Flooding is the natural hazard with the greatest economic and social impact in the United States, and these impacts are becoming more severe over time. Catastrophic flooding from recent hurricanes, including Superstorm Sandy in New York (2012) and Hurricane Harvey in Houston (2017), caused billions of dollars in property damage, adversely affected millions of people, and damaged the economic well-being of major metropolitan areas. Flooding takes a heavy toll even in years without a named storm or event. Major freshwater flood events from 2004 to 2014 cost an average of $9 billion in direct damage and 71 lives annually. These figures do not include the cumulative costs of frequent, small floods, which can be similar to those of infrequent extreme floods.
The Federal Emergency Management Agency (FEMA) helps communities with flood preparation, mitigation, response, and recovery. It carries out these functions by analyzing and mapping flood hazard, providing federal flood insurance, and disbursing grants and loans to individuals and businesses after presidentially declared flood disasters. FEMA’s flood hazard analysis and mapping focus on inundation from riverine and coastal flooding. Within cities, however, flood damage can occur anywhere, not just in floodplains along rivers and coasts. Impacts can be highly localized due to small-scale differences in topography, storm characteristics, storm water management infrastructure, and building design. In addition, with about 280 million people living in U.S. urban areas, the social and economic impacts of urban flooding can be particularly severe. With mounting costs, Congress and FEMA recognize that the causes and consequences of urban flooding need focused examination.
At the request of FEMA, the National Academies of Sciences, Engineering, and Medicine appointed a committee to hold workshops to gain an initial understanding of the causes and impacts of urban flooding in three to eight metropolitan areas, and to use that information to address three tasks:
- Identify any commonalities and variances among the case study metropolitan areas in terms of causes, adverse impacts, unexpected problems in recovery, or effective mitigation strategies, as well as key themes of urban flooding;
- Provide an estimate of the size or importance of flooding in those urban areas; and
- Relate, as appropriate, causes and actions of urban flooding to existing federal resources or policies, including but not limited to the National Flood Insurance Program, nondisaster grants, Stafford Act authorities, or others.
The objective was to contribute to existing knowledge by providing some real-world examples in specific places, based largely on regional workshops, and not to provide a comprehensive overview of urban flooding in the United States.
The workshop orientation ties directly to Task 1, but it posed problems for addressing Tasks 2 and 3. To estimate the magnitude of urban flooding (Task 2), the committee drew on published estimates of flood losses in the case study areas and also analyzed federal flood loss data. For Task 3—relating urban flooding causes and actions to federal resources—the committee focused on needs that were identified in the case studies or the committee’s loss calculations and that are strongly connected to federal resources and policies.
The committee’s definition of urban flooding and response to the three tasks are given below.
WHAT IS URBAN FLOODING?
Urban flooding is caused when the inflow of storm water in urban areas exceeds the capacity of drainage systems to infiltrate storm water into the soil or to carry it away. The inflow of storm water results from (a) heavy rainfall, which can collect on the landscape (pluvial flooding) or cause rivers and streams to overflow their banks and inundate surrounding areas; or (b) storm surge or high tides, which push water onto coastal cities. Floodwater inundation and movement are influenced by (a) land development, which disturbs natural drainage patterns and creates hardened surfaces that inhibit infiltration of storm water; and (b) storm water systems that are undersized for current needs and increase exposure to drainage hazards. In older cities, sewer systems carrying both storm water and wastewater can become surcharged during storms, causing sewer backups in homes—an often chronic and unseen form of urban flooding.
TASK 1: COMMONALITIES AND DIFFERENCES AMONG FOUR METROPOLITAN AREAS
A major thrust of this study involved visiting different metropolitan areas to examine urban flooding in different parts of the United States. The committee selected four disparate metropolitan areas to examine: Baltimore City and Baltimore County in Maryland, the City of Chicago and Cook County in Illinois, the City of Houston and Harris County in Texas, and the City of Phoenix and Maricopa County in Arizona. The committee visited each case study area and hosted a workshop with stakeholders, including city or county engineers responsible for flood management, nonprofit organizations, community groups, and residents. Such approaches emphasize idea generation and information exchange, rather than systematic evaluation.
