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Mapping the Zone: Improving Flood Map Accuracy
7
Mapping and Risk Communication: Moving to the Future
The Federal Emergency Management Agency (FEMA) has announced that in the next phase of its efforts to update and improve national flood hazard mapping it intends to “combine flood hazard mapping, risk assessment tools, and mitigation planning into one seamless program … to encourage beneficial partnerships and innovative uses of flood hazard and risk assessment data in order to maximize flood loss reduction.”1 FEMA envisions carrying out this RiskMap strategy by continuing to focus on improving and maintaining flood hazard data and maps while “delivering quality products and services to the right audience, using the right methods, at the right time,” and by increasing local mitigation actions to ultimately reduce losses of life and property. The committee believes that FEMA can achieve its objectives by modifying existing programs to improve the accuracy of flood data and maps, as outlined in Chapters 3 through 5, and by making a leap forward in communicating hazard and risk information. This will require FEMA to both improve the quality of existing flood maps and move on a new path of risk assessment and information dissemination. This chapter describes improvements that can be made to FEMA flood maps to improve flood risk communication.
IMPROVING COASTAL FLOODING DESIGNATIONS
Flood insurance rate zones are a primary way to communicate flood hazard because areas known or suspected to be subject to flood damage have higher premiums. Zone designations have evolved over the years as more is learned about how the built environment responds to flooding. For example, V (velocity) zones were added to coastal flood maps beginning in 1976 to account for the probability of damage in areas affected by waves and erosion. A case can be made to further refine coastal flood insurance rate zones, enabling a more accurate representation of coastal flood hazards.
Coastal A Zone
Two flood insurance rate zones apply to coastal areas: (1) V zones along the water’s edge, which are subject to damage from both inundation and wave heights greater than 3 feet; and (2) A zones further inland, which are subject to damage from inundation and waves of less than 3 feet (Figure 5.1). At some distance inland, the waves dissipate and damage is caused by inundation alone.
Historically, waves in the A zone have been assumed to be nondamaging. This assumption was challenged by a study of flood insurance claims from Hurricane Opal in 1995, which found that losses in some coastal A zones were more consistent with losses expected in V zones (EQE, 2000; Jones et al., 2001). The threshold for wave damage to buildings used to define the boundary between A and V zones is a 3-foot breaking wave, which was recommended by the U.S. Army Corps of Engineers (USACE) in 1975 (USACE, 1975). However, more recent tests suggest that building damage is likely from lower (1 to 2 feet) breaking waves (e.g.,
1
<http://www.fema.gov/plan/ffmm.shtm#1>.
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Tung et al., 2000). A number of reports have since recommended applying V-zone construction standards to the coastal A zone, defined as subject to breaking waves between 1.5 and 3 feet (ASCE, 2005a, 2005b; FEMA, 2005a, 2006c, 2006d; Wetmore et al., 2006).
The insurance losses in the coastal A zone following Hurricane Opal and recommendations to apply V zone construction standards suggest that the current zone boundaries do not adequately capture true coastal flood risk. Possible solutions include the following:
Lower the V zone boundary definition to a 1.5-foot breaking wave, which would expand V zone insurance rates and construction standards across the coastal A zone.
Retain the breaking wave threshold of 3 feet in the V zone and formally define the coastal A zone as areas subject to breaking waves between 1.5 and 3 feet.
FEMA is exploring both options. The first maps to include the extent of 1.5-foot waves were released in preliminary form for three coastal Mississippi counties in 2007. The boundary, called the “limit of moderate wave action delineation,” is not labeled a zone because it has no regulatory or insurance function, but simply provides guidance for reconstruction. Although this approach improves the portrayal of flood hazard in coastal A zones, it would not change construction standards and thus would not lower the risk of damage.
Recommendation. FEMA should redefine the V zone boundary based on a 1.5-foot breaking wave rather than the present 3-foot wave.
