Official reports released Monday say the catastrophic damage to Oroville Dam’s main spillway probably stemmed from swift water flows under the concrete chute, which was cracked and of uneven thickness. The observations, contained in consultants’ reports prepared for the state Department of Water Resources, echo much of an independent assessment made for UC Berkeley’s Center for Catastrophic Risk Management. An official verdict on the cause is not due until the fall, when a separate forensics team investigating the February spillway break will submit its report.