How can one effectively use sandbags to stop a flood?

To effectively use sandbags to stop a flood, place them in a staggered pattern to create a barrier against rising water. Make sure the sandbags are tightly packed and stacked at least two layers high. Additionally, place a plastic sheet or tarp behind the sandbags to prevent water from seeping through. Regularly monitor and reinforce the sandbag barrier as needed to maintain its effectiveness in preventing flooding.