ogy has matured to the point that it “is capable of producing a bare-earth elevation model with 2-foot equivalent contour accuracy in model terrain and land-cover types.” With geographic information system tools, overland flow on floodplains can be more accurately modeled from these data in a systematic, automated manner, with minimum human processing and few errors. Further, the state of practice of hydraulic modeling has advanced greatly since the early days of the NFIP, with two-dimensional (2-D) unsteady open channel and overland flow models now widely used. In fact, FEMA’s list of acceptable models for hydraulic analysis includes five such models.1 Data for estimation of consequence of inundation, particularly estimation of inundation damage, are readily available. These include, for example, property values, property type categorization, and property elevation data in FEMA’s Hazards-United States (Hazus) databases.2

The technical capability now exists for implementing and applying risk analysis as proposed in this report. In fact, other state and federal agencies with responsibilities for flood risk management have recognized the need for complete risk analyses to support their decision making, and have incorporated those analyses in their programs. To varying degrees, those analyses:

• Account for the full range of hazard, going beyond the one percent chance event, considering events as common as the 50 percent chance event and as rare as the 0.1 percent or 0.01 percent chance event.

• Include an explicit assessment of the consequences of inundation. Most analyses consider economic consequences, some life safety, and few environmental consequences.

• Evaluate all potential mechanisms for flooding areas that are protected by a flood protection system, including overtopping (without levee or structural failure), misoperation, breaching, and ponding. In addition, damages that may be caused by breaching itself, such as might be caused by the scour that occurs during the breach process, can be evaluated.

• Account for the performance of flood protection systems throughout the range of possible hazards. The analyses consider, for example, that a levee designed for the 10 percent chance event may reduce adverse economic consequence due to the one percent chance event, even though it will not eliminate all damage due to that event. They also represent the possibility, though unlikely, of failure of a FEMA “certified” levee during events that are smaller or larger than the one percent chance event.

• Acknowledge and evaluate the uncertainties about the various inputs to the analysis, including natural variability and lack of perfect understanding and ability to model the natural processes.

For example, all flood damage reduction studies conducted by the U.S. Army Corps of Engineers are required to analyze risk (USACE, 2006). The requirement applies to USACE studies that lead to feasibility reports, general design memorandums, and general re-evaluation reports. The regulation specifically prohibits traditional deterministic approaches—such as inclusion of freeboard to account for what are described as “hydrologic, hydraulic, and geotechnical uncertainties.” Instead, USACE analyses must use technical procedures presented in Engineering Manual 1110-2-1619 (USACE, 1996), Engineering Technical Letter 110-2-556, and companion documents.

In addition to the foregoing public-sector applications, private companies have developed flood risk analysis tools for evaluating insurance portfolios (AIR, 2012; RMS, 2012).


As discussed in Chapter 3, the use of risk analysis methods to evaluate flood risks and specifically to consider the performance of levees has been ongoing and improving for more than a decade. This includes the development of a probabilistic approach by USACE in the 1990s (USACE, 1996). More recently, after the events of Hurricane Katrina, there has been considerable development and application of risk analysis methods to evaluate the flood


1 See http://www.fema.gov/national-flood-insurance-program-flood-hazard-mapping/numerical-models-meeting-minimum-requirement-0.

2 See http://www.fema.gov/protecting-our-communities/hazus.

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