R655-11-4A. Inflow Design Hydrograph Determination

  A) In Utah, the IDF for all High and Moderate Hazard Dams will be the more critical SEF. It will be necessary to calculate both the 72 hour, general SEF using HMR49 with USUL as well as the 6 hour local SEF using HMR49 with USUS. These precipitation values need not exceed values calculated using HMR49 exclusively. Both of these hydrographs must be routed through the reservoir to determine which one represents the most extreme event.

  B) Once the critical SEF has been determined, it must be compared to a flood generated by the 100 year, 6 hour (for local storms), or 100 yr, 24 hour (for general storms) precipitation applied on a saturated watershed. If the routed 100 year event, including appropriate allowances for freeboard, is more critical than the SEF it must be used as the minimum IDF. This 100 year flood should also be used as the IDF for all Low Hazard Dams.