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Mapping the Zone: Improving Flood Map Accuracy
2 Flood Mapping and Flood Insurance
People have always settled near rivers and coasts, but population growth and the commensurate expansion of the built environment have increased their risk of losses to flooding over time. From the mid 1930s to the late 1960s, the federal government dealt with flood hazard primarily by building flood control structures, such as dams and levees. Flood insurance was not available because (1) the people most likely to buy it were those most prone to flooding, which meant that private companies could not profitably provide coverage at an affordable rate,1 and (2) existing data about flood extent were insufficient to accurately assess flood risk.
Escalating flood losses and disaster relief costs, particularly the widespread damage caused by Hurricane Betsy, led to the creation of the National Flood Insurance Program (NFIP) in 1968. The objectives of the NFIP, which is administered by the Federal Emergency Management Agency (FEMA), are to identify and map floodprone communities and to make flood insurance available in communities that adopt and enforce floodplain management regulations (e.g., zoning, building requirements, special-purpose floodplain ordinances). More than 20,400 communities currently participate in the NFIP.2 Although created for insurance and floodplain management purposes, FEMA’s Flood Insurance Rate Maps (FIRMs) are now used for many other purposes, including disaster mitigation, land use planning, and emergency response. This chapter describes how FIRMs are created and maintained and how information technology is used to update and share flood-related data.
FLOOD INSURANCE RATE MAPS
Flood Insurance Rate Maps delineate flood hazard areas, identify flood insurance rate zones within these areas, and may show elevation and other data related to flooding. The information that appears on individual maps (and the accuracy of those data) depends on the type of flood hazard (e.g., riverine, coastal) and the way the flood hazard was studied. The primary information portrayed on FIRMs is discussed below.
Flood Hazard Areas
Three types of flood hazard areas are shown on FIRMs:
Special Flood Hazard Areas (SFHAs) subject to a 1 percent or greater chance of flooding in any given year (44 CFR 59.1). The 1 percent annual chance flood, also known as the base flood or 100-year flood, is the NFIP standard for regulating new development in the floodplain and determining where mandatory flood insurance coverage is required.
Moderate flood hazard areas, including areas subject to a 0.2 percent annual chance (500-year)
1
The private sector stopped covering flood losses in 1929 after a series of devastating floods, including a 1927 flood of the Mississippi River, which inundated 13 million acres and killed several hundred people. See American Institutes for Research, 2002, A Chronologyof Major Events Affecting the National Flood Insurance Program, 78 pp., available at <http://www.fema.gov/library/viewRecord.do?id=2601>.
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Mapping the Zone: Improving Flood Map Accuracy
2
Flood Mapping and Flood Insurance
People have always settled near rivers and coasts, but population growth and the commensurate expansion of the built environment have increased their risk of losses to flooding over time. From the mid 1930s to the late 1960s, the federal government dealt with flood hazard primarily by building flood control structures, such as dams and levees. Flood insurance was not available because (1) the people most likely to buy it were those most prone to flooding, which meant that private companies could not profitably provide coverage at an affordable rate,1 and (2) existing data about flood extent were insufficient to accurately assess flood risk.
Escalating flood losses and disaster relief costs, particularly the widespread damage caused by Hurricane Betsy, led to the creation of the National Flood Insurance Program (NFIP) in 1968. The objectives of the NFIP, which is administered by the Federal Emergency Management Agency (FEMA), are to identify and map floodprone communities and to make flood insurance available in communities that adopt and enforce floodplain management regulations (e.g., zoning, building requirements, special-purpose floodplain ordinances). More than 20,400 communities currently participate in the NFIP.2 Although created for insurance and floodplain management purposes, FEMA’s Flood Insurance Rate Maps (FIRMs) are now used for many other purposes, including disaster mitigation, land use planning, and emergency response. This chapter describes how FIRMs are created and maintained and how information technology is used to update and share flood-related data.
FLOOD INSURANCE RATE MAPS
Flood Insurance Rate Maps delineate flood hazard areas, identify flood insurance rate zones within these areas, and may show elevation and other data related to flooding. The information that appears on individual maps (and the accuracy of those data) depends on the type of flood hazard (e.g., riverine, coastal) and the way the flood hazard was studied. The primary information portrayed on FIRMs is discussed below.
