APP LICATION OF THE INFLOW DESIGN FLOOD ANALYSIS ALTERNATIVE TO NRCS TR-60 DESIGN STORM CRITERIA FOR HIGH HAZARD DAMS

The United States Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) publication Earth Dams and Reservoirs TR-60 (2005) describes design procedures and provides requirements for planning and designing earth dams and associated spillways to ensure consistent performance of these dams. For high hazard dams, TR-60 specifies that the probable maximum precipitation (PMP) event be used to generate the freeboard hydrograph (FBH) storm. The FBH is used to analyze the integrity (erosion) of the earthen materials in the auxiliary spillway and set the height of the dam. TR-60 requirements may not be suitable for all sites, particularly where physical site constraints may preclude construction or rehabilitation of a high hazard dam with the capacity to pass the PMP generated FBH. For these situations, TR-60 permits the use of an inflow design flood (IDF) analysis to determine the FBH design storm based on an event smaller than the PMP for high hazard dams, “provided downstream land use controls exist to prevent voiding incremental risk assumptions after the dam is completed” (NRCS 2005). IDF should not be considered unless there is a limiting site constraint and may not be used solely for the purpose of building a smaller dam. Neither should IDF be considered if the inundation area downstream of a dam based on failure (breach) of the dam during a PMP rainfall event cannot be zoned or otherwise protected from development. IDF analysis is described in the Federal Emergency Management Agency (FEMA) Publication 94, Federal Guidelines for Dam Safety – Selecting and Accommodating Inflow Design Floods (FEMA 2004). FEMA 94 guidelines provide thorough and consistent procedures for selecting and accommodating the IDF. In IDF analysis, intensive and rigorous hydrologic evaluations are made of the dam for a without-failure condition and a with-failure condition. The results of these evaluations are compared to determine if the incremental increase in water surface elevation downstream due to failure of a dam presents an unacceptable threat. It is an iterative process whereby the procedure is repeated until the flood inflow condition is identified such that a failure at that flow, or larger flows (up to the probable maximum flood, or PMF), no longer result in unacceptable additional consequences. The resultant flood flow is the IDF for the project. The maximum IDF is always the PMF, but in many cases the IDF will be less than the PMF. In both TR-60 design and IDF analysis, it is taken to be understood that when translated to runoff, the estimated flood flow from the PMP rainfall is known as the PMF. In order to ensure consistency with NRCS policy, the NRCS approach to IDF analysis differs slightly in comparison to what is described in FEMA 94. This paper provides suggestions to aid