - which it usually is, generally in a remote and sparsely populated area, far from critical eyes - the downstream risk is magnified.

This is not an unprecedented risk, or even an unusual risk. Cascade failures have happened on numerous occasions over the last couple of centuries. The greatest was perhaps the collapse of the Henan Province dams in China in 1975. "As many as 230,000 people died in this domino-effect collapse of dams on the Huai River, some 85,000 in the flood waves and the rest from resulting epidemics and famine. The disaster began with the failure of the large Banqiao Dam in a typhoon, which resulted in the collapse of as many as 62 dams downstream."[6] The flood that was released in the collapse "created a wall of water 6 meters high and 12 kilometers wide ..... moving wall of water was 600 million cubic meters of more water." "The flood spread over more than a million hectares of farm land throughout 29 counties and municipalities."[9]

Consider the numbers. If a river system, like that above, has fifty, sixty or more dams on it, and each of those is, on average, holding back just the minimum large dam reservoir volume of three million cubic meters of water (the Banqiao Dam alone was designed to hold 492 million cubic meters), that entire system is holding back an amount of water equivalent to 3-million cubic meters times the number of dams. Fifty dams, one-hundred-fifty million cubic meters. In a cascade failure such as this, a person or community downstream is not at risk of inundation by the 3 million cubic meters in the dam nearest upriver from them. They are at risk from a cascade failure starting far upstream releasing a massive torrent of one-hundred-fifty million cubic meters of water. If that person/community is downstream from the dam lowest on the river - large population centres are more common at a river's mouth than along its course - that whole mass of water will come at them all at the same time in a wave that could be hundreds of feet high. Every dam downstream from the initial collapse, remember, has a design capacity of only 3-million cubic meters. It has a wave of water coming at it of 3-million cubic meters times the number of upstream dams already collapsed.

Of course, it is not just the massive volume of water behind a dam that rushes downstream as a dam collapses. The catchment area behind every dam gradually has an accumulated build-up of silt and debris. Over time any dam will completely silt-up. Some accumulate silt faster than others, largely a factor of geology and human activity upstream such as farming, lumbering and mining. When a dam collapses all of this silt and debris is also released. In addition the massive rush of water and debris scours the river banks and downstream river bottom and picks up even more debris as it progresses downstream. In floods it is usually the debris, not the water, that does the most damage. Flood water can carry boulders weighing many tons along as though they were pebbles.

Dams are not designed to withstand the pressures or the speed from the sudden influx of millions of cubic meters of water and debris such as this. They are designed to handle the build-up of water following heavy rainfall, or with the spring snowmelt, or the occasional collapse of a small bit of upstream river bank, or other normal events. As the report And The Walls Came Tumbling Down: Dam Safety Concerns Grow in Wake of Failures, Changing Climate says, "Building a totally safe dam is simply not possible. US dam-safety expert Robert Jansen says that dams “require defensive engineering, which means listing every imaginable force that might be imposed, examination of every possible set of circumstances, and incorporation of protective elements to cope with each and every condition.” This is clearly an unattainable target. In the real world, the degree of “defensive engineering” applied to the design of a dam will be decided by economics. ..... There will always therefore be pressure for dam builders to cut corners on safety."[6]

When a dam is designed to handle flood control (either alone or in conjunction with irrigation and/or hydro-electric generation) it must be designed with appropriate excess capacity (the Banqiao Dam was designed to accommodate 375 million cubic meters of flood storage)[8] and flood gates to handle the containment and controlled release of flood waters. "Flood gates are an expensive component of a dam's construction so engineers must consider a trade-off between the cost of the dam and the security it will provide. ..... The dam authorities must decide the proper excess capacity to maintain based on the trade-off they see between the value of stored water versus the value of flood control."[8]

There is another important component, as well, that has not been factored into the design of dams, most of which have been constructed in this past half century. Even dams currently being designed and built, however, share this shortcoming. That factor is global warming. As the above report notes, "Engineers design dams and their spillways to cope with the extreme floods that they predict using past records of streamflow and precipitation. It is vital that spillways are adequately sized – if a spillway is overwhelmed there is a high risk of a dam break. ..... But the assumption that we live in a stable climate no longer holds. Streamflow patterns are changing and are almost certain to continue to change, and at an accelerating rate, over the lifetime of the world’s dams. As noted in a World Commission on Dams’ background paper: “The major implications of climate change for dams and reservoirs are firstly that the future can no longer be assumed to be like the past, and secondly that the future is uncertain.”."[6]

