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WHAT HAPPENED AT OROVILLE DAM AND WHY – FINDINGS OF THE SPILLWAY INCIDENT FORENSIC INVESTIGATION John W. France, PE, D.GE, D.WRE Managing Member, JWF Consulting LLC, USA Irfan A. Alvi, PE President & Chief Engineer, Alvi Associates, Inc., USA Peter A. Dickson, PhD, PG Geotechnical Practice Leader, Stantec Consulting Services, Inc., USA Henry T. Falvey, Dr.-Ing, Hon.D.WRE President, Henry T. Falvey & Associates, Inc., USA Stephen J. Rigbey Director, SJR Consulting, Canada John Trojanowski, PE President, Trojanowski Dam Engineering, Limited, USA UNITED STATES OF AMERICA 1. INTRODUCTION On February 7, 2017, the gated service spillway (also known as the Flood Control Outlet or FCO Spillway) at Oroville Dam was being used to release water to control the Lake Oroville level according to the prescribed operations plan. During this operation, the service spillway’s concrete chute slab failed, resulting in the loss of spillway chute slab sections and deep erosion of underlying foundation materials. Subsequently, as the damaged service spillway was operated in an attempt to manage multiple risks, the project’s free overflow emergency spillway was overtopped for the first time since the project was completed in 1968. Significant erosion and headcutting occurred downstream of the emergency spillway’s crest structure, leading authorities to evacuate about 188,000 people from downstream communities. After the incident, the United States Federal Energy Regulatory Commission (FERC), one of two regulators for Oroville Dam, required the dam’s owner, the California Department of Water Resources (DWR), to retain an Independent Forensic Team (IFT) to investigate the Oroville Dam spillway incident and prepare 146