Flood Hazard Areas
Three types of flood hazard areas are shown on FIRMs:
Special Flood Hazard Areas (SFHAs) subject to a 1 percent or greater chance of flooding in any given year (44 CFR 59.1). The 1 percent annual chance flood, also known as the base flood or 100-year flood, is the NFIP standard for regulating new development in the floodplain and determining where mandatory flood insurance coverage is required.
Moderate flood hazard areas, including areas subject to a 0.2 percent annual chance (500-year)
1
The private sector stopped covering flood losses in 1929 after a series of devastating floods, including a 1927 flood of the Mississippi River, which inundated 13 million acres and killed several hundred people. See American Institutes for Research, 2002, A Chronology of Major Events Affecting the National Flood Insurance Program, 78 pp., available at <http://www.fema.gov/library/viewRecord.do?id=2601>.
2
See <http://www.floodsmart.gov/floodsmart/pages/about/community_preparedness_ratings.jsp>.
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flood (44 CFR 64.3) and SFHAs that are either small (drainage areas of less than 1 square mile), expected to flood less than 1 foot, or protected by levees from the 1 percent annual chance flood. Flood insurance is voluntary, although lenders may require flood insurance for structures. In addition, communities may choose to regulate land use and siting of critical services and emergency response facilities in these areas.
Areas in which flood hazards are minimal (e.g., less than a 0.2 percent annual chance of flooding) or undetermined, but still possible. These areas are not subject to federal regulations on insurance or land use, although communities and lenders may impose such requirements.
Each of these areas is divided into flood insurance rate zones, which designate the level and type of flood hazard (Box 2.1). The majority of SFHAs are either riverine and lacustrine (area along the shore of a lake or closed water basin) A zones (subject to a 1 percent annual chance flood) or coastal A zones and V zones (subject to storm surge where wave heights for the 1 percent annual chance flood are 3 feet or greater). Moderate flood areas are designated as shaded Zone X, and areas of minimal flood hazard include unshaded Zone X and zones for which flood hazard has not been determined. Example portions of FIRMs showing some of these zones in a riverine area and a coastal area are shown in Figures 2.1 and 2.2.
FEMA’s Map Modernization Program was intended to produce digital FIRMs for all of the nation’s 1 percent annual chance floodplains, but a midcourse adjustment gave priority to densely populated areas, where more lives and property are at risk (FEMA, 2006a). Risk-related priorities were based on total population, rate of population growth, number of housing units, number of flood insurance policies and claims, number of repetitive loss properties and claims, and number of declared flood disasters. This decision shifted emphasis from the risk of occurrence of a 1 percent annual chance flood to the risk of more significant flood damage.
BOX 2.1
Definitions of the Most Common Flood Insurance Rate Zones
Zone A: Special Flood Hazard Area (SFHA), defined as land subject to a 1 percent annual chance of flooding. The zone is divided into several subtypes, including
A (or unnumbered or approximate A): SFHA in which detailed analyses were not carried out and the base flood elevation is not shown.
AE, A1 through A30: SFHA in which the water surface elevation has been determined and is shown on the map.
Zone V: Coastal SFHA subject to high velocity wave action from storms or seismic sources. The zone is divided into several subtypes, including
V (or unnumbered V): Coastal SFHA for which water surface elevations are not shown.
V1 through V30, VE: Coastal SFHA with velocity hazard and water surface elevation determined and shown on the map. The VE designation is replacing the earlier numbered V designations.
Shaded Zone X, Zone B: Area of moderate flood hazard or future conditions flood hazard, generally defined as the 0.2 percent annual chance flood.
Unshaded Zone X, Zone C: Area of minimal flood hazard, commonly understood to have a lower probability of flooding than the moderate hazard area.
The numbers for zones A1 through A30 were determined by computing the difference between the 1 percent annual chance and 10 percent annual chance flood elevation, multiplying by 10, then applying a conversion factor (FEMA, 1983). The process was similar for numbered V zones, although different multiplication and conversion factors were used. Modernized maps have replaced the A1 through A30 designations with an AE designation, and the B and C designations with an X designation.
