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The FHF for a reach is the average weighted difference between the 10- and 100-year flood water-surface elevations rounded to the nearest one-half foot, multiplied by 10, and shown as a three- digit code. For example, if the difference between water-surface elevations of the 10- and 100-year floods is 0.7 foot, the FHF 1S 005; if the difference is 1.4 feet, the FHF is 015; if the differ- ence 1S 5.0 feet, the FHF is 050. When the difference between the 10- and 100-year flood water-surface elevations is greater than 10.0 feet, it is rounded to the nearest whole foot. 5.3 Flood Insurance Zones Flood insurance zones and zone numbers are assigned based on the type of flood hazard and the FHF, respectively. A unique zone number is associated with each possible FHF, and varies from 1 for an FHF of 005 to a maximum of 30 for an FHF of 200 or greater. Zone A: Special Flood Hazard Areas inundated by the 100-year flood, determined by approximate methods; no base flood elevations shown or FHFs determined. Zone AO: Special Flood Hazard Areas inundated by types of 100-year shallow flooding where depths are between 1. 0 and 3.0 feet; depths are shown, but no FHFs are determined. Zones Al-A16, Ala, and A2l: Special Flood Hazard Areas inundated by the lOO-year flood; w1th base flood elevations shown, and zones subdivided according to FHFs. Zone B: Areas between the Special Flood Hazard Areas and the limits of the 500-year flood; areas that are protected from the 100- or 500-year floods by dike, levee, or other local water-control structure; areas subject to certain types of 100-year shallow flood1ng where depths are less than 1.0 foot; and areas subject to lOO-year flooding from sources with drainage areas less than 1 square mile. Zone B is not subdivided. Zone C: Areas of minimal flood hazard; not sub- divided. Zone D: Areas of undetermined, but possible flood hazard. 38