In the hydraulic and hydrologic design of major erosion control measures (those whose tributary drainage area is greater than three acres) such as sediment basins and traps, diversions, and the like, the design frequency shall be commensurate with the risk of the design event being exceeded. The following design frequencies shall be regarded as minimum design frequencies for the construction period:
(A) For those projects whose construction period is less than six months then the storm event having a 50% chance (2-year event) of being exceeded in any year shall be used for design purposes;
(B) For those projects whose construction period is greater than six months but less than one year, the design frequency for major sediment basins shall be a rainfall event with a 20% (5-year event) chance of being exceeded in any one year; and
(C) For those construction projects expected to last more than one year major sediment basins shall be designed for a rainfall event with a 10% (10-year event) chance of being exceeded in any one year.
(Ord. 06-0383, passed 3-15-06)