SOURCE: 44 CFR 59.1 and 44 CFR 64.3.
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FIGURE 2.1 Extracted image from a paper map (FIRMette) for a riverine area in Greenville, South Carolina. The left side shows an approximate A zone (SFHA, shaded dark gray), where no elevation or floodway information is provided. The right side of the image shows an AE zone (SFHA, shaded dark gray) with lettered cross sections, base flood elevations (wavy lines with elevation), and floodway (hatched area bounded by heavy dashed lines), and a shaded Zone X (moderate flood hazard area, shaded light gray). The other areas are classified as unshaded Zone X (minimal flood hazard). SOURCE: FEMA’s Map Service Center, <http://msc.fema.gov/>.
Base Flood Elevations
The base flood elevation (BFE) is the computed elevation of a flood having a 1 percent chance of being equaled or exceeded in a given year (base flood). It accounts for the volume and velocity of water moving through the watershed and reflects the cumulative effects of topography, soils, vegetation, surface permeability, and other factors. The BFE is the regulatory standard for the elevation or floodproofing of structures, and the relationship between the BFE and the elevation of a structure also determines the flood insurance premium. In general, the higher the first floor elevation, the lower the insurance premium. Consequently, the accuracy of BFEs on the flood maps is important for
FIGURE 2.2 Example of a FIRMette for a coastal area near Myrtle Beach, South Carolina. The figure shows VE zones (SFHAs subject to coastal wave action) and associated elevations at the point on the ground to which the wave runs up during the 1 percent annual chance flood. Landward, the flood zones transition to Zone AE with their associated base flood elevations. SOURCE: FEMA’s Map Service Center, <http://msc.fema.gov/>.
both regulating and insuring properties commensurate with the true risk of flooding.
Despite the importance of accurate BFEs in Special Flood Hazard Areas, in unnumbered A and V zones they are generally only estimated using approximate methods (see “Types of Flood Studies” below), which estimate key variables such as water volume. The determination of flood risk is less certain in these areas, so local communities may require a safety factor (known as freeboard) above the estimated BFE for additional financial protection. However, even where BFEs are established with more certainty, communities may impose freeboard to help protect against damage resulting from multiple 1 percent annual chance floods in a given year or higher than expected flood waters.
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Future Hydrologic Conditions
Flood hazard information presented on FIRMs is typically based on conditions in the floodplain and watershed that existed when the map was made. In recent years, however, some growing communities have become interested in projecting how future land use and development in the watershed will affect the extent of the floodplain, and using those projections to regulate floodplain development. In response, FEMA issued a final rule in November 2001 that allows communities the option of showing future conditions floodplains based on land use change on the FIRM, along with the required existing conditions floodplain. The decision about how to use information on future conditions for regulatory decisions is left to the community. FEMA continues to use data on existing conditions for flood insurance purposes and has yet to consider the effects of climate change, long-term erosion of coastal areas, or long-term trends in hydrologic records on the determination of future conditions. By mid-century, the absolute flood elevations on structures along the Gulf Coast will be higher than at the time of their construction because of sea level rise and subsidence. The U.S. Army Corps of Engineers is including location-dependent adjustments in the design of structures to compensate for the expected rise.
FLOOD MAP PRODUCTION
The process for producing flood maps involves three main phases (Figure 2.3):
Scoping, including identifying flood risk, assessing immediate and future needs (e.g., development of floodprone areas), and determining what type of flood study is feasible with available resources. This step is carried out by FEMA in conjunction with state and local officials.
Development, including collecting technical data, modeling, creating a preliminary map, and performing quality control and quality assurance. Modeling and map production are carried out by a FEMA mapping partner (e.g., contractor, state or local government employee). Once the technical work has been completed, it is reviewed by a FEMA contractor, then preliminary maps are prepared and released to the relevant communities for review.
