Oroville Dam Showed Them Where It Would Break for 48 Years

In February 2017 the floor of Oroville Dam's service spillway came apart in eighty minutes, under about 52,500 cubic feet per second. The same chute had carried about 160,000 in 1997, and held. We read the Independent Forensic Team report — 584 pages, 5 January 2018 — and this is what it says, with the page numbers. The chute slab was designed to be at least 15 inches thick (p. 11). But the herringbone drains did not sit underneath it: they protruded up into the concrete section, and where a pipe crossed, the slab above it came down to a minimum of 7 inches (p. 11). During construction the drains were enlarged from 4-inch to 6-inch pipe (pp. 11–12). Of 110 other spillway chutes designed between 1955 and 1975, about 8 percent had drains inside the slab — and none had them taking up as large a fraction of the thickness (p. 48). From the first time the spillway ran in 1969, water poured out of the drain outfalls. The inspection record of that year calls those flows "mystifying" (p. 50). The cracks over the drains and the drain flows came to be accepted as normal, inspection after inspection, for forty-eight years. A 1962 geology report had already written: "weathered rock will of course be subject to relatively accelerated erosion; where this is critical, the rock should be protected" (pp. 53–54). The detailed borehole logs appear in the 1962 and 1964 interim geological reports and are omitted from the final geological report (p. 54). A 2005 DWR memorandum concluded that the emergency spillway stood on "bedrock that will not erode" (p. 54). A 2009 study assumed 3 to 4 feet of erodible cover, while a table in the same report documents soil and weathered rock down to 36.5 feet (p. 54). In 2014 a potential failure mode analysis essentially identified the failure mode that initiated in February 2017 — and ruled it out as a Category IV, because it would not cause an uncontrolled release of the reservoir (p. 51). It did not release the reservoir. It put about 188,000 people under an evacuation order (p. 26). The forensic team can find no record that, in the entire operating history of the project, there had ever been a detailed review of the design, construction and performance of that spillway slab (p. 52). Twenty-one days after the report, the federal regulator wrote to every dam owner it regulates, saying the potential-failure-mode process "focuses too heavily on uncontrolled release of the reservoir… and too often does not reconsider PFMs that have been dismissed previously" (FERC, 26 January 2018, p. 2). — SOURCES — Independent Forensic Team Report, Oroville Dam Spillway Incident, 5 January 2018 (584 pp.) FERC, Division of Dam Safety and Inspections, letter of 26 January 2018 Page numbers for every claim, and the discrepancies we could not resolve, are in the pinned comment. — IMAGES — All photographs: California Department of Water Resources (Kelly M. Grow, Dale Kolke, Florence Low, Paul Hames, William Croyle), 2011 and 2017. Public domain under PD-CAGov — California Public Records Act, Gov. Code § 7920 et seq. — MUSIC & SOUND — Original score and sound design composed for this episode by Collapse Mechanism. The pulse of the bed is 4.93 Hz: the rate at which the flow crossed a line of herringbone drains (20 ft spacing, p. 11; velocity 98.6 ft/s, p. B-19). WATCH NEXT • The Tacoma Narrows Report Already Said It Wasn't Resonance. In 1941. —    • The Tacoma Narrows Report Already Said It ...   • Silver Bridge: One Hidden Crack Was All It Took —    • Silver Bridge: One Hidden Crack Was All It...   • Every episode, in order —    • Collapse Mechanism — Every Episode   #OrovilleDam #EngineeringDisaster #CivilEngineering