Flood damage is not typically covered by a homeowners insurance policy.
There are some homeowner endorsements that will cover a back-up or overflow of sewers or drains or a sump-pump. These type losses sometimes do occur during flooding events. These endorsements will often have dollar limits substantially less than the flood policy
Flood insurance is required by law for some high risk homes but only a minority of homes fall under this category. Congress passed some laws requiring that all federally-insured or regulated lenders, for instance mortgages backed by Fannie Mae or Freddie Mac require flood insurance to protect their investment in the property.
90% of all natural disasters have some form of flooding.
Overflows from sewer systems or sump pumps are not considered a flood unless a flood has caused the backup and overflow.