Coastal E Zone
The National Flood Insurance Program has the authority to identify erosion (E) zones in coastal and riverine environments but has not acted on it. A 1990 National Research Council (NRC) report recommended mapping coastal E zones to more accurately reflect the hazards of storm-induced and long-term erosion (NRC, 1990). Following debate in the House and Senate in 1994, Congress declined to approve FEMA erosion mapping and directed FEMA to study the coastal erosion problem.2 In 2000, the Heinz Center recommended that “Congress should instruct the Federal Emergency Management Agency to develop erosion hazard maps that display the location and extent of coastal areas subject to erosion” (Heinz Center, 2000). To date, Congress has taken no action on this recommendation and FEMA has not moved on its own.
A coastal E zone would be a special area within the V zone, and its seaward side would define the area where significant flood-related and long-term beach and dune erosion is expected to occur. This area is partially identified in the course of FEMA’s modeling procedures but is not currently drawn on the resulting coastal flood maps. Long-term erosion is measured by state or federal government agencies, but is not factored into flood maps, even when erosion rates are high compared to the lifetime of buildings. For example, the average rate of oceanfront erosion in North Carolina has been about 2 to 3 feet per year over the last 50 years.3
Flood-related and long-term erosion increases wave heights, so buildings in erosion zones need deeper and higher foundations than buildings outside erosion zones. However, current standards call for foundations to extend to a minimum depth of –10 feet orth American Vertical Datum 1988 (NAVD 88) for the entire V zone (ASCE, 2005b). As a result, foundations may be overdesigned (and more costly than necessary) in areas of low erosion and potentially underdesigned in areas of high erosion. This problem is likely to become more acute with climate change, which is expected to lead to sea level rise and more frequent or intense storms and thus to increase coastal erosion (IPCC, 2007). Similarly, insurance premiums are uniform throughout the V zone, but studies have shown that flood damage is greater in areas subject to both erosion and waves than areas further inland that are subject to waves alone (Rogers, 1990; USACE, 2005). Mapping an E zone could yield more actuarially realistic flood
2
Congressional Record, National Flood Insurance Reform Act of 1994, House of Representatives, May 3, 1994; Congressional Record, Community Development Banking and Financial Institutions Act of 1993, Senate, March 17, 1994.
3
Based on data from <http://dcm2.enr.state.nc.us/Maps/ER_1998/SB_Factor.htm>.
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insurance rates because the hazards are represented more accurately.
Recommendation. FEMA should begin mapping E zones to better serve insurance and floodplain management needs.
MAPPING FLOOD RISK
Risk is defined as the product of the probability of an event and the consequences of its occurrence (Einstein, 1988). For there to be a risk, there must be a hazard consisting of an initiator event, a receptor, and a pathway linking the two. For example, in the event of heavy rainfall (the initiator), floodwater may propagate across the floodplain (the pathway) and inundate housing (the receptor) that may suffer material damage (the consequence). If the consequences of an event can be mitigated by some intervening measure (e.g., presence of a levee, floodwall, or other structure), the probability that the intervening measure will function as designed must be factored into the risk equation.
Hazard and risk maps are essential tools for helping the public understand the challenge it faces by living in a flood hazard area. They can also help communicate the inundation risks associated with global warming and sea level rise. However, although much has been written on risk communication in general, little formal research has been done in the United States on effective ways to use maps to communicate flood risk to those in the floodplain. What studies exist indicate continued problems of low market penetration (Dixon et al., 2006) and communication associated with FEMA’s flood hazard maps (e.g., the annual chance terminology is still not commonly used by government officials, the media, or the public; Galloway et al., 2006) and the potential benefits of risk mapping (IPET, 2008).
A hazard map shows the location and probability of a hazard. FEMA’s paper and digital Flood Insurance Rate Maps (FIRMs) are hazard maps because they show floodplain boundaries that indicate different flooding probabilities (i.e., 1 percent and 0.2 percent annual chance floods). A risk map not only shows the hazard probability, but also includes the probability that protection systems (e.g., levees, dams) will operate properly and the consequences of failure of the system for a given event. While DFIRMs will be needed for the National Flood Insurance Program for some time, the communication of flood risk to better inform the public and support an effective mitigation program will require FEMA to shift its risk mapping communication focus to a higher and more technical level. Geographic Information System (GIS) and database technologies and the widespread availability of the Internet offer opportunities to leverage the Map Modernization investment to effectively communicate risk through improved maps and websites. Tools such as Hazards U.S. (HAZUS) provide communities, private companies, and others with an understanding of GIS the opportunity to learn more about the risks they face.
