International Geologiical Congress - Oslo 2008

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GHZ-11 Rock slope movements and early warning of catastrophic failure and related tsunamis

 

Landslides on accumulation reservoirs - A field applicable hazard and risk evaluation scheme

 

Manfred Thuering, University of Applied Sciences of Southern Switzerland (Switzerland)
Massimiliano Cannata, University of Applied Sciences of Southern Switzerland (Switzerland)
Juerg Hammer, DRM Inc. (Switzerland)
 

 

Slope instabilities are a threat to accumulation reservoirs and corresponding down-stream areas due to possible slope collapse, generation of flood waves, dam overtopping and flooding of downstream areas. A simple and field-applicable assessment scheme, based on existing approaches, is presented to evaluate hazards and risks due to landslide collapse, in order to obtain a first and preliminary overview. The outcomes are used to support planning of further investigations.
The hazards that arise from the presence of landslides on the slopes of accumulation reservoirs are: (i) The slope instability damages the dam itself and buildings and infrastructure. (ii) The masses of a partial or complete slope collapse reach the reservoir lake and cause a pulse wave, which travels to and damages the dam and buildings and infrastructures. (iii) The pulse wave overtops the dam and causes flooding in the areas below the dam. These three hazard situations are discussed briefly and field-applicable assessment schemes are presented to preliminarily evaluate the hazards and estimate the connected risks in terms of worst case scenarios.
In a first step the primary hazard needs to be assessed, estimating the characteristics of the landslide mass and its path down slope. This is mainly done by field evidence. Key information needed is type of material of landslide, volume, elevation above lake level, inclination of trajectory and width of landslide. The maximum run-out of the hazard is estimated by the Fahrböschung-approach, evaluating if the slope instability reaches the reservoir lake. With the previously obtained information the characteristics of the pulse wave which may be generated due to a slope collapse is estimated, using simple calculations. For this purpose a map view of the reservoir and a cross section of the dam is needed, where dam height and geometry, water depths, freeboard, and distances can be read from. From the calculations it is possible to estimate if dam overtopping occurs.
If the calculated wave is likely to overtop the dam, the consequences of dam collapse and flooding are estimated using methodologies that predict flood heights and velocities in downstream areas. Data obtained from maps or by field inspections is needed, such as valley geometry, roughness and inclination. The vulnerability assessment of the flooded areas is done based on maps or field inspections, such as number and type of flooded objects. The assessment gives an estimate of values and fatalities at risk.
The scheme is field-applicable with data usually available or that can be produced on-site and delivers a preliminary assessment of hazards and risks connected to the presence of landslides on the slopes of accumulation reservoirs. We present and discuss the scheme and show its application in a project in Romania, where a preliminary screening on a number of reservoirs was conducted to identify key characteristics in terms of hazard and risk.

 

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