First, determine "What's Your Number?" by learning the flood stage at the stream gage nearest you. This information is available through the National Weather Service's Advanced Hydrologic Prediction Services website.
Then, determine the level at which floodwaters begin to affect your property. This step may take research or personal experience to determine, such as talking to neighbors to find out how high the river was during recent floods, and at what point flooding began in your neighborhood. Each neighborhood and each property has its own unique terrain and placement to consider when determining this factor, and it is safest to err on the side of caution.