All day, every day and into the night, crews for the U.S. Army Corps of Engineers pour concrete into walls, pack dirt into hills and ram steel into the earth. They are scrambling to undo the damage Hurricane Katrina inflicted on the region's levee system. Their task is urgent: Hurricane season begins June 1. But even when the holes are plugged — a $2 billion endeavor — the entire 350-mile protection system remains flawed, the corps now admits. Flood walls are too weak in some places; earthen levees are too short in others. Locals say the only thing that will save the low-lying region from more flooding this summer is not getting hit with a strong storm.