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1 INTRODUCTION BACKGROUND Prior to the 1960's, the national thrust in flood damage mitigation and relief was to provide structural flood protection works, such as dams, levees, and seawalls, and disaster assistance to remaining flood victims. Unfortunately in many instances, flood protection works generated a false sense of security and magnified the impacts of future disasters by encouraging floodplain development. The National Flood Insurance Program (NFIP) was established by the National Flood Insurance Act of 1968 in response to a continued increase in annual flood losses. This act formally realigned the federal flood control program toward an emphasis on nonstructural approaches to flood hazard mitigation. The act established a program to provide flood insurance coverage within communities willing to implement a floodplain management program featuring (1) land use regulations to guide development away from flood-prone locations and (2) minimum building construction standards to reduce future flood losses to structures in inundated areas. Section 1360 of the act required the identification of all areas having special flood hazards within 5 years. The act also required determinations of flood risk probabilities throughout all flood-prone areas within 15 years of enactment. The Federal Emergency Management Agency (FEMA) has adopted the 1 percent annual chance of exceedance (100-year) flood as the standard for identifying special flood hazard areas, and it is within the 100-year floodplain that floodplain management programs are required and provisions for lender notification and mandatory insurance are enforced. FEMA has chosen maps as the mechanism for identification and publication of floodplain and risk information. Flood Hazard Boundary Maps (FHBMs) are used initially to identify special flood hazard areas from approximate information. From priorities set according to a community's existing floodplain development and future development potential, a detailed engineering study is made to establish 100-year flood elevations and assemble risk information necessary for floodplain management and the determination of actuarial flood insurance rates. This information is published on Flood Insurance Rate Maps (FIRMs), which fulfills the second requirement of the act. Because of the large number of communities (approximately 20,000) identified as having some areas of special flood hazard, the FHBM -1-
program was accelerated to meet statutory deadlines. For the conduct of this mapping effort, FEMA did not provide guidance for evaluation and acceptance of local flood protection works, such as levees, as sufficient for removal of special flood hazard designations. The assessment of a particular protection work was left to the judgment of the agency or consultant who produced the map. In most instances, because of time and cost constraints, levees with crown elevations exceeding estimated 100-year flood levels were credited with providing protection against the 100-year flood. Little or no consideration was given to freeboard requirements, structural stability, or maintenance. As a result of the Flood Disaster Protection Act of 1973, the mapping to define flood risk zones, called flood insurance studies, was also accelerated. The lack of a specific policy for treating leveed areas carried over into the rate mapping program, and the study contractor's evaluations were made primarily on the basis of a field reconnaissance and a comparison between levee crown elevations and computed 100-year flood levels. In 1975, following the initiation of a significant number of rate studies, the need for a formal levee policy became more apparent. The Federal Insurance Administration (FIA) began receiving requests from study contractors for guidance in the evaluation of levees. Land developers began requesting FLA to provide standards that levees must meet in order for the special flood hazard designation to be removed from an area. Various groups were constructing 100-year design levees for the sole purpose of freeing property from the floodplain management, lender notification, and insurance purchase requirements under the NFIP. Thus, by crediting levee systems for providing protection against the 100-year flood, FEMA may be indirectly encouraging levee construction to no more than this minimum top elevation standard, with essentially no control on design, construction, operation, inspection, and maintenance. This situation has generated concern within FEMA for the following significant reasons, as indicated by FEMA: 1. It is estimated that levee overtopping or failure is involved in approximately one-third of all flood disasters. 2. The 100-year flood is generally found to be a low design standard for structures protecting densely populated areas because of the relatively low cost of raising or upgrading the levee compared to the damages that can be prevented. 3. Only a fraction of all earthen levees built with crown elevations at the computed 100-year flood elevation can be expected to provide protection to the true 1 percent event because of (1) the uncertainty involved in establishing flood elevations, (2) changing hydrologic conditions, and (3) the possibility of structural failure before overtopping. 4. The degree of protection to be expected from a 100-year design levee is less than that obtained by elevating individual buildings to the 100-year flood elevation because of the possibility of levee failure during smaller floods and the greater depths of -2-
flooding experienced in unelevated structures upon levee overtopping or failure. 5. Crediting a levee system with protection against the 100-year flood would, under present interim procedures, remove essentially all floodplain management requirements, lender notification requirements, and insurance purchase requirements within the leveed area (provided that flooding from interior drainage did not trigger such requirements). This could violate the spirit of the National Flood Insurance Act by encouraging development in areas subject to major flood damage. It could be financially burdening for the program should the people in the area purchase flood insurance at rates that apply outside special flood hazard areas. 6. Results of the U.S. Army Corps of Engineers nonfederal dam inspection program suggest that a large percentage of private or locally built levees as well as dams are or can be expected to be poorly designed and maintained. Because of these concerns, FEMA contracted with the National Research Council to recommend provisions for a comprehensive levee policy for use in administering floodplain management, insurance, and risk mapping aspects of the National Flood Insurance Program (NFIP). NATIONAL RESEARCH COUNCIL APPROACH AND OBJECTIVES The Commission on Engineering and Technical Systems of the National Research Council established a committee of recognized experts with experience in hydrologic, hydraulic, and geotechnical engineering; levee and dam design; construction and operation of flood control works; flood plain management; insurance; emergency planning and management; and law. Representatives from federal, state, and local agencies involved in the above areas were also consulted and kept aware of the study's progress as liaison representatives to the committee. A workshop, held in St. Louis, Missouri, on January 20-23, 1982, brought together the committee and other individuals interested in FEMA's levee policy program. These invited individuals gave their time and experience to assist the committee in sorting out its tasks and gathering additional information. Invitees included persons with varying expertise as regards levees such as emergency preparedness planners, hydrologists, hydraulic engineers, geotechnical engineers, environmental engineers, engineers familiar with Corps of Engineers' practices regarding levee design and construction, and public administrators representing viewpoints of various geographical areas where levees exist. This report was developed from the experience of the committee and the deliberations at the workshop. The study was structured to address the following five specific task areas identified by FEMA as critical to a levee policy: -3-
1. Minimum design standards (covering level of protection, structural integrity, interior drainage, etc.), for recognition of the levee in the program. 2. The nature and extent of the inspection and evaluation to be conducted by or for FEMA to assure conformance with the minimum design standards at the time a levee is accepted (recognized) in the program. 3. Requirements FEMA should place on communities with recognized levees as to (1) levee system operation, inspection, testing, and maintenance; (2) floodplain management in the protected area; and (3) contingency planning. The first requirement pertains to the agency responsible for the levee; the second and third pertain to communities enrolled in the NFIP. 4. Estimation of risk in areas protected by levees for use in setting insurance rates. 5. Floodplain mapping of levee protected areas to portray special hazard areas, degree of flood risk, and evacuation routes. INTERIM FEMA POLICY ON LEVEES In February 1981 FEMA established an interim policy on levees for the NFIP. This policy and subsequently developed policy criteria are included in Appendix B. -4-
