Inflow Design Flood Analysis for NRCS High Hazard Dam Rehabilitation Projects: Case Studies

The United States Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) publication Technical Release 60 (TR-60), Earth Dams and Reservoirs (NRCS, 2005), provides NRCS requirements for planning and designing earth dams and associated spillways and describes NRCS design procedures. For high hazard dams, TR-60 specifies stability (resistance of vegetation to stripping) of earthen auxiliary spillway materials be based on the sum of the one percent chance (100-year) storm plus a ratio of the difference between the probable maximum precipitation (PMP) event and the one percent chance event. To analyze the integrity (erosion resistance of the earthen materials) of the auxiliary spillway, TR-60 specifies that the PMP event be used to generate the freeboard hydrograph (FBH) storm, which is used to set the height of the dam.
Physical site constraints may preclude rehabilitation of a high hazard dam with the capacity to pass the PMP generated FBH. In these situations, TR-60 does permit the use of inflow design flood (IDF) analysis to determine the FBH design storm. IDF analysis is described in the Federal Emergency Management Agency (FEMA) Publication 94 (FEMA 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. IDF 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 (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.
To ensure consistency with NRCS policy, the NRCS approach to IDF analysis differs slightly from that described in FEMA 94. This paper presents the results of several different projects where IDF analysis was used. These examples document the analysis and the considerations that were given to accepting or rejecting the IDF approach by NRCS and/or project sponsors. 10 pp. 9 references.