Twelve years ago, the idea of a giant mudslide barreling down the Arroyo Seco in Pasadena was considered only a distant possibility. Nevertheless, the Devil's Gate Dam underwent a major enhancement that took two years to complete and enabled one of the oldest dams in Los Angeles County to withstand a potential massive debris flow. At the time, engineers thought the improvements might be overkill because conditions would never be extreme enough to test the dam's strength, at least not in their lifetimes. "Not a chance," recalled Brad Boman, engineering manager of water services at Pasadena Water and Power. But...