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  • #16
    re: Lake Sarez, here are the high points of my notes from the talk last week:

    Vital stats:
    - The dam was formed by a landslide caused by an earthquake in 1911, magnitude approximately 7.4
    - It is ~5km long along the crest, ~4 km wide, and 600 m high
    - The dam currently impounds ~17 km^3 of water
    - It currently leaks from a point about 400m up from the downstream toe (about 200m down from the top), this leakage forms a medium-sized to large river
    - Despite the leak, the lake fills faster than it empties and has been filling since 1911. It is currently rising at a rate of 20-30 cm per year, but this rate is slowing because the reservoir gets wider as it gets higher.
    - The river and erosion from the leakage is not a risk to the stability of the dam
    - The leaks are stable and do not appear to be eroding soil from inside the dam
    - There is currently 20-50 meter of freeboard (the height of the dam above the water level), it is estimated that it will be hundreds of years before the dam overtops naturally

    Risk mitigation:
    - Funding from the Swiss government, administered by the World Bank (Swiss government is the largest source of foreign aid in the region)
    - There are 18 villages in the immediate river valley, and countless cities in both Tajikistan and Afghanistan between the dam and the ocean
    - The failure of the dam would cause the largest flood in history, and probably the largest in pre-history as well.

    Major risks:
    - landslide into lake could cause a sietch (a tsunami in a lake) that could overtop the dam
    - the dam itself could have a landslide the damages it
    - an earthquake could damage the dam
    - natural erosion/overtopping could damage the dam

    - Of these, a landslide into the lake is the most serious
    - There is very little geologic info, the only soil boring was done by the USSR in the 1980s to investigate an ongoing landslide behind the dam (see attached diagram). The boring stopped at 200 m because they used all of the well casing they brought with them. There are no borings of the material making up the dam itself.
    - The location is very remote, it is a 20km hike (no roads) from the nearest village to the dam site. That village is at the end of a constantly eroding/landsliding jeep trail, approximately one week of driving from the nearest city.
    - The slide investigated by the 1980s Soviet soil boring is on the right bank upstream from the dam. The size of this slide is unknown; the 200m boring was not deep enough to reach the slip surface.
    - This landslide is the greatest risk to the dam, it is estimated that it would cause a wave 19-40m high in the lake that would run up 50-100m higher when it reached the dam.

    - There is currently a manned monitoring station near the dam with many sensors to detect earthquakes, rainfall large waves, increased streamflow downstream, etc. This monitoring station can set off sirens via satellite at locations downstream of the dam if it detects a problem.
    - The nearby villages have evacuation plans and trails made leading to high ground.

    - Main way to reduce the risk is to lower the lake level
    - Possible schemes:
    - divert the river upstream of the lake so it stops filling
    - drill bypass tunnels around the dam to lower lake level
    - Estimated costs for these projects range from $121 million to $300 million, of which $100 million is from only transportation costs to get construction equipment out to the remote location

    - In Sept 2006 another area of leaks opened up in a different location from the old leaks, these also appear to be stable and not an immediate risk to the dam.
    - The increased river flow from these new leaks triggered a small landslide downstream of the dam which formed its own small lake before eroding away and causing another larger flood downstream.

    Last edited by withak; 02-12-2007, 09:57 PM.