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3 ENGINEERING CRITERIA FOR LEVEE RECOGNITION GENERAL This chapter recommends minimum engineering criteria to be used in the evaluation of new or existing levee systems for recognition by the National Flood Insurance Program (NFIP). These criteria will enable FEMA to determine the acceptability of a levee system for recognition as achieving NFIP purposes. In order to be recognized, a levee should be (1) hydraulically able to contain an acceptably large flood and (2) structurally sound when that flood occurs. The committee considered both causes of failure and a number of factors affecting probabilities for failures of each type. Major among the engineering factors were: level of protection and the hydrologic and hydraulic analyses required to support this level, embankment geometry, seepage control, slope protection, freeboard, closure devices, and interior drainage facilities. Additionally, the committee recognized that a number of institutional and social realities need to be considered in setting levee criteria. Examples are restrictions and requirements of the NFIP statutes, FEMA's fiscal and administrative limitations, socioeconomic implications on affected communities, and the past performances of existing levees during floods. In general, a levee project can be said to have failed when the area that the project was intended to protect is flooded as a result of water passage across the line of the levee. This failure can occur due to overtopping of flood waters from the riverward side of the levee or from structural failure of the levee owing to piping, erosion, or structural instability. Additionally, a project should be considered to have failed (but with lesser damage) when accessorial facilities (i.e., pipes, pumps, ponds, and valves) fail to prevent flooding from interior drainage. It is believed that the committee's guideline-level criteria encompasses all modes of levee failure. Since early 198l, FEMA has had an interim policy concerning treatment of levees in the NFIP (see Appendix B). An important feature of this interim policy is a requirement that, to be recognized in the NFIP, a levee must provide protection equal to or exceeding the 1-percent chance of exceedence on an annual basis (100-year) flood with an additional height for freeboard, essentially 3 feet. From the point of view of levee evaluation, the policy is directed at obvious structural deficiencies. The committee reviewed this interim policy -10-
and believes that the interim policy may be overly rigid in some cases and not rigid enough in others. For the purposes of this report, the committee considered that floodwalls are substitutes for levees and that the criteria applied to levees should, as appropriate, be applied to floodwalls. RECOGNITION OF LEVEES IN THE NATIONAL FLOOD INSURANCE PROGRAM Prior to developing design criteria, one very basic issue had to be addressed by the committee: what levees, if any, should be recognized for purposes of modifying NFIP requirements currently applied to the area protected or to be protected by the levee. Existing Levees Many existing levees have, for a great number of years, provided varying degrees of protection to the occupants of the floodplains behind them. This proven history of successful performance commends these levees for some consideration in the implementation of the NFIP. This recognition should reflect, as nearly as possible, actual degrees of protection provided by the levees. While it is possible that recognition of these levees in the NFIP might encourage additional development, such development would take place in compliance with the provisions of the NFIP. In the opinion of the committee, existing levees at the 25-year level or higher, plus freeboard, provide, to facilities behind them, sufficient protection to be credited in setting insurance rates within the NFIP. Typically these levees represent substantial engineered construction efforts designed for reasonable structural safety. Often levees built to less than the 25-year level are poorly constructed and have been developed, over time, on an ad hoc basis, with little or no attention having been given to acceptable design criteria. Review of the large number of levees below the 25-year level would involve a major administrative problem for FEMA in handling both the sheer volume and the difficult engineering analyses that would be encountered in evaluating the smaller and, typically, less well engineered levees. The economic and safety returns from bringing such levees into the program would be minimal. Limiting recognition to levees providing a greater degree of protection (e.g., 50 years) would, in the opinion of the committee, be an unnecessary financial burden on communities that have and must continue to pay for carefully engineered levee projects that substantially reduce the average annual flood damages experienced. The committee recommends: ⢠Existing levees should be recognized for the purpose of reducing insurance rates where they provide protection against 25-year or larger -11-
floods and where they meet specified structural design criteria, including requisite freeboard. New Levees About 16 percent of the nation's urban land area is located in floodplains susceptible to 100-year floods. Many communities, in fact some entire counties, lie within the floodplains of major rivers, and it would be unreasonable to halt all future development in these areas. When use of the floodplain is necessary to ensure the continued vitality of an area, a feasible and prudent approach may be the use of levees to protect the floodplain areas. Under circumstances where the community lies both within and outside the floodplain, economics will frequently dictate which approachâwise use of the floodplain or construction on higher groundâis more prudent. The NFIP now permits construction in the floodplain when lowest floor elevations are above 100-year flood levels, or, in the case of nonresidential structures, where flood proofing provides for watertight, stable, and nonbouyant buildings. Recognition of new levees provides consistency with this key tenet of the NFIP and permits growth behind appropriately constructed levees. The 100-year flood has