In a flood situation, follow local recommendations. Listen to local officials on the radio or check their Web site to see if they're recommending that you plug drains. Plugging drains is necessary if there are problems with the sanitary and/or storm systems, but, even in a local flood situation, the two systems may be in fine shape. However, if you have to evacuate, be sure to plug drains in the lower levels of your home in case the situation changes while you're gone. Sump pumps should be discharging water outside, definitely not into the sewage system. Keeping that excess water out of the sewer will help keep that system working.