The four case study workshops, site visits, and interviews showed that each metropolitan area has its own flavor of urban flooding. Baltimore participants did not identify with the term urban flooding. Although they noted that flooding occurs along Jones Falls and the low-lying areas near the harbor, the workshop participants seemed more concerned about basement flooding and sinkholes. In contrast, Houston area residents have been living with severe flooding for generations. Even though the Houston workshop was held before Hurricane Harvey, the participants demonstrated a high awareness of urban flooding and its impacts. The Chicago metropolitan area is also prone to several sources of flooding, including sewer backups into basements. Workshop participants had a high awareness of urban flooding as well as government and neighborhood efforts to lesson flood impacts. The Phoenix metropolitan area suffers from comparatively few floods, and many of these are flash floods. In lieu of a workshop with a variety of local stakeholders, the committee met with subject matter experts to discuss some innovative approaches to flood control in an arid climate.
Discussions at the workshops and meetings were organized around four dimensions of urban flooding: (1) physical—the built and natural environments, (2) social—impacts on people, (3) information—data used to understand or communicate flood events, and (4) actions and decision making—steps and policies for managing flooding. Key similarities and differences among the four metropolitan areas identified by the case study participants are summarized below.
Physical Dimensions. Each of the case study areas has a unique urban flood hazard defined by its natural environment (e.g., land cover, topography, soil type, and rainfall), history and pattern of land development (e.g., sprawling or dense), and type of storm water and sewage systems. A key difference among the case study areas was the sources of flooding: riverine (Baltimore), coastal (Baltimore, Houston, and Chicago [Great Lakes]), flash (Phoenix), and pluvial flooding (all four areas), as well as sewer backups (Chicago and Baltimore). Decisions on land development and design or maintenance of infrastructure were seen to amplify the intensity and influence the location of flood impacts in each metropolitan area.
Social Dimensions. Flooding crosses the economic spectrum, but workshop sessions and interviews paint a clear picture that the poor, racial and ethnic minorities, the elderly, renters, non-native English speakers, and those with mobility challenges were disproportionately affected by floods in each area. A major difference across the metropolitan areas was the level of citizen empowerment, which ranged from highly organized neighborhood and citizen groups (Chicago) to low levels of citizen engagement (Baltimore).
Information Dimensions. Stakeholders in all four metropolitan areas lamented a lack of data on urban flood hazard, flooding at local scales, or the economic costs and social impacts of urban flooding. In the absence of better information, managers and residents are using FEMA’s Flood Insurance Rate Maps to estimate where flooding will occur in urban areas. Some metropolitan areas are working to fill these gaps. For example, Harris County has developed local flood models, and Maricopa County and its partners are producing flood maps for transportation and flood warning purposes.
Actions and Decision-Making Dimensions. People in each metropolitan area wanted ongoing urban flood management efforts (e.g., buyouts of chronically flooded properties in Houston), and they noted the importance and the challenges of working toward solutions for urban flooding across jurisdictional divides. The range of collaborative projects across and among jurisdictions resulted in substantial engineering projects (the Tunnel and Reservoir Plan [TARP] in Chicago or Brays Bayou in Houston), large-scale blue-green solutions for flood (Indian Bend Wash in Maricopa County), and information-sharing efforts for flash flood warning systems (Arizona Department of Transportation Resilience Program).
Finding: Each of the study areas (Baltimore, Houston, Chicago, and Phoenix) has a unique flood hazard and manages urban flooding in its own way, using a tailored mix of federal, state, local, and nongovernmental financial and information resources. In each metropolitan area visited, the impacts of flooding are particularly felt by disenfranchised populations. All four dimensions (physical, social, information, and actions and decision making) are needed to understand and manage urban flooding.
TASK 2: SIZE OR IMPORTANCE OF URBAN FLOODING
Task 2 sought to answer the question, “how big is the problem of urban flooding?” Workshop participants in each metropolitan area felt that urban flooding is important in their area. However, quantifying the magnitude of urban flooding is challenging because flooding can result in a wide variety of economic, social, and ecological impacts, all of which vary geographically. Impacts include the following:
- Direct impacts—Immediate effect of the disaster (e.g., loss of life; damage to buildings, roads, agriculture, and infrastructure; monetary loss).
- Indirect impacts—Result from the direct impacts in the medium to long term (e.g., increased morbidity due to lack of sanitation facilities; unemployment and reduced income due to business and transportation interruption).
- Tangible impacts—Impacts that have a market value and can generally be measured in monetary terms (e.g., structural losses).
- Intangible impacts—Nonmarket impacts (e.g., health, natural resources, cohesion of a social group or community).
Although indirect and intangible impacts can be substantial, direct and tangible impacts are easier to measure. Thus, direct and tangible impacts are more commonly used to estimate the magnitude of urban flooding.
The two main methods for estimating the magnitude of flooding are (1) a descriptive or statistical assessment of historical flood impact data (retrospective estimate) and (2) an urban flood risk assessment (prospective estimate).
