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Mapping the Zone: Improving Flood Map Accuracy (2009)

Chapter: 7 Mapping and Risk Communication: Moving to the Future

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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Suggested Citation:"7 Mapping and Risk Communication: Moving to the Future." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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7 Mapping and Risk Communication: Moving to the Future T he Federal Emergency Management Agency suspected to be subject to flood damage have higher (FEMA) has announced that in the next phase premiums. Zone designations have evolved over the of its efforts to update and improve national years as more is learned about how the built environ- flood hazard mapping it intends to “combine flood ment responds to flooding. For example, V (velocity) hazard mapping, risk assessment tools, and mitigation zones were added to coastal flood maps beginning in planning into one seamless program . . . to encourage 1976 to account for the probability of damage in areas beneficial partnerships and innovative uses of flood affected by waves and erosion. A case can be made hazard and risk assessment data in order to maximize to further refine coastal flood insurance rate zones, flood loss reduction.” FEMA envisions carrying enabling a more accurate representation of coastal flood out this RiskMap strategy by continuing to focus on hazards. improving and maintaining flood hazard data and maps while “delivering quality products and services to the Coastal A Zone right audience, using the right methods, at the right time,” and by increasing local mitigation actions to Two flood insurance rate zones apply to coastal ultimately reduce losses of life and property. The com- areas: (1) V zones along the water’s edge, which are mittee believes that FEMA can achieve its objectives by subject to damage from both inundation and wave modifying existing programs to improve the accuracy of heights greater than 3 feet; and (2) A zones further flood data and maps, as outlined in Chapters 3 through inland, which are subject to damage from inundation 5, and by making a leap forward in communicating and waves of less than 3 feet (Figure 5.1). At some dis- hazard and risk information. This will require FEMA tance inland, the waves dissipate and damage is caused to both improve the quality of existing flood maps and by inundation alone. move on a new path of risk assessment and information Historically, waves in the A zone have been assumed dissemination. This chapter describes improvements to be nondamaging. This assumption was challenged that can be made to FEMA flood maps to improve by a study of flood insurance claims from Hurricane flood risk communication. Opal in 1995, which found that losses in some coastal A zones were more consistent with losses expected in IMPROVING COASTAL FLOODING V zones (EQE, 2000; Jones et al., 2001). The threshold DESIGNATIONS for wave damage to buildings used to define the bound- ary between A and V zones is a 3-foot breaking wave, Flood insurance rate zones are a primary way to which was recommended by the U.S. Army Corps of communicate flood hazard because areas known or Engineers (USACE) in 1975 (USACE, 1975). How- ever, more recent tests suggest that building damage <http://www.fema.gov/plan/ffmm.shtm#1>. is likely from lower (1 to 2 feet) breaking waves (e.g., 89