become a widely accepted, applied, and institutionalized standard, and the establishment of the 100-year minimum degree of protection as a standard for new levees would be consistent with the base flood level of the National Flood Insurance Program guidelines. Requiring buildings in the floodplain to have their base elevations at or above the 100-year level is part of the NFIP, and the committee supports this policy. If such a level of protection is appropriate for new buildings, then at least that amount is appropriate as a minimum level of protection for new levees that protect many buildings. A recent survey of state floodplain managers indicates that they generally share this view. Provision of less than 100-year protection would be inconsistent with the thrust of Executive Order 11988 and the NFIP. On the other hand, under certain circumstances, for example in densely urbanized areas containing critical facilities, considerations for loss of life and economics might dictate more than the 100-year level of protection. The committee recommends: ⢠New levees should be recognized for the purpose of reducing insurance rates where they provide protection against 100-year or larger floods and where they meet specified structural design criteria including freeboard. All levees on which construction begins after a date to be determined by FEMA should be considered new levees. -12-
A counter argument to this recommendation is contained in Appendix D. STRUCTURAL CRITERIA Once a determination has been made that an existing or proposed levee is eligible, from a level of protection, standpoint, for recognition by the NFIP, the structural characteristics of the levee must be evaluated. This section discusses the important considerations, but not the engineering details, in light of the committee's opinion that it is inadvisable in this report to specify engineering criteria in depth. Standard references cover each of the design aspects in extensive detail and should be consulted. Evaluations of the suitability of the engineering of a specific levee must be based on sound technical analyses made by professional engineers skilled in structural, geotechnical, hydrologic, and hydraulic engineering, with specific experience in levee design. Exceptions to the general design criteria discussed below should be permitted only when information presented by professional engineers shows that the risk of flooding will not increase as a result of the exceptions. The committee recommends: ⢠All levees (existing and new) to be given credit for reducing flood risk in the NFIP must meet standard minimum engineering criteria with respect to geometric parameters, freeboard, soils and foundations, interior drainage, closure devices, and rights of way. Geometric Criteria Height. The height of a levee or floodwall is the difference between its top elevation and the land surface elevation upon which it is built. The top elevation must be at the design high water level plus an allowance for freeboard. Top width. The top width of a levee must be capable of supporting one way vehicular (truck) traffic. Appropriate provision must be made for turnarounds. The top width criteria is necessary to ensure that heavy equipment and personnel can be brought to all parts of the levee during flood fights. Slopes. The specific slopes used for the embankment on the land and water sides of levees must be determined safe by stability analyses but should not normally be steeper than 1 vertical on 3 horizontal. -13-
Side slopes for earth-fill levees should ordinarily not be steeper than one vertical on three horizontal. In cross section, access ramps should be constructed in addition to, rather than infringing upon, minimum levee side slopes. Some levees with steeper side slopes can satisfy stability analyses and perform satisfactorily. However, experience has shown that flatter side slopes (i.e., less than 1 on 3) are more amenable to proper maintenance. Freeboard Freeboard provides a margin of safety for those situations that cannot be rationally quantified in design flood profile computations. Freeboard requirements should apply equally to floodwalls and levees, since the lack of precision associated with flow line calculations pertains to both floodwalls and levees. Both existing and new levees and floodwalls must have a freeboard of 3 feet, or 1 foot plus wave height plus runup, whichever is greater. At bridges and other hydraulic obstructions, additional freeboard may be required. With the presentation of detailed information substantiating the accuracy of the forecast water surface level (e.g., levee next to a lined channel) or where the 3-foot requirement represents a substantial percentage of the levee height, freeboard requirements may be reduced. But, for new construction, in no case should freeboard be less than 2 feet. The 3 foot freeboard is consistent with that specified by the Corps of Engineers in Engineering Manuals 1110-2-1913 (Design and Construction of Levees) and 1110-2-1601 (Hydraulic Design of Flood Control Structures). These guidelines are only slightly more stringent than those specified for type I structures by the Soil Conservation Service (SCS National Engineering Standard 356-1). Soils and Foundations Design or construction of any levee or floodwall must be based on use of recognized soil mechanics analyses and techniques. In evaluation of existing levees or floodwalls, site conditions must be investigated by a professional engineer. In determination of the acceptability of plans for new levees, appropriate seepage analyses must be conducted, provisions should be made to accommodate any seepage problems, subsidence should be fully accounted for in design where applicable, and margins of safety should be identified. Designs must provide for adequate vegetative cover or other protection to prevent erosion. All justified penetrations of a levee must be made so as to maintain the -14-
