Eleven years ago, as three environmental groups were urging the state and federal government to require that concrete be used to armor the emergency spillway at Oroville Dam — an earthen spillway whose near collapse caused the evacuation of 200,000 people this week — state water officials said that was unnecessary. “The spillway is a safe and solid structure founded on solid bedrock that will not erode,” the state Department of Water Resources concluded in a May 26, 2006, filing to federal officials.