Base Flood Depth - the depth shown on Flood Insurance Rate Maps for Zone AO that indicates the depth of water above the highest adjacent grade resulting from a base flood (1% annual chance flood). Unlike other flood zones that show elevation, Zone AO shows flood depth in feet. This depth measurement is used to determine the required elevation for new construction and to calculate flood insurance premiums. Properties in Zone AO must be elevated so that the lowest floor is above the highest adjacent grade by at least the BFD shown on the FIRM, plus any additional freeboard required by local ordinance.