Adoption, including periods for public comment and appeal. FEMA, contractors, and state and local government agencies involved in the process must respond to comments made within the appeal period. Once the protest and appeal process is completed and any outstanding issues are resolved, the maps are finalized and FEMA issues a Letter of Final Determination. The local community then has up to six months to adopt the new map and update its floodplain management ordinances, if necessary, before the map becomes effective (i.e., the most current legal map for regulatory and insurance purposes).
Data for Digital FIRMs
Digital Flood Insurance Rate Maps (DFIRMs) are built from three layers of information (Figure 2.4).
FIGURE 2.3 Flood map production process. SOURCE: Courtesy of Michael Godesky, FEMA.
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FIGURE 2.4 Major components of DFIRMs. SOURCE: Modified from Maune (2007). Reprinted with permission from the American Society for Photogrammetry and Remote Sensing.
The base map imagery (orthophoto or vector) shows planimetric features such as roads, rivers, and buildings. Digital elevation data are overlain to give each feature in the base map image a vertical position. Finally, flood hazard data, collected and modeled by surveyors and engineers, are overlain to produce the DFIRM.
Methods for Mapping Flood Hazard
FEMA’s methods for mapping the most common flood hazards are summarized below and discussed in more detail in Chapters 4 and 5.
Riverine Flooding. Overbank flooding, the most common type of flooding in our nation, occurs when downstream channels receive more water than they can accommodate due to rain, snowmelt, blockage of channels by ice or debris, or dam or levee failure. Mapping riverine flood hazards requires hydrologic and hydraulic studies to determine ground elevations, the depth of floodwaters, the width of floodplains, the amount of water that will be carried by watercourses during flood events, and obstructions to water flow (FEMA, 2003, V. 1 and Appendix C). Cross sections, based on topographic data collected in the field or scaled from U.S. Geological Survey topographic quadrangle maps, are taken to define the floodplain. The locations of these cross sections are chosen to capture variations in topography and possible obstructions to flow.
Coastal Flooding. The coasts of the Great Lakes and the oceans are subject to severe flooding from storm surge, the result of high winds and air pressure changes that push water toward the shore. Coastal flood studies assess the effects of storm surge and wave action and determine base flood elevations (FEMA, 2003, V. 1 and Appendix D). The study process is similar to that for riverine flooding, except that instead of cross sections, transects are surveyed perpendicular to the coastline, yielding onshore and offshore ground elevations. The elevations are then used to compute the expected height of wave crests and wave runup that are added to the storm surge as it approaches the shoreline.
Shallow Flooding. Even a minimal rise in water level can lead to extensive inundation in relatively flat areas
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TABLE 2.1 Types of Flood Study Methods
Detailed (Riverine)
Detailed (Coastal)
Limited Detailed
Approximate
Redelineation
Base mapa
Orthophotography or vector
Orthophotography or vector
Orthophotography or vector
Orthophotography or vector
Orthophotography or vector
Hydrology (flows)
Regression equations, stream gage data, or rainfall-runoff models
Historical water marks and tide gage data
Regression equations or stream gage data
Analysis not technically reviewed
Uses previously published flow information
Hydraulics (flood elevations)
Modeled (steady state or dynamic) with detailed structure survey data
Modeled storm surge, waves, erosion, and wave runup
Modeled (steady state) without survey information on bridge or culvert structures
Analysis not technically reviewed
Uses previously determined elevations
Mapping presentation
Typical zone representations include AE with floodway
Typical zone representations include AE and VE
Zone representation limited to AE
Typical zone representations include A and V
New floodplain boundaries matching new base map information; Letters of Map Change (LOMCs)
Study report
Provides flow estimates, floodway data tables, and flood elevation profiles
Provides shoreline profiles and stillwater data tables
Provides flood elevation and profile information
Not applicable
Republishes flood study
Cost per mileb
$10,000-$25,000 (typically $13,500)
Approximately $9300
$1500-$5000 (typically $3000)
$250-$2000 (typically $900)
aAll flood study methods use best available base map at the time of production; the current FEMA minimum standard is digital orthoquarter quadrangles.
bSOURCE: Paul Rooney, FEMA.
such as Florida. The low relief and absence of channels in these areas can cause water to flow in sheets across the land surface, often in unpredictable directions. Drainage ditches and stormwater management facilities may be overloaded by storms more severe than the 10 percent annual chance floods for which they are usually designed. Ponding of rainfall in depressions often creates local floods, which may be alleviated by infiltration, evaporation, or mechanical pumping. Shallow flood studies yield a uniform depth of flooding, which is either added to the ground elevation or used to determine a single base flood elevation for a large area (FEMA, 2003, V. 1 and Appendix E). When adequate topographic data are not available, cross sections may be taken to determine storage volume for areas subject to ponding and average flood depths for areas subject to sheet flow.