90 MAPPING THE ZONE Tung et al., 2000). A number of reports have since rec- the coastal erosion problem.  In 2000, the Heinz ommended applying V-zone construction standards to Center recommended that “Congress should instruct the coastal A zone, defined as subject to breaking waves the Federal Emergency Management Agency to between 1.5 and 3 feet (ASCE, 2005a, 2005b; FEMA, develop erosion hazard maps that display the location 2005a, 2006c, 2006d; Wetmore et al., 2006). and extent of coastal areas subject to erosion” (Heinz The insurance losses in the coastal A zone follow- Center, 2000). To date, Congress has taken no action ing Hurricane Opal and recommendations to apply V on this recommendation and FEMA has not moved zone construction standards suggest that the current on its own. zone boundaries do not adequately capture true coastal A coastal E zone would be a special area within flood risk. Possible solutions include the following: the V zone, and its seaward side would define the area where significant flood-related and long-term 1. Lower the V zone boundary definition to a beach and dune erosion is expected to occur. This 1.5-foot breaking wave, which would expand V zone area is partially identified in the course of FEMA’s insurance rates and construction standards across the modeling procedures but is not currently drawn on coastal A zone. the resulting coastal flood maps. Long-term erosion 2. Retain the breaking wave threshold of 3 feet is measured by state or federal government agencies, in the V zone and formally define the coastal A zone but is not factored into flood maps, even when erosion as areas subject to breaking waves between 1.5 and rates are high compared to the lifetime of buildings. 3 feet. For example, the average rate of oceanfront erosion in North ­Carolina has been about 2 to 3 feet per year over FEMA is exploring both options. The first maps the last 50 years. to include the extent of 1.5-foot waves were released Flood-related and long-term erosion increases in preliminary form for three coastal Mississippi wave heights, so buildings in erosion zones need deeper counties in 2007. The boundary, called the “limit of and higher foundations than buildings outside erosion moderate wave action delineation,” is not labeled a zones. However, current standards call for founda- zone because it has no regulatory or insurance func- tions to extend to a minimum depth of −10 feet North tion, but simply provides guidance for reconstruction. American Vertical Datum 1988 (NAVD 88) for the Although this approach improves the portrayal of entire V zone (ASCE, 2005b). As a result, foundations flood hazard in coastal A zones, it would not change may be overdesigned (and more costly than necessary) construction standards and thus would not lower the in areas of low erosion and potentially underdesigned in risk of damage. areas of high erosion. This problem is likely to become more acute with climate change, which is expected Recommendation. FEMA should redefine the V zone to lead to sea level rise and more frequent or intense boundary based on a 1.5-foot breaking wave rather storms and thus to increase coastal erosion (IPCC, than the present 3-foot wave. 2007). Similarly, insurance premiums are uniform throughout the V zone, but studies have shown that Coastal E Zone flood damage is greater in areas subject to both erosion and waves than areas further inland that are subject to The National Flood Insurance Program has the waves alone (Rogers, 1990; USACE, 2005). Mapping authority to identify erosion (E) zones in coastal and an E zone could yield more actuarially realistic flood riverine environments but has not acted on it. A 1990 National Research Council (NRC) report recom- mended mapping coastal E zones to more accurately Congressional Record, National Flood Insurance Reform Act reflect the hazards of storm-induced and long-term of 1994, House of Representatives, May 3, 1994; Congressional erosion (NRC, 1990). Following debate in the House Record, Community Development Banking and Financial Institu- and Senate in 1994, Congress declined to approve tions Act of 1993, Senate, March 17, 1994.  Based on data from <http://dcm2.enr.state.nc.us/Maps/ FEMA erosion mapping and directed FEMA to study ER_1998/SB_Factor.htm>.

