Its looking like an "engineering political solution" is coming back to haunt the future. In the 1960's, rather than make the Original combined spillway design of 620,000 cfs work, the intervention of DWR's decision to split the combined spillway into two separate designs has forced a "lock-in" of the new "fixes" (see post link below). Yet another seeming "engineering political solution", possibly related to an expensive price tag, has the consequence of catastrophic destruction of a hillside triggering a "chain of events". This "chain of events" could lead to a swirling back flow that could erode the "toe" of the main dam, thus risking a failure collapse of the whole dam itself. This "failure mode" has been identified by a world renowned leading dam expert Scott Cahill. Scott identifies a dangerous erosion scenario that could cause the "loss of the Oroville dam" (via "eddy swirls" at the toe of the main dam).
The New "Board of Consultants (BOC)", of advising experts to DWR, has stated a requirement of restoration of the rated 646,000 cfs flood capacity of the combined spillway flows (see March 10, 2017 report below). HOWEVER, the new "upped" rating of a huge 396,000 cfs requirement of a new Emergency Spillway includes the caveat: "damage below the Emergency Spillway to be expected". This directly implies that there will be erosive damage. Given the known geology and the actual alarming erosion of "rotten rock" & debris from the last Emergency Spillway incident, the only method to prevent a repeat of this is to armor the hillside all of the way to the Feather River.
We will see how this "damage to be expected" is translated into a new design. Is the "damage" limited to the "armoring itself" or is it damage to an unconsolidated hillside further below an "armoring apron" (or both)? The latter is what could engender a very problematic sequence. (1) If the max rated 396,000 cfs destroys the hillside, a "dam" would form in the Feather River channel. (2) The main spillway chute is upstream, thus flows from the main spillway likely would "backflow" towards the "toe" of the earthen dam. (3) This backflow would form an "eddy swirl" at the "toe" terminus. (4) dangerous destabilizing erosion of the backside toe of the earthen dam would occur. (5) to prevent this "eddy swirl" or partially limit its intensity, the main spillway would need to be slowed or shut off. (6) Further erosion would occur from the Emergency Spillway handling "flood control". This is an inverse repeat of what is currently going on at the dam - BUT- with the exception of a lesser eddy back flow towards the dam (has since been mitigated by the excavation of most of the 1.7 million cubic yards of rock/debris.
The only clean design that "preserves the safety of redundancy" is by fully armoring the Emergency Spillway for the full length of the hillside to the Feather River. But the associated costs for this may become a difficult "political" funding issue.
…The 1960's DWR decision is still front and center today…
Expert panel of "Board of Consultants" requiring new Emergency Spillway to handle an upward revised 396,000 cfs down the hillside. "Damage Below the Emergency Spillway To Be Expected".