Hazard and Risk Maps
Considerable effort is underway in the United States and abroad to take advantage of new mapping capabilities to portray up-to-date information that floodplain occupants need and will use. Maps can integrate information about the flood hazard with information about the economic, social, or environmental consequences of flooding. In 2008, the European Commission published an atlas of flood maps that provides examples of the best mapping techniques used in 19 European countries, the United States, and Japan.4 The atlas contains examples of maps designed to support risk communication, land use planning, emergency notification and response, insurance rating, and historical analysis. The maps reflect involvement at the national, regional, and local levels and public-private partnerships.
The Czech government, in cooperation with Swiss Re, an international insurance company, and MMC, a European GIS company, has developed the Flood Risk Assessment Tool, an interactive system that identifies up to six different risk zones within the floodplain. The tool is similar to FloodSmart prepared under FEMA’s Map Modernization Program, but has a higher level of discrimination. Users are able to enter a database and extract information about an area of interest. Figure 7.1 illustrates a map with four risk zones.
The German state of Rheinland-Pfalz has developed maps for the Mosel River Basin that portray
4
<http://ec.europa.eu/environment/water/flood_risk/flood_atlas/index.htm>.
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FIGURE 7.1 Czech flood insurance rate map, in the area of Roudna. Four zones are designated to communicate risk, from safe zones (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 percent chance of occurring in any given year). SOURCE: Swiss Re. All rights reserved. Used with permission. See also, <http://ec.europa.eu/environment/water/flood_risk/flood_atlas/index.htm>.
possible danger zones. The degree of hazard is expressed by the “intensity” of a flood event, as measured by the relationship between water depth and flow velocity. In Figure 7.2, red represents substantial hazard to persons, animals, and property; orange represents moderate hazard; and yellow represents minor hazard.
Flood maps may be used to illustrate the impact of flooding on future land development. For example, the European Space Agency shows the relationship between flooding and land use descriptively by overlaying the extent of historical flooding on planned urban development (Figure 7.3).
Work on improving flood risk communication through maps has also been taking place in the United States. The National Oceanic and Atmospheric Administration’s (NOAA’s) Advanced Hydrologic Prediction Service has developed prototype maps of the observed and/or forecast water level and depths of inundation at or near a stream gage, using National Weather Service forecasts, models and map inundation libraries produced by states, and U.S. Geological Survey (USGS) stream gages. The maps display anticipated water levels extending from flood stage through record or major flooding, whichever is greater. Modeling and accuracy constraints (e.g., in floodwater elevation, terrain data) limit coverage to river reaches within a mile and a half of an existing stream gage.
Where developed, the flood inundation maps can be used by local emergency managers and other decision makers to plan for disasters and guide actions during floods. An interactive website enables users to choose which features to show on the inundation map, including water depth, FEMA 100-year and 500-year floodplain boundaries and floodways, and roads. Potential impacts are identified for different depths of inundation. For example, at a flood stage of 23 feet (moderate to major flooding) in the Goldsboro, North Carolina, area, the strobe lights beyond the end of the runway at Seymour Johnson Air Force Base are flooded (Figure 7.4).
As part of the examination of post-Hurricane Katrina hazards, the USACE has recently published risk maps showing the depths of inundation for different flood scenarios in the New Orleans area (Figure 7.5). Because the maps were developed incorporating the probability of a flood event, the probability that the flood protection system (e.g., levees, floodwalls) will perform as designed, the probabilities of overtopping or failure of the structures, and the consequences of the flood event (inundation), these products are true risk maps rather than hazard maps. Other USACE risk maps have been prepared displaying economic consequences and loss of life.
Flood maps may also be used to portray changing situations. For example, the government of France
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FIGURE 7.2 Danger zones along the Mosel River, Germany, showing substantial flood hazard (red), moderate hazard (orange), and minor hazard (yellow). SOURCE: <http://www.gefahrenatlas-mosel.de/>, EXCIMAP Atlas of Flood Maps. Used with permission.