As it looks at the moment, allowance for climate change is not likely to be built into the design of new dams anytime soon, let alone upgrading the existing dam inventory. There seems to be a large dose of denial amongst those involved in the dam designing/building industry. "While the climatic future is indeed filled with uncertainties, one trend upon which climatologists almost universally agree is that we will see (and indeed are already seeing) more extreme storms and increasingly severe floods. And yet, alarmingly, the vast majority of dam proponents and operators deny that climate change is even relevant for dam safety. The president of a major dam engineering firm told this author last year that climate change is "a problem for dams in 20 or 30 years, but not now."."[6] Even were that the case, that 20 to 30 years is exactly the time when the combined impact of global warming and oil depletion will severely hamper our ability and desire to upgrade dams to a safe level. Even to bring the world's dams up to levels currently considered safe that investment would be sizable. "But if securing US dams would cost $30 billion [some estimates, in fact, exceed $100 billion for U.S. dams] and the US has an estimated 10% of the world’s dams, a ballpark figure for the global under-investment in dam safety would be $300 billion."[6]

That state of denial also manifests itself at government levels, sometimes in the extreme. The most common and obvious form of this government denial, of course, is in insufficient budget allocations to maintain the dam inventory at safe levels. Following the catastrophic Henan Province cascade dam failure that killed an estimated 230,000 people in 1975, however, "The Chinese government kept the incident secret for about 20 years, but information on the disaster was eventually leaked to the outside world."[6] If this was possible, even in a closed totalitarian state, in an age of instant global communication, what might happen with those catastrophes in 20 to 30 years time in a very changed, power-reduced world?

Few countries have, or can even afford, comprehensive dam inspection/maintenance safety programs. Most, especially in underdeveloped countries, were built with inordinately expensive borrowed funds, monies which are not sufficient to cover future maintenance which may not be needed for 20-30 years after the dam's completion. "Despite the massive risk to human life and property posed by large dams, few countries have comprehensive dam safety legislation. Such laws should cover the engineering criteria that new dams must meet; the regular inspection and repair of old dams; and the preparation of emergency evacuation plans for people living downstream. ..... Studies in the US have shown that where early warning systems and evacuation plans are in place, the fatalities caused by dam bursts are on average reduced by a factor of more than 100. However, such plans have been made for only a handful of the world's dams, mostly in the US, Canada and Australia...."[6]

Even where safety legislation and programs exist, however, it generally treats dams on a one by one basis. Each dam is designed, built, inspected, maintained as though it were a structure in isolation. But most large river systems have, as noted earlier, multiple dams along their course. The excess capacity of any dam is designed to accommodate a particular volume of water from floods or other designed-for events. But they are designed assuming that all other conditions are normal and that the combined infrastructure of dams on the river will remain intact through the event (If I did not already know it instinctively, thirty years of system design experience would have taught me that you never design a system with the assumption it will work perfectly). In other words, a dam designed with a flood containment capacity of 300 million cubic meters assumes that that volume will be delivered by nature. The fact that there is a dam upstream with a capacity of 500 million cubic meters, or a series of dams with a total capacity of a billion cubic meters, is irrelevant in the design.

As we pass peak oil and the budgets and abilities to properly maintain our massive dam inventory diminish over time (time in which those dams continue to age and require increased, not decreased, maintenance) this design shortcoming will become critical for those water courses with multiple dams, which includes most of our large river systems. The risk to any dam on such systems is not the once-in-a-hundred-years or once-in-a-thousand-years flood that the dam is designed to accommodate but rather the combined capacity of all of the dams upstream from that dam plus the hundred-year or thousand-year flood. No one, especially those living along the banks of such river systems, should take any solace from the fact that such events may be twenty or thirty years in the future. That should, in fact, be more a cause for serious concern than solace. That future in which those failures increase in probability is a future of declining energy and infrastructure maintenance budgets and increased climatic extremes, a potentially deadly combination.

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Additional reading:

1) Fragmentation Of Riparian Floras In Rivers With Multiple Dams
2) Simulation of Dam Failures in Multidike Reservoirs Arranged in Cascade
3) NOTE: The following emails are reproduced in chronological order ...
4) Federal Guidelines for Dam Safety
5) Revised Criteria for Assigning Hazard Potential Ratings to BLM Dams
6) And The Walls Came Tumbling Down: Dam Safety Concerns Grow in Wake of Failures, Changing Climate
7) 17 Large Dams Under Construction by Basin - Watersheds of the World
8) The Amazon Rainforest
9) The Catastrophic Dam Failures in China in August 1975