Application of Inflow Design Flood Analysis to the Analysis and Design of High Hazard Dams in NRCS

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. For high hazard dams, TR-60 specifies the probable maximum precipitation (PMP) event be used to generate the freeboard hydrograph (FBH) storm. TR-60 requirements may not be suitable for sites 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. 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). This paper provides suggestions to aid the modeler in making an IDF analysis consistent with NRCS policies, addresses the considerations that should be given to using an IDF approach, and suggests appropriate documentation.