Types of Flood Studies
The four main approaches used to study riverine flood hazard are (1) detailed studies, (2) limited detailed studies, (3) approximate studies, and (4) redelineation. Each approach yields different information, and the decision about which to use depends on the type of flood hazard, the resources available, and the risk of flood damage. Coastal flood mapping is currently done using the equivalent of detailed studies. Table 2.1 compares the information used and presented in the four study types.
Detailed studies are most expensive and provide the most information about flood hazards, establishing base flood elevations, special and moderate flood hazard areas, and where appropriate, floodways.3 Limited detailed studies provide a reasonable representation of the floodplain limits and often a base flood elevation. Structures such as bridges or culverts are represented in the models, but their dimensions and elevations are not verified in the field. Approximate studies yield
3
A floodway is the river channel and adjacent land areas required to discharge the base flood without significantly increasing flood heights. Coastal high hazard areas and tidal rivers, which experience regular fluctuations in water surface elevations, do not have designated floodways.
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an approximate outline of the floodplain, but no base flood elevations, floodways, moderate hazard areas, or other details. Although comparison of the floodplain boundaries to a topographic map provides an estimate of the base flood elevation, this estimate is inadequate for regulatory purposes. FEMA provides written guidance (FEMA, 1995) and a computer program for calculating approximate water surface elevations on open channels based on specified field measurements (see Appendix A for a list of methods used to estimate BFEs in approximate studies).
Redelineation studies are aimed at producing digital representations of flood maps as part of a national digital flood layer. Redelineation uses existing flood elevation information and redraws the floodboundaries on new or updated topographic maps. All approved changes to the flood maps (see “Map Maintenance” below) are incorporated, resulting in an updated map that reflects the most current effective flood elevation and hazard information. In contrast, the digital conversion method simply scans the flood boundaries shown on paper maps and transfers them to a new digital map. Fifty-four percent of the stream miles mapped until 2007 were the result of the digital conversion process.4 This approach was discontinued for new studies following FEMA’s midcourse adjustment (FEMA, 2006a) and prior to issuance of a new floodplain boundary standard (see below).
FEMA’S MAP MODERNIZATION PROGRAM
The nation has floodplains along approximately 3.5 million miles of rivers and coasts (FEMA, 2006a). Prior to 2003, only 1 million miles had been mapped, often at a lower quality than meets NFIP needs, and most flood maps and related products were outdated and available only in paper form. FEMA’s Map Modernization Program was established to collect new flood data in unmapped areas, to update or validate existing flood data, and to create digital flood maps. The federal government invested about $1 billion in this 2003-2008 mapping effort, and considerable matching funds were provided by FEMA’s state government and local community partners. This investment in more accurate maps was intended to benefit communities that use the maps to establish zoning and building standards; insurance companies, lenders, real estate agencies, and property owners who use the maps to determine whether flood insurance is required; and government officials who use the maps to support infrastructure, transportation, and other planning and to prepare for and respond to flooding.