MAPPING AND RISK COMMUNICATION 91 insurance rates because the hazards are represented needed for the National Flood Insurance Program for more accurately. some time, the communication of flood risk to better inform the public and support an effective mitigation Recommendation. FEMA should begin mapping program will require FEMA to shift its risk mapping E zones to better serve insurance and floodplain communication focus to a higher and more tech­nical m ­ anagement needs. level. Geographic Information System (GIS) and database technologies and the widespread availability MAPPING FLOOD RISK of the Internet offer opportunities to leverage the Map Modernization investment to effectively communicate Risk is defined as the product of the probability risk through improved maps and websites. Tools such of an event and the consequences of its occurrence as Hazards U.S. (HAZUS) provide communities, (Einstein, 1988). For there to be a risk, there must be a private companies, and others with an understanding hazard consisting of an initiator event, a receptor, and of GIS the opportunity to learn more about the risks a pathway linking the two. For example, in the event of they face. heavy rainfall (the initiator), floodwater may propagate across the floodplain (the pathway) and inundate hous- Hazard and Risk Maps ing (the receptor) that may suffer material damage (the consequence). If the consequences of an event can be Considerable effort is underway in the United mitigated by some intervening measure (e.g., presence States and abroad to take advantage of new mapping of a levee, floodwall, or other structure), the probability capabilities to portray up-to-date information that that the intervening measure will function as designed floodplain occupants need and will use. Maps can must be factored into the risk equation. integrate information about the flood hazard with Hazard and risk maps are essential tools for helping information about the economic, social, or environ- the public understand the challenge it faces by living in mental consequences of flooding. In 2008, the Euro- a flood hazard area. They can also help communicate pean Commission published an atlas of flood maps the inundation risks associated with global warming that provides examples of the best mapping techniques and sea level rise. However, although much has been used in 19 European countries, the United States, and written on risk communication in general, little formal Japan. The atlas contains examples of maps designed research has been done in the United States on effective to support risk communication, land use planning, ways to use maps to communicate flood risk to those emergency notification and response, insurance rating, in the floodplain. What studies exist indicate contin- and historical analysis. The maps reflect involvement ued problems of low market penetration (Dixon et al., at the national, regional, and local levels and public- 2006) and communication associated with FEMA’s private partnerships. flood hazard maps (e.g., the annual chance terminology The Czech government, in cooperation with is still not commonly used by government officials, the Swiss Re, an international insurance company, and media, or the public; Galloway et al., 2006) and the MMC, a European GIS company, has developed the potential benefits of risk mapping (IPET, 2008). Flood Risk Assessment Tool, an interactive system A hazard map shows the location and probability that identifies up to six different risk zones within the of a hazard. FEMA’s paper and digital Flood Insur- floodplain. The tool is similar to FloodSmart prepared ance Rate Maps (FIRMs) are hazard maps because under FEMA’s Map Modernization Program, but has a they show floodplain boundaries that indicate different higher level of discrimination. Users are able to enter a flooding probabilities (i.e., 1 percent and 0.2 percent database and extract information about an area of inter- annual chance floods). A risk map not only shows est. Figure 7.1 illustrates a map with four risk zones. the hazard probability, but also includes the prob- The German state of Rheinland-Pfalz has devel- ability that protection systems (e.g., levees, dams) will oped maps for the Mosel River Basin that portray operate properly and the consequences of failure of <http://ec.europa.eu/environment/water/flood_risk/flood_atlas/ the system for a given event. While DFIRMs will be index.htm>.

92 MAPPING THE ZONE Description of the zones: Zone 1 – out of probable max. flood Zone 2 – up to possible max. flood Zone 3 – up to average 50 years flood Zone 4 – up to average 20 years flood FIGURE 7.1  Czech flood insurance rate map, in the area of Roudna. Four zones are designated to communicate risk, from safe zones Figure 7-1 cropped.eps (white), which are outside areas of probable maximum flooding, to high-risk zones (green hatched), which are subject to inundation from an average 20-year flood (a flood that has a 5 percentw/ someoccurring in any given year). SOURCE: Swiss Re. All rights bitmap image chance of vector type reserved. Used with permission. See also, <http://ec.europa.eu/environment/water/flood_risk/flood_atlas/index.htm>. p ­ ossible danger zones. The degree of hazard is expressed Where developed, the flood inundation maps can by the “intensity” of a flood event, as measured by the be used by local emergency managers and other decision relationship between water depth and flow velocity. In makers to plan for disasters and guide actions during Figure 7.2, red represents substantial hazard to persons, floods. An interactive website enables users to choose animals, and property; orange represents moderate which features to show on the inundation map, including hazard; and yellow represents minor hazard. water depth, FEMA 100-year and 500-year floodplain Flood maps may be used to illustrate the impact boundaries and floodways, and roads. Potential impacts of flooding on future land development. For example, are identified for different depths of inundation. For the European Space Agency shows the relationship example, at a flood stage of 23 feet (moderate to major between flooding and land use descriptively by overlay- flooding) in the Goldsboro, North Carolina, area, the ing the extent of historical flooding on planned urban strobe lights beyond the end of the runway at Seymour development (Figure 7.3). Johnson Air Force Base are flooded (Figure 7.4). Work on improving flood risk communication As part of the examination of post-Hurricane through maps has also been taking place in the United Katrina hazards, the USACE has recently published States. The National Oceanic and Atmospheric Admin- risk maps showing the depths of inundation for differ- istration’s (NOAA’s) Advanced Hydrologic Prediction ent flood scenarios in the New Orleans area (Figure 7.5). Service has developed prototype maps of the observed Because the maps were developed incorporating the and/or forecast water level and depths of inundation at probability of a flood event, the probability that the or near a stream gage, using National Weather Service flood protection system (e.g., levees, floodwalls) will forecasts, models and map inundation libraries pro- perform as designed, the probabilities of overtopping duced by states, and U.S. Geological Survey (USGS) or failure of the structures, and the consequences of stream gages. The maps display anticipated water levels the flood event (inundation), these products are true extending from flood stage through record or major risk maps rather than hazard maps. Other USACE flooding, whichever is greater. Modeling and accuracy risk maps have been prepared displaying economic constraints (e.g., in floodwater elevation, terrain data) consequences and loss of life. limit coverage to river reaches within a mile and a half Flood maps may also be used to portray chang- of an existing stream gage. ing situations. For example, the government of France

