* Former geologist here. What I’ve gleaned from Steve’s commentariat (but emphatically not from following the mainstream news) is that the multiple failures at Oroville are not uncorrelated, because they trace back to a fundamental issue: the bedrock at the dam site is highly fractured.
When we think of bedrock, we typically think of the granite of El Capitan, or impressive roadcuts like Sideling Hill. However, some rock is inherently weak (unconsolidated), or is prone to fracture and weathering, or has been subject to immense tectonic forces (e.g. pictures of conjugate fractures in rock at the San Andreas Fault here).
These are not observations that are new to the 21st Century, or to the 20th Century. For example, read the first paragraph of UCSB faculty Douglas Burbank & Brian Clarke’s ~2011 essay The Role of Rock Fracture in Erosion.
The engineers who designed Oroville Dam in the 1980s were obviously not blind to the implications of building on weak bedrock, or of the facts on the (chosen) ground. There are sure to be damning memos in the archives. But for whatever reasons, the dam was built the way that it was.
1. Failure at the middle of the main spillway — The concrete of the spillway cracked because it was undermined — the rock beneath it must have been eroded away by water flow over the decades that the structure has been in use. This happened because that rock was unconsolidated.
2. Risk that sustained water release at 100,000 to 150,000 cfm will cause the top part of the spillway to break up, undermining the spillway gate itself — Again, unconsolidated rock.
3. Loss of the ability to release 14,000 cfm through the power station — Result of #1.
4. Inability of the auxiliary spillway to operate at 12,500 cfm, 5% of its design capacity of 250,000 cfm — Photos taken Monday show that the emergency spillway flow eroded a deep channel in the hillside in a matter of days, and it was quickly expanding uphill, towards the spillway dam. In addition, the water cresting that 30 foot concrete spillway was likely to excavate spaces underneath it, risking its failure. Both are consequences of the unconsolidated nature of the bedrock at the site.
* Or, they could rally university SJWs to raise awareness about how fluid flow is just a social construct and then implement a program to break down flawed stereotypes about “erosion” and replace the discredited conventional demotics of dam dynamics with progressive intersectional theories that embrace a new sense of fluid diversity.