Historical Estimates of Urban Flood Losses
FEMA collects the most complete, consistent, and accessible data on historical flood losses. FEMA data include claims for property losses insured by the National Flood Insurance Program, loans for Small Business Assistance, and grants to cover immediate unmet recovery needs of individuals (Individual Assistance), assistance for publicly owned facilities (Public Assistance), and hazard mitigation projects and buyouts (Hazard Mitigation Grant Program). Data are available at the county level.
To estimate urban flood losses, the committee summed the dollar amounts from these five FEMA data sets over a 10-year period (2004–2014), then adjusted the figures to 2014 prices. For the 10 analyzed years, the total payouts, grants, and loans for case study counties were $2.7 billion for Harris County, $1.8 billion for Cook County, $38 million for Baltimore County, and $11 million for Maricopa County. Losses for Harris County would be considerably higher if the data set included Hurricane Harvey. Across the United States, FEMA data show that flood losses are greatest in heavily populated coastal counties, driven by coastal storm-induced flooding. For example, the significant flood events of Hurricane Katrina and Superstorm Sandy drove the urban flood losses for New York and Louisiana, which received payouts, loans, and grants on the order of $10 billion from 2004 to 2014.
Flood Risk Assessments
Flood risk assessments comprise four main components: (1) flood hazard—the probability and magnitude of the urban flood hazard, (2) exposure—the population and economic assets at risk, (3) vulnerability—the damage relationship between hazard and exposure, and (4) performance—accounting for any flood mitigation measures such as levees. Flood risk assessments offer a more comprehensive, but still incomplete, picture of urban flooding than historical estimates because they include a wider range of flood probabilities and some nonproperty damages. As a result, their estimates of flood damage and population affected are higher, sometime substantially, than estimates based on historical data. The few published risk assessments for the case study metropolitan areas found that 2.8 million people are exposed to flooding, more than triple the number estimated by FEMA. In addition, studies estimate that average annual losses are $3.3 billion for both Chicago and Houston and $76 million for Baltimore, more than 20 times higher than historical estimates.
Both historical estimates and flood risk assessments likely underestimate flood losses. Much of the historical data is derived from presidentially declared flood events, which miss impacts from less extreme but more frequent flood events. They also exclude uninsured property and indirect losses. Many flood risk assessments do not consider pluvial flooding, which is an important source of urban flooding, and they include only a few nonproperty damages.
Finding: Existing data are inadequate to provide an accurate monetary estimate of the magnitude of urban flooding. Historical loss estimates for the counties that include Chicago and Houston average $200 million per year (for 2004–2014) in each county. However, losses likely far exceed these estimates—possibly on the order of a few billion dollars per year—when pluvial flooding, uninsured property and indirect losses, declines in gross domestic product, and the millions of urban residents exposed to flooding are considered in a flood risk assessment. Although historical flood losses are lower in the counties that include Baltimore and Phoenix (few million dollars per year), actual losses are likely much higher when the other contributing factors are considered.
TASK 3: CONNECTION OF FEDERAL RESOURCES TO URBAN FLOODING
Task 3 was to relate FEMA and other federal resources to causes and actions related to urban flooding. Key needs with a strong federal connection concern understanding and communicating urban flood hazard and flood risk, understanding and mitigating the social impacts of urban flooding, and coordinating activities of organizations with a role in managing urban flooding.
Urban Flood Hazard
A key need identified in the case study areas was a better understanding of urban flood hazard. FEMA has established methods for analyzing several types of flood hazard, such as riverine or coastal flood hazard. However, methods for analyzing urban flood hazard will have to incorporate urban components, such as the capacity of storm water systems, as well as the small topographic variations, local drainage patterns, and site-specific structural designs that drive the granular nature of urban flood impacts.
Finding: An established method for analyzing urban flood hazard is needed. FEMA is well positioned to take a leading role in guiding this development effort by virtue of its mission and expertise in analyzing various types of flood hazards. Important partners include local government agencies, which know their storm water systems and local land characteristics, and organizations developing hydrologic or hydraulic models that account for pluvial flooding and other factors. Urban flood hazard analyses would also contribute to urban flood risk assessments being developed by academic researchers and private companies.
