 The large spillway tunnels have been used only twice , for testing in 1941 and because of flooding in 1983 . During both times , when inspecting the tunnels after the spillways were used , engineers found major damage to the concrete <unk> and underlying rock . The 1941 damage was attributed to a slight <unk> of the tunnel <unk> ( or base ) , which caused <unk> , a phenomenon in fast @-@ flowing liquids in which vapor bubbles collapse with explosive force . In response to this finding , the tunnels were patched with special heavy @-@ duty concrete and the surface of the concrete was polished mirror @-@ smooth . The spillways were modified in 1947 by adding flip buckets , which both slow the water and decrease the spillway 's effective capacity , in an attempt to eliminate conditions thought to have contributed to the 1941 damage . The 1983 damage , also due to <unk> , led to the installation of <unk> in the spillways . Tests at Grand <unk> Dam showed that the technique worked , in principle . 
