Thomas Dietler, Martin Wieland and Martin Fuchs from Poyry give an insight into the natural hazard considerations in hydro power development.
The assessment of natural hazards is a prerequisite for the safe design of all hydropower projects. Hydroelectric power's vulnerability to natural hazards is a direct consequence of factors needed for its viability. Many of the world's hydroelectric power generation plants are situated in extreme landscapes with acute drops in height over a relatively short distance. The topography is inherently hazardous and highly prone to natural hazard events. Added to this, hydropower plants are typically located in areas which receive high amounts of precipitation. The need for abundant water resource brings with it additional risk from flooding and ever evolving landscapes. Combined, these same conditions that are so conducive with hydropower are also the source of risk.
A hazard is defined as a potentially damaging physical event. Such an event may cause the loss of life or injury, property damage; social and economic disruption or environmental degradation. There is not simply one type of hazard and many hazards are not mutually exclusive. The list of hazards outlined here is not exhaustive and individual cases should always be assessed differently to ascertain risk factors.
One of the most commonly occurring natural hazards impacting hydropower facilities are landslides. The movement of large amounts of materials close to a site can lead to the burial of structures, weakening of foundations and dumping of large volumes of material into the water reservoir. One recent, and severe, example of this was the large landslide that took place at the Zipingpu reservoir following the 2008 Wenchuan earthquake. The earth’s shuddering caused a large mass of material to slide down the adjoining hill side and into the reservoir below. Even if a single landslide impact isn't as disastrous, the cumulative effect of small scale slides can significantly damage a site and reduce the productive water volume. Landslides, however, do not occur only as a consequence of earthquakes, but also as standing alone events due to general slope instabilities.
Earthquakes can affect hydroelectric facilities in a multitude of ways, both directly and via the secondary impacts they cause. The main hazard is the shuddering of the ground leading to cracks in the wall or the failure of the dam forming the reservoir. In severe instances, the rupturing of the earth's surface along a fault line has the capacity to stress or even split any building siting within the rupture zone. Secondary impacts include rockfalls and landslides, impulse waves in the reservoir and the creation of landslide dams. Earthquake activity that is large enough to cause serious damage is rare, however, it does happen. For example, the same earthquake mentioned above – the Richter magnitude 8 Wenchuan earthquake of 12 May 2008 – caused significant damage to 1,803 concrete and embankment dams and reservoirs and 403 hydropower plants were damaged.
Although generally less dramatic than Landslides or Earthquakes, primary mass movements – for example slope failure and rock fall-caused by general degradation of the topography are as important sources for natural hazards. This type of continual low level hazard has the potential to significantly impact hydro power facilities by damaging equipment and putting personnel at risk.
Hydropower facilities can often be found in areas of high flood exposure. Flood induced failures of dams and other hydropower structures may result in catastrophic events causing enormous losses in the downstream areas. Proper assessment of design floods is of critical importance since under-design of spillways might lead to overtopping of dams and potential dam failure during extreme floods. However overtopping of dams is not always the result of river floods but can also be triggered by strong winds or landslides plunging into reservoirs. Furthermore dam breaks might also be caused by earthquakes or geological failure of dam foundation or dam abutment. Comprehensive analysis of natural hazards is therefore considered an essential part of modern dam safety assessment.
The assessment of risk
With the potential for severe consequences, the reduction of risk and the assessment of natural hazards is a vital element of all hydropower construction. Conventionally, risk is expressed by the notation: Risk= Hazards x Vulnerability. In this case, vulnerability is defined as the conditions determined by physical, social economic, and environmental factors or processes, which increase the susceptibility of a community to the impact of hazards.
Assessing and quantifying all of these factors is a procedure. The approach used by Poyry follows the methods developed and used successfully in infrastructure projects in Switzerland and elsewhere, across a range of difficult geological, topographical and climatic environments. Firstly, a thorough risk assessment is carried out, followed by a process of defining the protection goals and design criteria before planning measures according to sustainability principles. The final stage is integral to the process as it applies an integrated approach through partnering with all project stakeholders and authorities. Today, risk-based approaches for particular hazards such as flood and earthquakes are gaining importance.
Following this preliminary assessment, a procedure on continual mitigation, response and recovery must be adhered. Preventative measures are taken to mitigate harm, responses are prepared in case of harm and lessons are taken following a harmful event in order to learn lessons about mitigation.
The primary goal of risk assessment is to answer three questions. Firstly, what can happen and where will it happen? This can be referred to as the identification of hazards. Secondly, how often. and how intense will the event be, along with how much damage can be expected? This is the central question for the analysis of hazards, vulnerabilities and risk. Finally, what are the most efficient ways to protect people and assets? This falls within the realm of planning measures.
The bases for answering all of the questions above are a series of map types. Depending on the scale and complexity of the project, different map types are applied for the assessment. These maps fall broadly into four categories.
- Historical mapping. This type of maps records phenomena documenting events that have occurred in the past. This helps to indicate where any future, potential events may occur.
- Hazard mapping. This type of map shows where a hazardous process could occur, even if it hasn't in the past, and then shows the intensity and probability of the process. For example, a map of this sort could show the probabilistic earthquake or flooding hazard of the area. Mapping of this variety is used as a primary management tool. It helps to justify structural protection measure and is the basis for site monitoring, planning and risk assessment.
- Damage mapping. This type of map lays out what is at risk in a given area. This includes economic assets, an average loss per event! year or the number of deaths per event / year. This type of mapping is the basis for the chronological and financial prioritisation of protection methods and is the most appropriate tool for decision making about structural and non-structural measures.
- Intensity mapping. Intensity maps provide the spatial extent and the corresponding intensities of a natural event, having a specific return period or probability.
The benefit of a methodological approach
Following the methodological approach outlined here leads to a range of benefits for hydropower construction, but can also be applied with the same benefit to any other infrastructural projects. The well-defined, standardised working steps allow for successful application to a range of different hazards and projects. The standardization of these steps enables transparency and reproducibility of the result for all stakeholders. The approach is efficient and effective. Only hazards with real impact on the project are treated, with mitigation measured tailored to the risk.
Hydropower generation is intrinsically linked with the risk of natural hazards due to the shared characteristics of both. Therefore, assessing the associated risk of these hazards is paramount to the security of all hydropower facilities or any other infrastructures. The use of a methodological approach, based on a series of maps is the most effective way of achieving successful outcomes. Likewise, risk assessment and management should be viewed as a continuous process. Mitigation, response and recovery actions must all be intertwined.
Dr. Thomas Dietler, Senior Expert Geology; Dr Martin Fuchs, Section Head Hydro Consulting; and Dr Martin, Senior Dam and Earthquake Engineer, Pöyry.