(a) For the purposes of this Code section:

(1) "Acre-feet" means the measurement of the impounding capacity of a dam as described in paragraph (4) of Code Section 12-5-372.

(2) "Height" means the height of a dam determined in the manner described in paragraph (4) of Code Section 12-5-372.

(3) "PMP" means probable maximum precipitation as determined by the United States Weather Service to be the greatest amount of rainfall of a six-hour duration which would be expected for a given location.

(b) All category I dams identified by the commission pursuant to Code Section 12-5-375 shall be subclassified by the director as follows:

(1) Small dams -- Those with a storage capacity not exceeding 500 acre-feet and a height not exceeding 25 feet.

(2) Medium dams -- Those with a storage capacity exceeding 500 acre-feet but not exceeding 1,000 acre-feet or a height exceeding 25 feet but not exceeding 35 feet.

(3) Large dams -- Those with a storage capacity exceeding 1,000 acre-feet but not exceeding 50,000 acre-feet or a height exceeding 35 feet but not exceeding 100 feet.

(4) Very large dams -- Those with a storage capacity exceeding 50,000 acre-feet or a height exceeding 100 feet.

(c) Permits issued pursuant to this part shall require minimum spillway design as follows:

(1) Small dams.............................................25 percent PMP

(2) Medium dams..........................................33.3 percent PMP

(3) Large dams.............................................50 percent PMP

(4) Very large dams.......................................100 percent PMP

Based on visual inspection and detailed hydrologic and hydraulic evaluation, including documentation of competent original design and construction procedures, up to a 10 percent lower requirement (22.5, 30, 45, 90) can be accepted, at the discretion of the director, provided the project is in an acceptable state of maintenance. The design storm may also be reduced if the applicant's engineer can successfully demonstrate to the director by engineering analysis that the dam is sufficient to protect against probable loss of human life downstream at a lesser design storm.