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>> “This is a very serious situation.” <<
Indeed!
But not one that cannot be addressed with fairly routine procedures.
Bentonite clay is frequently used to deal with such seepage, but if there is turbulence from flows going over the weir it could be difficult to apply.
The work is typically done by divers.
Another concern is that a possible earthquake, up to 5.0 magnitude has been predicted by a freelance seismologist that has a fairly good record.
There are going to be lots of jitters around here for the next few days.
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https://www.metabunk.org/oroville-dam-spillway-failure.t8381/page-9