integrity of the levee; however, penetrations should be avoided if at all possible. Appropriate design and construction techniques must be followed to prevent adverse effects of seepage along penetrations through or beneath the levee. Interior Drainage Provisions must be made for interior drainage, i.e., the discharge of runoff from the area on the land side of the levee. If interior drainage flooding can result in loss of life or extensive property damage, protection should be provided against 100-year interior flood conditions. This flood level should be determined based on the joint probability of interior and exterior flooding as estimated by considering the interior hydrograph (including baseflow and seepage through and beneath the levee), the river stage, and the hydraulics of facilities for evacuating the water. The interior drainage system may include storage areas, gravity outlets, pumping plants, or combinations thereof. These provisions are necessary in view of the high potential for damage created by flooding behind levees during periods when the river stages do not permit gravity flow and the flow into the leveed area exceed what can be discharged through pumps. In many situations interior flooding may be as damaging as flooding from outside the levee. Closure Devices All openings in floodwalls must be provided with appropriately engineered closure devices. Sand bagging of openings should not be allowed. Rights of Way Sufficient rights of way and easements must be provided to accomplish maintenance of the levee and to insure the proper functioning of landside seepage collection or pressure relief systems. Activities on the riverside of the levee must be monitored and appropriate controls must be taken to preclude all activity riverside of the levee that would increase the design high water flow in the area of the levee. -15-
OTHER FACTORS TO BE CONSIDERED Coastal Levee Systems This document does not provide specific guidance on coastal levee recognition or design. There may be important differences in engineering requirements for coastal levees as compared with riverine levees. For example, differences might be made in the allowance for waves and seepage requirements between a riverine levee, where the water level may remain high for extended periods and the water flows parallel to the levee, as opposed to a coastal levee, where the duration of high waters is limited but the full dynamic impact of waves from the sea strikes the levee perpendicularly. In the design of coastal levees, the engineering criteria should be documented by a professional engineer with justifications for any departures from the engineering criteria for riverine levees. Floodwall Design The design of floodwalls should adhere generally to the applicable criteria provided for levees. In those cases where levee criteria are not applicable (e.g. slope, width, etc.), the design of the floodwall should be in accordance with recognized professional engineering standards for reinforced concrete structures. Hydrologic and Hydraulic Analyses For evaluation of the plans for new levees and the continued soundness of existing levees, FEMA must require the application of standard, scientifically acceptable methodologies accounting for the relevant factors in the computation of discharge frequencies and water surface profiles (as an example with many applications, the HEC-2 "Water Surface Profiles" computer program). It is especially important to consider the possibilities of ice and debris causing higher flood peaks. Further, where circumstances dictate, regional hydraulic analyses of the cumulative effects of construction of several levees on flood flows and stages should be required in order to determine the impacts of the total system of existing levees and/or proposed new levees on levee flow lines. In cases where the 10-, 50-, 100-, and 500-year flood levels have already been determined in the NFIP study for communities, where levees already exist, the committee suggests that FEMA recognize a practical and economical alternative method of determining the level of protection. The existence of these four flood profiles constitutes an adequate base for the development of elevation-frequency curves, and levels of protection can be determined adequately therefrom. Flood stages for a given flow can change over time due to a variety of factors, and any change will alter the hydrologic risk. FEMA must -16-
be cognizant of these factors as they may drastically affect design levels and the NFIP as a whole. Four cases of change are: 1. Increased land use change that results in increased runoff volumes, shorter times of concentration, and greater peak discharges for events having the same meteorological characteristics. 2. Removal of natural valley storage and conveyance due to excess encroachment in floodplains, including construction of levee systems, resulting in higher stages and discharges. 3. Construction of reservoirs that modify the flows so that historical records cannot be used for current risk assessment without hydrologic reanalysis. 4. Changes to river bed or bank geomorphology or vegetative cover that significantly alter stage-discharge relationships and flood elevations. An obvious impact of such changes is that levels of protection may change to greater or lesser levels. The committee recommends: ⢠FEMA should monitor watershed and channel changes where hydrologic risk is increasing and respond to significant changes with restudies and subsequent map and rate revisions. Such surveillance can be accomplished as part of FEMA's periodic restudy process, as part of a national monitoring program, perhaps using advanced remote sensing systems, or through reports from federal, state, or local agencies and NFIF participants. EXCEPTIONS In the application of the principles presented in this report, levee proponents may, because of unusual circumstances or advances in technologies or construction techniques, request exceptions to the criteria and guidelines. As with any specifications, there must be cutoff points, and the committee carefully considered each criterion and guideline to ensure that the chosen cutoff points (elevation, slope, etc.) were dictated by sound engineering practice and represented, in the opinion of the committee, the "bottom-line." Since each exception granted becomes a precedent, extreme care must be taken to ensure that all requests are fully documented and well justified and that the precedent-setting impacts of each decision are carefully analysed prior to the decision by FEMA to grant a waiver. -17-
