Because your property is included on a map that shows it is at risk of flooding. Homes in flood zones that have been paid for with federally-backed mortgages are required to carry flood insurance
In the early 1970s the City of Menlo Park followed FEMAs procedure to develop a map showing where floods were most likely to occur. The maps were based on historic records of rainfall, tides and volume of water flowing down local creeks. The maps have been updated five times since then and are due to be updated again in 2017.
The National Flood Insurance Program, through your homeowners’ insurance company, uses these maps to decide who needs flood insurance and how much it should cost.