Your homeowner policy does not protect you in the event you suffer a loss caused by flood damage. Flood coverage is provided under a separate policy through a select group of insurance companies that offer this protection that is managed by the government. Premiums vary depending upon your location, if you are in a high (standard) or low (preferred) risk flood zone. The low risk or preferred zones offer very affordable priced policies.
There is a 30-day waiting period after your application has been submitted unless you are closing on a loan so be sure to apply in advance of when you want your coverage to take effect.
Two informative sites to visit are www.floodsmart.gov and www.fema.gov to learn more about flood insurance.