Originally Posted by etudiant
Consider me skeptical. Sand bags do add bulk, but not strength or water impermeability.
The spillway cladding was clearly undercut and has given way, leaving the earthen dam below exposed.
Impermeability is conferred by the clay core of the dam, which must still be in place or the dam would have given way on the night of the occurrence. The sand bags simply replace the (permeable) earth washed away after the spillway failed, the purpose of which is to hold the clay core in place by gravity. They
do add strength and it’s a good repair as long as the broken part of the spillway isn’t overtopped again, which is why they started by placing a wall of sandbags at the top (with concrete grouting) to divert any water onto the good part of the spillway before pumping the level down. Removing the rest of the water is about reducing the load on the dam sufficiently for the temporary repair to be removed and placed with a permanent one.