A few days before Hurricane Katrina struck New Orleans, Ninth Ward resident Kimberly Roberts bought a used videocam for $20. Unable, like many others, to evacuate before the levees failed, she instead documented the disaster and its aftermath, capturing searing images of her experience: huddling in an attic as waters rose inexorably below; discovering a body in an uninspected house weeks later; seeing a crowd being turned away at gunpoint from a vacant naval base on high ground. Her remarkable footage conveys the reality of Katrina with an immediacy and a power that professional coverage cannot. The film earned a Best Documentary Oscar nomination. Thursday, April 23, at 8:30 p.m., HBO