The summation of these factors is that, under current FEMA policies and flood hazard analyses, the flood risk for many lands is not described completely, for a variety of reasons. This is not unknown to FEMA or relevant stakeholders including policy makers. In its 2006 report, the Interagency Levee Policy Review Committee recommended that within 5 years FEMA should, in the evaluation of levees, begin to use the risk-based hydrologic techniques being employed by USACE (ILPRC, 2006). In 2007, USACE, in coordination with FEMA, established the National Flood Risk Management Program, emphasizing the intention of the USACE to move to risk-based approaches to flood management. In 2009, with the creation of the Risk MAP program, FEMA identified risk-based analysis as a critical element in efforts to accurately identify the risk to those who occupy flood-prone areas.3 As indicated in Chapter 2, the Biggert-Waters Act placed emphasis on risk analysis in NFIP activities and directed the formation of the USACE-FEMA Flood Protection Structure Accreditation Task Force to better coordinate the data development of the two agencies in levee accreditation.4
In this report, a modern flood risk analysis is defined as a risk analysis using the best available science and analytical methods to evaluate flood risk. This is more precisely defined as an analysis where the likelihood of adverse consequence (loss, personal injury) is quantitatively evaluated, taking into account
1. the likelihood of flood events occurring (the randomness of future events);
2. the likelihood that the capacity of levee and other flood protection systems may be overwhelmed by the flood (be overtopped) or fail (the randomness of structure performance for given levels of loading), leading to flooding of protected areas; and,
3. given flooding has occurred, an assessment of the consequences that occur.
Each element of the analysis is subject to uncertainty; neither the chance that different-size flood events can occur nor their magnitude can be estimated exactly—they are uncertain. These uncertainties and the related implications to the estimate of risk are quantitatively evaluated in a modern risk-based analysis.
There are a number of clear and specific attributes of a flood risk analysis that are not inherent to the definition, but at the same time, define what it means to perform such an analysis. For instance, the analysis considers the occurrence and consequences for a full range5 of flood events that can occur, impact a community, and affect the assessment of insurance rates; it models and analyzes the hydrologic system, including levees and flood protection systems as systems, physically and logically integrated features that impact the outcome of future events; and it quantitatively evaluates the sources of uncertainty that affect the results and the estimation of future losses.
The ILPRC (2006) and many others (Appendix F) recommend that the NFIP should move to implement a more complete approach to the risk analysis that is the cornerstone of its mapping and insurance program. The committee concurs with these recommendations. A modern risk analysis would take advantage of new computational and mapping capacity to produce state-of-the art risk estimates for all areas that are vulnerable to flooding hazards. The analysis would provide a foundation for the NFIP to directly evaluate flood hazards and the performance of levee and other flood control systems and assess the consequences of flooding using sound engineering assessment and scientific and probabilistic methods. Specifically, the modern risk analysis would
3 Although Risk MAP is focused on new and better analysis to support existing criteria and methodologies of the NFIP, this focus does not prevent the program from considering risk in the broader sense. The committee considers this program an important step in FEMA’s attempt to move toward risk-based analysis.
4 Section 100221, Interagency Coordination Study.
5 The term “full range” is used in general terms to reflect that the likelihood of small as well as large flood events are considered in the analysis, not just a single event. The range that is considered should be broad enough to include the frequency and consequences of events that are meaningful to floodplain management and determination of insurance rates.