Mapping costs and map accuracy are directly related, and funding for the Map Modernization Program was insufficient to produce high-quality maps of the entire nation (GAO, 2004). Moreover, the Government Accountability Office, Congress, and stakeholders were concerned about the accuracy of the mapped floodplain boundaries that were to be digitized (FEMA, 2006a). In response, FEMA made a midcourse adjustment to the Map Modernization Program. Two criteria were used to quantify map and engineering accuracy: (1) a floodplain boundary standard and (2) validation guidelines for flood data and engineering analyses used to delineate floodplains. The floodplain boundary standard is a statistical measure of the vertical discrepancy between the water surface elevation at the boundary of the floodplain and the land surface elevation at that location (FEMA, 2007c). The measure is computed at a sequence of points along the floodplain boundary and a specified percentage of these points must lie within defined error ranges that are more strict for maps produced from detailed studies than for maps produced from approximate studies. The standard is aimed at ensuring that the flood maps match the topographic data used, although adherence to the standard does not itself validate the topographic data. The validation guidelines for flood data and engineering analyses are a set of rules which define whether a flood study done in the past is adequate for current use or whether physical, hydrologic, or methodological changes since the time of the original study are sufficiently great to warrant an updated study (FEMA, 2007b). The intention of these changes was to improve the percentage of studies meeting these criteria while relaxing the original program goal of complete digital flood map coverage of the nation. Doing so is consistent with stakeholders’ comments on the midcourse adjustment that “The goal of digitization of the nation’s flood maps … should not outweigh the goal of achieving accuracy on the newly updated maps” (FEMA, 2008c, p. 22). A map of the data quality standards achieved for U.S. counties by March 2008 is shown in Figure 2.5.
4
Presentation to the committee by Patrick Sacbibit, FEMA, on November 8, 2007.
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FIGURE 2.5 Data quality standards achieved by individual counties as of March 31, 2008. Green counties (21 percent of the population) meet or exceed the floodplain boundary standard and the engineering analysis standard. Yellow counties (47 percent of the population) meet either the floodplain boundary standard or the engineering analysis standard or part of either standard but below thresholds. In red counties (1 percent of the population), the maps have been updated digitally and a digital product has been issued. Compliance with data quality standards was not required for such digital conversions, although a limited FEMA audit suggests that some portions of these counties meet the standards. In beige counties (26 percent of the population), modernized maps have not yet been issued because the first phase of map production (scoping) has not been completed or quality data do not exist. No study is planned in white counties (5 percent of the population). SOURCE: Paul Rooney, FEMA.
The adjusted goal is to have 65 percent of the U.S. continental land area and 92 percent of the U.S. population covered by digital flood maps (Table 2.2; FEMA, 2006a). For 30 percent of the mapped stream and coastal miles covering 40 percent of the population, the maps should meet the engineering analysis standard. For 75 percent of the mapped stream and coastal miles covering 80 percent of the population, the maps should meet the floodplain boundary standard. These figures illustrate the challenges of increasing flood map accuracy: even if the goals articulated in the midcourse adjustment are achieved, 70 percent of the mapped stream miles will not have validated engineering analyses supporting the flood map, and 25 percent will not meet the floodplain boundary standard. In addition, this standard ensures that the maps match existing topographic data within defined error tolerances, but it does not ensure the accuracy of the topographic data.
MAP MAINTENANCE
A map records the conditions that existed when the data for its compilation were gathered. By the time the data are gathered and analyzed and the map is published, it may already be outdated. Corporate boundaries and other non-flood-related features can change, affecting regulation of floodplain development. Ground elevations in the floodplain can change—for example, when fill is placed in the floodplain to raise building sites or when a new flood control project introduces levees, reservoirs, or stream channel modifications—affecting the spread of floodwater. Small projects, such as clearing channels or building retention basins in new subdivisions, commonly do not have a measurable effect on the base flood and thus do not warrant a map change on their own. Cumulative effects of small projects, however, may be significant.
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TABLE 2.2 Adjusted Targets for FEMA’s Map Modernization Program
Performance Measure
Original Target (%)
Adjusted Target (%)
Percentage of continental U.S. land area covered by digital flood maps
100
65
Percentage of U.S. population covered by digital flood maps
100
92
Percentage of mapped stream and coastal miles with new, updated, or validated engineering analysis
22
30
Percentage of population covered by maps with new, updated, or validated engineering analysis
15
40
Percentage of mapped stream and coastal miles that meet the 2005 floodplain boundary standard
57
75
Percentage of population covered by maps that meet the 2005 floodplain boundary standard
32
80
SOURCE: FEMA (2006a).
Finally, better topographic data, models, or statistical data on hazard events may become available, potentially improving the depiction of the flood hazard.