Socially Vulnerable Populations
A point raised repeatedly in the case study workshops and interviews is that while severe storms fall on the rich and poor alike, the capacity to respond to and recover from flooding is much lower in socially vulnerable populations that even in the best of times are struggling to function. This point is supported by research on social vulnerability and flood hazard impacts. However, the social dimensions of urban flooding are far less studied and understood than the physical dimensions. Academic studies focused on communities affected by urban floods would yield valuable insights. Data on intangible impacts (e.g., health or community cohesion), indirect impacts (e.g., unemployment due to business interruption), and additional vulnerability drivers (e.g., risk perception and social capital) would help improve urban flood risk assessments. Data collection and analysis could also reveal ways to build effective social networks or to support civic organizations that help residents increase their social agency, capacity, and capability for adjusting to flood hazards.
Finding: Greater investments are needed to research, understand, and develop interventions to mitigate the social impacts of urban flooding and their disparate effects across populations. Although the National Science Foundation is the primary funder of social science research, FEMA, the U.S. Army Corps of Engineers, and the Centers for Disease Control and Prevention have promoted accounting for and engaging socially vulnerable populations in the planning and response to hazard events.
Communicating Urban Flood Hazard and Flood Risk
The case study workshops showed that people want to know and understand their flood risk—the flood hazard as well as the consequences of it occurring, such as property damage and business, school, and transportation disruptions. Maps and visualizations are a primary means of communicating flood risk. Maps that show relative risk rather than probability, or that offer improved searchability or address lookup would be useful to the public.
A comprehensive flood risk map would portray information on both the flood hazard (e.g., depth and extent of flooding expected under different scenarios) and the consequences of flooding (e.g., building damage and population exposure). Urban flood risk maps also need to portray other information, such as land cover, the distribution of socially vulnerable and other populations, the location of previous flood problems, and the age, design capacity, and condition of storm water networks, drainage systems, and roads. Geographic information systems offer one means for integrating these observations with predictions of flood inundation.
Finding: A new generation of flood maps and visualizations that integrate predictions and local observations of flood extent and impact is needed to communicate urban flood risk. Improved methods for updating the maps to keep pace with urbanization and climate change are also needed. Federal contributions for such an undertaking include flood hazard analysis (discussed above) and data on flood damage (FEMA), precipitation and climate change (National Oceanic and Atmospheric Administration), social vulnerability (National Science Foundation), population and demographics (U.S. Census Bureau), and information from community development grants (Department of Housing and Urban Development). Other contributors include public and private organizations developing visualization techniques, especially for flood risk.
Coordination of Agencies Managing Urban Flooding
The challenges and successes of collaboration illustrated in the case studies underscore the need for coordination among entities that manage urban flooding. Depending on the metropolitan area, more than a dozen organizations and agency departments may be involved in urban flood preparation, response, recovery, and mitigation. FEMA’s National Response Framework follows a tiered response approach in which responses are handled at the lowest jurisdictional level capable of handling the problem. For major floods, FEMA is statutorily obligated to coordinate mitigation, response, and short-term recovery operations. However, many urban floods are too small to trigger federal resources and are managed at the state or local level.
The coordinating structures described in the National Response Framework are intended to be adaptable to meet the unique needs, capabilities, and circumstances of affected communities. For example, several agencies are involved with floods in urban areas, and these agencies may include those responsible for storm water and sewer systems or for deploying tide gauges to monitor tidal flooding and sea level rise. These differences complicate federal, state, and local government agency coordination for urban flooding. Nevertheless, the high concentrations of people and assets at risk add urgency for these organizations to work together quickly and efficiently.
Finding: Stronger coordination is needed across agencies that have a role in managing small or large urban floods. Such coordination will be both vertical (e.g., federal, state, local) and horizontal (e.g., local agencies responsible for storm water systems, flood control, and removal of damaged property; federal agencies responsible for severe storm warnings, evacuation, community redevelopment, and flood mitigation in urban areas).
Urban flooding is a complex problem that manifests across multiple dimensions. The particular combination of physical environment, types of flood sources, and development patterns results in distinct impacts on different urban centers and neighborhoods. Impacts vary across the social spectrum, with vulnerable populations at higher risk, yet less protected by insurance or the social safety net. Data and information on the causes and impacts of urban flooding are sparse, incomplete, and inconsistent. Although it is clear that urban flooding is costly in some places (particularly in coastal cities), the shortage of suitable data and models make it difficult to adequately quantify losses. Finally, responsibility for managing urban flooding is distributed across federal, state, and local government agencies and nongovernmental entities.
The current costs and impacts of urban flooding merit national attention. Further, flood problems are likely to get worse with continued urban development and population growth in urban areas, as well as with climate change, which is increasing sea-level rise and the frequency of heavy precipitation events. Multiagency and cross-jurisdictional efforts are needed to analyze urban flood hazard, advance understanding of social impacts, and communicate urban flood hazard and flood risk.