Failure of the spillway structure on the Oroville Dam in California, US on 7 February last year was a result of a chain of factors rather than a single root cause, according to investigators.
The incident occurred after the Department of Water Resources (DWR) started to discharge water over both the main and emergency spillway in order to control water levels impounded by the dam.
High water flows led to both spillways being damaged and concerns that the erosion could led to the collapse of the earth embankment dam, which is the highest in the US at 230m, and resulted in over 180,000 people being evacuated from their homes.
An Independent Forensic Team Report into the failure of the DWR structure has pointed to construction flaws, inspection failures and poor planning leading to overfilling as the main issues that caused the incident last year.
In investigation found that the concrete chute slab cracked soon after construction in the 1960s but the failure was deemed normal and ongoing repairs were accepted by DWR as part of the maintenance routine. Inspections over the last 50 years also failed to identify the weakness within the structure.
The reservoir impounded by the dam was allowed to overfill in February last year following heavy rain in a bid to avoid flooding of a powerhouse but the water levels led to water being sent down an untested earthen spillway which immediately started to fail.
According to the report, there were many opportunities to intervene and prevent the incident but “the overall system of interconnected factors operated in a way that these opportunities were missed”. The report describes the dam safety culture within DWR as “insular” and “shortsighted”.
The Independent Forensic Team Report calls for all dam owners to develop and maintain “mature dam safety management programmes which are based on strong ‘top-down’ dam safety culture” with one executive specifically charged with dam safety. The report also calls for more frequent inspections and periodic reviews of the original design.