MAPPING AND RISK COMMUNICATION 93 FIGURE 7.2  Danger zones along the Mosel River, Germany, showing substantial flood hazard (red), moderate hazard (orange), and Figure 7-2 (new).eps minor hazard (yellow). SOURCE: <http://www.gefahrenatlas-mosel.de/>, EXCIMAP Atlas of Flood Maps. Used with permission. bitmap image

94 MAPPING THE ZONE FIGURE 7.3  Flood risk map of a section of the Moselle River, near the village of Cattenom, France. Hatched blue indicates areas that are historically floodprone. The gray shows Figure 7-3the SERTIT.eps 1960s, and red shows subsequent urban and the extent of - urban area in the bitmap image industrial development. The map is superimposed on a false-color composite SPOT image, showing vegetation (green) and bare soil (pink). Maps such as this make it easier to decide where to build structures or flood control measures. SOURCE: Processed by SERTIT, <http://www.eomd.esa.int/booklets/booklet172.asp>. Used with permission. has taken steps to make maps with real-time weather represent the only near nationwide coverage, albeit information available to the public. Using a new flood limited, of flooding and, given the significant federal warning system (Adaptation of Geographical Infor- investment in the flood map program, provide a logical mation for Flood Warning [AIGA]), Cemagref and base for extensions into new areas. Still to be resolved is Météo France monitor real-time flows and streamflow how to incorporate uncertainty into mapping products. changes on selected rivers in the Mediterranean region Work on visualizing uncertainty of geospatial data is of France, and provide maps of the risk connected beginning to be done (e.g., MacEachren et al., 2005), with rainfall and runoff. The level of risk is portrayed but a consensus does not yet exist. by colors, with red indicating “a disaster event that is likely to isolate and endanger a large number of homes,” Finding. FEMA’s transition to digital flood mapping orange indicating that a large number of roads are going during the Map Modernization Program creates to be cut off and movement by road will be difficult opportunities to develop a variety of hazard and risk and dangerous, and yellow indicating that “property maps. damage is likely and that the highest level of caution is recommended” (Figure 7.6). AIGA can also provide Finding. Combining the appropriate attributes of information on hydrologic risk, such as the nature of FEMA DFIRMs with attributes of NOAA inundation the flows. maps, USACE risk maps, and the innovative mapping The above maps illustrate the wide variety of excit- techniques developed by state and local entities and ing products that offer significant improvements in the other countries would significantly enhance the com- ability to communicate risk to those in the floodplain. munication of flood risk information to those who live In the United States, FEMA’s paper and digital FIRMs in floodplains or manage floodplain development.

MAPPING AND RISK COMMUNICATION 95 FIGURE 7.4  NOAA flood inundation map of a segment7-4.eps Figure of the Neuse River near Goldsboro, N.C., showing the extent of flooding when water levels are forecast to rise to a stage of 23 feet (blue) and the location of the 1 percent annual chance floodplain (blue bitmap image green) from a FEMA map. The darker the blue, the greater is the depth of inundation. The water depth is 0 to 2 feet near the edge of the Seymour Johnson Air Force Base runway (red arrow). The green circle shows the USGS stream gage where the National Weather Service provides the river forecast. The topographic data, digital elevation models, and hydraulic models underlying the map were produced by the USGS office in Raleigh and the North Carolina Floodplain Mapping Program. SOURCE: <http://www.weather. gov/ahps/inundation.php>.