FEMA has four approaches to changing flood maps:
Restudy, in which a new Flood Insurance Study is carried out to establish new flood profiles, data tables, and flood boundaries when development has substantially changed stormwater runoff conditions or when growth is occurring in a floodprone area that lacks base flood elevations. Restudies can be completely new work or new analysis of existing data using different models, and they result in addition of or adjustment to the BFEs, addition of the 0.2 percent annual chance floodplain, and/or changes in the horizontal extent of the SFHA.
Limited map maintenance projects, which are restudies that are limited in size and cost. They are frequently used to increase detail in approximate studies in unnumbered A zones.
Revisions, which are made after a flood map is published to reflect changes in the horizontal or vertical extent of the floodplain. Revisions may add or adjust the BFE; add, remove, expand, or contract the mapped floodplain; and/or add or remove a defined floodway.
Amendments, which are made to correct mapping inaccuracies, including non-flood-related map elements (e.g., north arrows, graphic scale) and inadvertent inclusion of higher areas in the mapped floodplain. Inadvertent inclusions are commonly found through more accurate or detailed topographic study; when they are too small to depict graphically, they are only correctable in Letters of Map Amendment.
Amendments and revisions generally result in the issuance of a Letter of Map Change (LOMC), and revisions may also result in a physical map revision. Letters of Map Change originated when the production of FIRMs was an expensive photographic-based process, and it was less expensive to issue a letter than to publish a new version of an affected map panel. Applications for LOMCs are approved if computer models and ground surveys technically demonstrate that the ground surface (and the lowest floor elevation, depending on the type of LOMC) is a tenth of a foot above the established BFE, even though current mapping methodologies are not that accurate. Approved LOMCs are used with the associated FIRMs for floodplain regulation and insurance purposes.
Despite ongoing changes in the floodplain, FEMA flood maps are not updated on a regular schedule. Requests for changes are made irregularly and physical map revisions are infrequent due to funding constraints. Priorities must be set, and FEMA developed the Mapping Needs Assessment Process and the Map Needs Update Support System (MNUSS) to document and rank map update needs nationally. However, even high-priority updates (e.g., areas with known unmapped flood hazards, communities that are undergoing rapid growth or that can contribute to the map update) may not be made. Moreover, the time lag between approving and publishing LOMCs and physical map revisions lengthened when FEMA directed funds from map maintenance to digital conversion of paper maps during the Map Modernization Program. As a result, some parcels and structures may not be regulated or insured properly, even though the change in risk is known.
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FLOOD MAP INFORMATION TECHNOLOGY
In the early days of the NFIP, data were published and revised in the form of paper maps, Flood Insurance Study reports, and Letters of Map Change—a costly,5 inefficient, and time-consuming process. Initial steps toward a less paper intensive process led to the creation of FEMA’s Map Service Center website in the late 1990s and the development of new mapping products. Through this website, users can extract images from a full-sized paper map to create FIRMettes (e.g., Figure 2.1) that are legally equivalent to the original paper product. The recent availability of LOMCs and Flood Insurance Study reports online has made data even more accessible. Yet although more products are available and distribution has improved, digital updating processes have lagged.
FEMA created the Mapping Information Platform (MIP)6 on a secure website to allow its mapping partners (e.g., communities, engineers, surveyors, flood control districts, Cooperating Technical Partners) to submit data for review and share work responsibilities. With this system, map information (e.g., flood study data, LOMCs) is being shared, rather than the maps themselves. This system of information sharing shows what might be possible for map updates, which are often slow to be integrated with other map information.
Recommendation. FEMA should ensure that new flood information, revisions, and Letters of Map Change are incorporated into the digital Flood Insurance Rate Maps as soon as they become effective.
The digital environment could also facilitate communication of metadata—information about how flood data were generated. A variety of study methods are often used along a stream reach or coastline. For example, different segments of the same stream flowing through two adjacent communities may have been studied using different techniques and in different years. This distinction was commonly lost when the information was consolidated in the Map Modernization Program. Documenting how each mile was studied—including what input data, mapping, and modeling methods were used, the date of mapping, the contractor, and the starting and ending points of each study segment—would help users better understand the reliability and accuracy of the data. Many of these metadata are not currently included in Flood Insurance Study reports, particularly to this level of detail. However, metadata can easily be linked with digital flood map information, enabling users to examine data age, gathering, and analysis techniques to decide whether the flood data are suitable for the intended use. This is especially important, given that FEMA flood data are increasingly being used for land use planning, emergency response, and risk assessment, in addition to the insurance and regulatory purposes for which they were collected.
