When Superstorm Sandy slammed into New York and New Jersey last fall, it sent massive floods through the streets of coastal towns and cities across the Northeast, turning areas like Toms River, N.J., into something like a war zone. But nearly a year later, residents there and in many other coastal communities across the U.S. face a potentially far more devastating menace: a nationwide revamp of flood insurance rates, forcing premiums that were once around $500 per year into the $5,000-, $10,000- and even $20,000-a-year range and higher. "The adverse effect of [this] would be more devastating than Hurricane Katrina,"...