96 MAPPING THE ZONE FIGURE 7.5  Flood risk maps for New Orleans. Water surface elevations are mean values, with a sensitivity of ±2 feet. The Figure 7-5.eps maps assume 50 percent pumping capacity. SOURCE: USACE, New Orleans District, <http://www.mvn.usace.army.mil/hps2/ hps_risk_depth_map.asp>. bitmap image FIGURE 7.6  EOS-AIGA map of flood risk around Nimes, France. SOURCE: Météo France, Institut Géographique National. Used ����������������������������������������������������������� ����� with permission. Figure 7-6.eps bitmap image

MAPPING AND RISK COMMUNICATION 97 Elevation and Risk of 2.3 feet or more during the next 30 years.” For this to work, elevation information for individual struc- On many flood maps, the likelihood of flooding is tures, base flood elevations for the floodplain area, and based on location relative to the horizontal extent of information about the probability that any protection the floodplain. However, using floodplain boundaries structures will perform as designed must be kept up suggests that every building inside the Special Flood to date and accessible via the web. The probability of Hazard Area (SFHA) may flood and that every build- system failure would also be computed, and a personal- ing outside is safe. In fact, there is no magic boundary ized, quantified risk of flooding could then be provided that separates those subject to flooding from those to individuals. not at risk; one-third of flood insurance claims are for areas outside the SFHA. Moreover, risk within the HAZUS floodplain is not uniform because of variations in the elevation of land and structures. At its FloodSmart.gov A critical component of the risk equation is deter- website, FEMA provides a tool that enables individuals mination of the consequences of flooding, including to type in an address and see whether the property is at which buildings are likely to be damaged by floods of low, moderate, or high risk of flooding. The assessment different magnitudes and the extent of the damage. To is based on the location of the property relative to the standardize estimates of potential losses from natural 1 percent and 0.2 percent annual chance floodplain hazards including floods, FEMA developed and is con- boundaries on digital FIRMs. tinuously improving the Hazards U.S. Multi-Hazards The elevation of structures relative to the expected (HAZUS-MH) software. The GIS software facilitates height of floodwaters offers a finer discrimination of loss estimation from floods by integrating spatial analy- risk. Some countries are beginning to use elevation sis, database management tools, and a suite of hazard, to communicate risk. For example, a website in the damage, and loss estimation modules (Figure 7.7). N ­ etherlands enables users to identify ground level rela- The flood module addresses both coastal and riverine tive to mean sea level by entering a postal code. The flooding and can be operated at three different levels of elevation difference provides a sense of potential flood increasing complexity and detail (Table 7.1). In addition risk in the event of a dike failure. In the United States, building elevation information tied to latitude, longi- tude, and street addresses is available from Elevation Certificates, although these are not yet electronically accessible (see Chapter 3, “Surveying Structure Eleva- tions”) and Elevation Certificates are not available for every structure in and near the floodplain. Similarly, base flood elevations and system performance informa- tion are not available for all floodplains. However, the GIS technology needed to provide an individualized risk assessment based on system performance and the difference between the lowest floor elevation and the base flood elevation does exist. If complete risk infor- mation were available, individuals would be able to enter an address on the web, click on “flood risk,” and see something like: “The building at 123 Main Street has a 26 percent chance of being flooded to a depth  <http://www.floodsmart.gov/floodsmart/pages/flood_facts. jsp>. <http://www.floodsmart.gov/>. FIGURE 7.7  Components of FEMA’s HAZUS-MH flood module. <www.ahn.nl/hoogtetool>. SOURCE: FEMA E13 Basic HAZUS course material, 2008. Figure 7-7.eps bitmap image