Recommendation. FEMA should require that every flood study be accompanied by detailed metadata identifying how each stream and coastline reach was studied and what methods were used to identify the magnitude and extent of the flood hazard and to produce the map.
FLOOD DATA AND A NATIONAL HYDROLOGIC INFORMATION SYSTEM
The FEMA Map Modernization Program is by far the largest investment that the nation has made in hydrologic information in recent years. It is also the largest effort that the nation has ever made to digitally describe the morphology of its streams and rivers. This investment could have many benefits beyond flood mapping. The flood models could be used for flood management and planning studies or for building real-time flood inundation mapping systems. The digital terrain and stream channel information could be used for water quality studies of contaminant transport in streams. FEMA is one of several federal agencies generating spatial hydrologic information and it is reasonable to ask how the data and models compiled during the Map Modernization Program could be made part of a National Hydrologic Information System.
Each of FEMA’s flood studies covers a geographic region, often a county. Within that region, each stream reach is considered a separate entity with its own flood
5
FEMA distributes more than 1 million paper maps each year, and the average cost of producing maps for a typical county is $250,000 to $500,000. Presentation to the committee by Paul Rooney, FEMA, on August 20, 2007.
6
See <https://hazards.fema.gov/femaportal/wps/portal>.
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discharge estimate, stream cross sections, and BFE. The floodplain boundaries of individual reaches are merged to delineate the Special Flood Hazard Area on a map panel. The digital information describing a single flood study is stored in hundreds or even thousands of files, which must be compiled for each county mapped in the nation. A key purpose of FEMA’s MIP is to store these files so that they will be available for later retrieval. Two types of files are involved: the files that comprise the flood map (DFIRMs) and files of raw field data analyzed in engineering studies to define the BFE (Data Capture Standard database; FEMA, 2003, Appendix L).
Walker and Maidment (2006) examined the design of a geodatabase model to store flood map information. They showed that the most critical parts of the data capture standards are the stream centerlines and cross sections used in the flood hydraulics model. If accurate geographic information system (GIS) files of these are maintained along with the flood hydraulics model, the model could be georeferenced and used in subsequent applications. This involves preserving data defining the connection between two coordinate systems: the Cartesian (x, y, z) coordinate system used to record the meandering of the channel through the landscape and the (s, n, z) coordinate system used in the river hydraulics model, in which s represents stationing distance along the river and n represents the distance across a particular cross section in the river. In effect, the hydraulic model “straightens” the channel by ignoring the bends and considering only how far along and transverse to the stream centerline the water flows. Unless both sets of coordinates are stored in the archived map and model information, it will be difficult or impossible at a later date to place a hydraulic model cross section at the correct map location along the stream.
One limitation of FEMA studies is that they are done county by county and there is no requirement that the underlying streamlines match across county boundaries. This difficulty can be overcome if FEMA streamline data are matched with those of the U.S. Geological Survey (USGS) National Hydrography Dataset (NHD).7 The NHD is a seamless, digital representation of streams and water bodies at map scales of 1:24,000 and 1:100,000 in the continental United States.8 Walker and Maidment (2006) showed that for Fayette County, Texas, the 1:24,000 NHD streamlines cover all the streams mapped in the Map Modernization Program, and that each FEMA-mapped stream segment could be located in a corresponding position on the NHD. Thus, the flood study data collected by FEMA could be linked to and become a part of the nation’s larger repository of hydrologic information, enabling it to be used for much more than flood mapping.
Recommendation. FEMA should reference all stream and coastal studies within its Mapping Information Platform to the USGS National Hydrography Dataset.
7
Presentation to the committee by Sally McConkey, Association of State Floodplain Managers, on November 8, 2007.
8
See <http://nhd.usgs.gov/>.
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