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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 the extent of the urban area in the 1960s, and red shows subsequent urban and 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 information available to the public. Using a new flood warning system (Adaptation of Geographical Information for Flood Warning [AIGA]), Cemagref and Météo France monitor real-time flows and streamflow changes on selected rivers in the Mediterranean region of France, and provide maps of the risk connected with rainfall and runoff. The level of risk is portrayed by colors, with red indicating “a disaster event that is likely to isolate and endanger a large number of homes,” orange indicating that a large number of roads are going to be cut off and movement by road will be difficult and dangerous, and yellow indicating that “property damage is likely and that the highest level of caution is recommended” (Figure 7.6). AIGA can also provide information on hydrologic risk, such as the nature of the flows.
The above maps illustrate the wide variety of exciting products that offer significant improvements in the ability to communicate risk to those in the floodplain. In the United States, FEMA’s paper and digital FIRMs represent the only near nationwide coverage, albeit limited, of flooding and, given the significant federal investment in the flood map program, provide a logical base for extensions into new areas. Still to be resolved is how to incorporate uncertainty into mapping products. Work on visualizing uncertainty of geospatial data is beginning to be done (e.g., MacEachren et al., 2005), but a consensus does not yet exist.
Finding. FEMA’s transition to digital flood mapping during the Map Modernization Program creates opportunities to develop a variety of hazard and risk maps.
Finding. Combining the appropriate attributes of FEMADFIRMs with attributes of NOAA inundation maps, USACE risk maps, and the innovative mapping techniques developed by state and local entities and other countries would significantly enhance the communication of flood risk information to those who live in floodplains or manage floodplain development.
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FIGURE 7.4 NOAA flood inundation map of a segment 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 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>.
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FIGURE 7.5 Flood risk maps for New Orleans. Water surface elevations are mean values, with a sensitivity of ±2 feet. The maps assume 50 percent pumping capacity. SOURCE: USACE, New Orleans District, <http://www.mvn.usace.army.mil/hps2/hps_risk_depth_map.asp>.
FIGURE 7.6 EOS-AIGA map of flood risk around Nimes, France. SOURCE: Météo France, Institut Géographique National. Used with permission.
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Elevation and Risk
On many flood maps, the likelihood of flooding is based on location relative to the horizontal extent of the floodplain. However, using floodplain boundaries suggests that every building inside the Special Flood Hazard Area (SFHA) may flood and that every building outside is safe. In fact, there is no magic boundary that separates those subject to flooding from those not at risk; one-third of flood insurance claims are for areas outside the SFHA.5 Moreover, risk within the floodplain is not uniform because of variations in the elevation of land and structures. At its FloodSmart.gov website, FEMA provides a tool that enables individuals to type in an address and see whether the property is at low, moderate, or high risk of flooding. The assessment is based on the location of the property relative to the 1 percent and 0.2 percent annual chance floodplain boundaries on digital FIRMs.6
The elevation of structures relative to the expected height of floodwaters offers a finer discrimination of risk. Some countries are beginning to use elevation to communicate risk. For example, a website in the Netherlands enables users to identify ground level relative to mean sea level by entering a postal code.7 The elevation difference provides a sense of potential flood risk in the event of a dike failure. In the United States, building elevation information tied to latitude, longitude, and street addresses is available from Elevation Certificates, although these are not yet electronically accessible (see Chapter 3, “Surveying Structure Elevations”) and Elevation Certificates are not available for every structure in and near the floodplain. Similarly, base flood elevations and system performance information 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 information 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 of 2.3 feet or more during the next 30 years.” For this to work, elevation information for individual structures, base flood elevations for the floodplain area, and information about the probability that any protection structures will perform as designed must be kept up to date and accessible via the web. The probability of system failure would also be computed, and a personalized, quantified risk of flooding could then be provided to individuals.
