R655-11-4B. Freeboard Requirements

  All high and moderate hazard dams must have a normal freeboard above the crest of the principal spillway capable of 1) routing the IDF, 2) containing the maximum wave action, 3) containing the combined precipitation and wind event detailed in this paragraph and 4) the normal freeboard will be no less than three feet. Wave action will be determined considering site wind-duration and fetch control characteristics. Wave action includes wave height, maximum runup, and reservoir setup against the embankment slope. Unless otherwise justified by specific data acceptable to the State Engineer, the maximum wave action will be based on a wind velocity (fastest mile) over land of 100 miles per hour. In addition, while routing the 100 year precipitation event through the spillway, sufficient residual freeboard must be available to control wave action from a fetch controlled 50 miles per hour wind. Low hazard dams must have sufficient freeboard to allow the spillway to route the applicable 100 year flood. The State Engineer may reduce the three feet minimum freeboard requirement for low hazard dams based upon a review of the relative increase in risk associated with this reduction.