Page - 25 - in Book of Full Papers - Symposium Hydro Engineering

reservoir level in accordance with the prescribed operations plan. During this operation, the FCO spillway suffered a catastrophic failure of the lower chute area eventually resulting in the loss of approximately 427 m (1400 ft) of the lower chute, including the scour of more than 1.2 million m3 (1.6 million yd3 of soil and rock materials. On February 11, 2017, the Emergency Spillway (Fig. 1) was used for the first time since the project was completed in 1968. During this operation, significant erosion and scour caused by the Emergency Spillway overflow led authorities to fear for the safety of the spillway structures, resulting in the activation of the Emergency Action Plan and evacuation of about 188,000 persons from downstream communities. Fig. 1 Flood Control Outlet (FCO) Structure and Emergency Spillway, March 2011 25