HAZUS
A critical component of the risk equation is determination of the consequences of flooding, including which buildings are likely to be damaged by floods of different magnitudes and the extent of the damage. To standardize estimates of potential losses from natural hazards including floods, FEMA developed and is continuously improving the Hazards U.S. Multi-Hazards (HAZUS-MH) software. The GIS software facilitates loss estimation from floods by integrating spatial analysis, database management tools, and a suite of hazard, damage, and loss estimation modules (Figure 7.7). The flood module addresses both coastal and riverine flooding and can be operated at three different levels of increasing complexity and detail (Table 7.1). In addition
FIGURE 7.7 Components of FEMA’s HAZUS-MH flood module. SOURCE: FEMA E13 Basic HAZUS course material, 2008.
5
<http://www.floodsmart.gov/floodsmart/pages/flood_facts.jsp>.
6
<http://www.floodsmart.gov/>.
7
<www.ahn.nl/hoogtetool>.
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TABLE 7.1 Flood Hazard Module Use Levels in FEMA’s HAZUS-MH
HAZUS Level
Base Elevation
Estimates of Flood Hazard
Loss Estimates
Inland
Coastal
Hydrology
Hydraulics
Wave Model
Inventory
Damage Function
1. Default databases
Any available NED
USGS regression
Default resistance equation
Default 1-D wave model
Census track data
Default damage curves
2. User-modified data
User supplied
User-supplied Qp at river reach
Default resistance equation
Default 1-D wave model
Modify inventory
Modify parameters
3. Expert-supplied data
User supplied
Hydrologic model output at reaches
Hydraulic model output (predefined BFE surface grid)
Modify wave parameters
Detailed building or facility types
Community-based damage functions
NOTES: 1-D = one-dimensional; NED = National Elevation Dataset.
to simple hydrologic, hydraulic, and wave models, which are suitable only for preliminary analyses, HAZUS-MH allows the user to supply model output, building inventory data, and localized building and facility level damage curves. The higher levels of HAZUS require disciplinary expertise, as well as significant expertise in database management and operations.
HAZUS is used by federal, state, and local governments to estimate potential flood damage. For example, it formed the basis for damage information developed as part of the risk and reliability sections of the recently completed Interagency Performance Evaluation Task Force (IPET, 2008) report on risks in the New Orleans area following Hurricane Katrina. The availability of HAZUS, combined with information already gathered as part of floodplain mapping, places FEMA’s floodplain mapping program in a position to develop effective hazard-consequence flood maps.
Finding. The mapped location of buildings inside or outside an SFHA does not adequately convey a sense of flood hazard. Flood risk can be assessed and communicated more effectively in terms of the relative elevations of the structures and facilities in the flood hazard area.
CONCLUSIONS
The principal product created by FEMA’s Map Modernization Program is digital flood maps to replace paper flood maps. In some cases, this conversion was made using updated or new hydraulic, hydrologic, and topographic data. These maps represent an improvement in the quality of flood hazard information provided to the public. Where paper maps have merely been converted to digital representations, the value added has been minimal, and these maps will have to be updated to communicate flood hazard more accurately. This task must be accomplished to fully meet the objectives of the FEMA Map Modernization Program.
New technologies offer FEMA the opportunity to vastly improve the accuracy and thus the utility of digital maps. Current procedures for producing riverine and coastal maps can be improved, and these improvements are economically and socially justified. Improving the accuracy of flood maps by using higher-quality topographic data as well as updated hydrology and hydraulics enables communities to more accurately portray flood hazard and mitigate the risk to existing structures. Coastal flood mapping has revealed hazards beyond simply inundation—buildings can be damaged by wave action and by erosion of their foundations. Refining current coastal flood zone definitions to correspond more closely to actual flood damage during coastal flood events could lead to more accurate and consistent insurance ratings and thus to a better sense of flood hazard.
FEMA’s RiskMap goals open the door to the possibility of significantly improving the communication of risk to those in the most hazardous areas as well as those responsible for mitigating the risk. New technologies will enable FEMA to portray information about the flood hazard and flood risk through multiple means and to tailor the information to meet the specific
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needs of government, business, and the public at large. The variety of map products that can be generated and the availability of web tools to provide personalized information to floodplain occupants will enable them to make decisions that ultimately will reduce national risk in the floodplain.
Recommendation. FEMA should commission a study on technology and metrics to analyze and communicate flood risk.
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