98 MAPPING THE ZONE TABLE 7.1  Flood Hazard Module Use Levels in FEMA’s HAZUS-MH Estimates of Flood Hazard Inland Coastal Loss Estimates HAZUS Level Base Elevation Hydrology Hydraulics Wave Model Inventory Damage Function 1. Default databases Any available USGS regression Default resistance Default 1-D wave Census track data Default damage NED equation model curves 2. User-modified User supplied User-supplied Qp Default resistance Default 1-D wave Modify inventory Modify parameters data at river reach equation model 3. Expert-supplied User supplied Hydrologic model Hydraulic model Modify wave Detailed building Community-based data output at reaches output (predefined parameters or facility types damage functions BFE surface grid) NOTES: 1-D = one-dimensional; NED = National Elevation Dataset. to simple hydrologic, hydraulic, and wave models, which topographic data. These maps represent an improve- are suitable only for preliminary analyses, HAZUS-MH ment in the quality of flood hazard information allows the user to supply model output, building inven- p ­ rovided to the public. Where paper maps have merely tory data, and localized building and facility level been converted to digital representations, the value damage curves. The higher levels of HAZUS require added has been minimal, and these maps will have to be disciplinary expertise, as well as significant expertise in updated to communicate flood hazard more accurately. database management and operations. This task must be accomplished to fully meet the objec- HAZUS is used by federal, state, and local govern- tives of the FEMA Map Modernization Program. ments to estimate potential flood damage. For example, New technologies offer FEMA the opportunity it formed the basis for damage information developed to vastly improve the accuracy and thus the utility as part of the risk and reliability sections of the recently of digital maps. Current procedures for producing completed Interagency Performance Evaluation Task riverine and coastal maps can be improved, and these Force (IPET, 2008) report on risks in the New Orleans improvements are economically and socially justified. area following Hurricane Katrina. The availability of Improving the accuracy of flood maps by using higher- HAZUS, combined with information already ­gathered quality topographic data as well as updated hydrology as part of floodplain mapping, places FEMA’s flood- and hydraulics enables communities to more accurately plain mapping program in a position to develop effec- portray flood hazard and mitigate the risk to existing tive hazard-consequence flood maps. structures. Coastal flood mapping has revealed hazards beyond simply inundation—buildings can be damaged Finding. The mapped location of buildings inside or by wave action and by erosion of their foundations. outside an SFHA does not adequately convey a sense Refining current coastal flood zone definitions to cor- of flood hazard. Flood risk can be assessed and com- respond more closely to actual flood damage during municated more effectively in terms of the relative coastal flood events could lead to more accurate and elevations of the structures and facilities in the flood consistent insurance ratings and thus to a better sense hazard area. of flood hazard. FEMA’s RiskMap goals open the door to the pos- CONCLUSIONS sibility of significantly improving the communication of risk to those in the most hazardous areas as well as The principal product created by FEMA’s Map those responsible for mitigating the risk. New tech- Modernization Program is digital flood maps to replace nologies will enable FEMA to portray information paper flood maps. In some cases, this conversion was about the flood hazard and flood risk through multiple made using updated or new hydraulic, hydrologic, and means and to tailor the information to meet the specific

MAPPING AND RISK COMMUNICATION 99 needs of government, business, and the public at large. Recommendation. FEMA should commission a The variety of map products that can be generated and study on technology and metrics to analyze and com- the availability of web tools to provide personalized municate flood risk. information to floodplain occupants will enable them to make decisions that ultimately will reduce national risk in the floodplain.

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Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps portray the height and extent to which flooding is expected to occur, and they form the basis for setting flood insurance premiums and regulating development in the floodplain. As such, they are an important tool for individuals, businesses, communities, and government agencies to understand and deal with flood hazard and flood risk. Improving map accuracy is therefore not an academic question--better maps help everyone.

Making and maintaining an accurate flood map is neither simple nor inexpensive. Even after an investment of more than $1 billion to take flood maps into the digital world, only 21 percent of the population has maps that meet or exceed national flood hazard data quality thresholds. Even when floodplains are mapped with high accuracy, land development and natural changes to the landscape or hydrologic systems create the need for continuous map maintenance and updates.

Mapping the Zone examines the factors that affect flood map accuracy, assesses the benefits and costs of more accurate flood maps, and recommends ways to improve flood mapping, communication, and management of flood-related data.

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