analytical part of flood risk management that provides information about or quantifies consequences and probabilities of a flood event.
The current NFIP flood hazard analysis is a partial risk-based analysis used with respect to performance of levee systems, where many parts of the analysis are deterministic in fashion. A levee system that is not accredited is not considered in the analysis used to quantify flood risk, even though it provides some (potentially considerable) protection against floods. This encourages communities to construct levee systems that protect only to the one percent annual exceedance flood, enabling new development in areas with significant, but unquantified exposure to catastrophic flood risk. Thus, protection against the one percent annual chance flood event is the de facto design standard for most levees seeking accreditation in the United States, with limited attention given to the consequences (“residual risk”) should a levee fail or be overtopped. Furthermore, levee systems that only marginally meet certification standards are vulnerable to loss of accreditation status. If not properly maintained, the performance of levee systems degrades over time due to erosion, rodent damage, subsidence, and other factors. Further, the frequency and magnitude of flood hazards can increase over time due to natural and anthropogenic causes. Loss of accreditation status can be very disruptive to the affected communities in terms of safety and insurance cost.
Thus, the current NFIP approach to flood hazard analyses leads to an incomplete description of the flood hazard in many areas. This is not unknown to FEMA or relevant stakeholders, including policy makers. A more modern approach to flood risk analysis addresses problematic aspects of the without-levee analysis.8 The elements of a modern risk-based approach include:
• flood hazard analysis or an estimate of the frequency and magnitude of flooding;
• levee and other system component fragility analysis, which estimates structure performance as a function of flood levels;
• systems analysis, which includes all features that impact flooding along a levee system;
• levee breach and inundation assessment; and
• consequence analysis that estimates the damage to structures that are inundated in a flood event.
All elements of this analysis are subject to uncertainty; the evaluation of these uncertainties is also integral to a modern risk analysis.
The modern risk-based analysis offers several advantages. It would provide an in-depth technical evaluation of flood hazards and directly account for the performance of levee flood protection systems whether accredited within the NFIP or located within an NFIP community. It would account for all features of the flood protection system (levees, gates, other structures) that significantly affect flood risk, as long as they meet minimum design, operation, and maintenance standards. Hence credit could be given for establishing flood insurance rates for flood protection systems that do not protect to the one percent annual chance standard, but at the same time provide some level of protection. In other words, the modern risk-based approach acknowledges that risk across a floodplain varies across the landscape. It has the impact of softening, but not eliminating, the one percent annual chance decision line.
Also, a risk-based approach would estimate flood risk for lands protected by levee systems that do protect to the one percent annual chance standard or higher. In these circumstances, the risk analysis results would inform communities of the flood protection system limitations and potential vulnerabilities, the actual flood risk being faced, and inform flood insurance ratings. Risk analysis results would also differentiate between protection provided by one percent annual chance levees and 0.2 percent levees. It would be able to recognize difference between low areas behind levees and higher ground. It would also provide guidance to the communities for development of new regulations that would encourage floodplain management. The NFIP should move to a modern risk analysis that makes use of modern methods and computational mapping capacity to produce state-of-the-art risk estimates for all areas that are vulnerable to flooding.
8 In this report the term “modern” is used to indicate characterization and analysis of risk using the best science available to the risk management field.