FEMA released new flood maps of Long Island online Wednesday, adding more than 25,000 homes to the new flood zones.

James Callahan, Nassau County's commissioner of Emergency Management, says the new zones are based on the elevation of the land around the homes, rather than proximity to the ocean. Under FEMA's new guidelines, residents with property less than 13 feet above sea level must purchase flood insurance.

Officials say homeowners who purchase flood insurance before Sept. 10 will be able to get at least one year of coverage for under $400. Most flood insurance costs between $1,500 and $2,000 a year.

Nassau County Flood MapSuffolk County Flood Map