WHAT IS URBAN FLOODING?

Urban flooding is the accumulation of floodwaters that result when the inflow of storm water exceeds the capacity of a drainage system to infiltrate water into the soil or to carry it away. When a natural landscape is transformed by urban development, its drainage pattern is disturbed. The natural landscape, where flow gradually accumulates through small hollows and channels into local streams, is replaced by a graded landscape where streets carry surface water flow and become an important part of the drainage network. Storm sewer inlets drain water from the street system and convey the flow through subsurface pipes to discharge points at downstream locations. A combination of transmission systems—channels, streets, and pipes—conveys precipitation falling on the city to its outfall locations in larger streams or to the coast. Floodwaters accumulating in larger streams can overwhelm the capacity of the stream channel and inundate surrounding areas, particularly in downstream areas that receive floodwaters from developed areas in the upstream part of the watershed. Coastal storm surge from storm winds can flow into coastal cities, causing direct flood damage and inhibiting drainage from inland flooding. Even in the absence of substantial wave surge or precipitation, coastal water can encroach on urban landscapes at high tide. Smaller, chronic floods can also occur in older cities with combined sewer systems carrying both storm water and wastewater. These systems can become surcharged during storms, causing sewer backups in homes and discharge of untreated wastewater into streams. Aging and inadequate drainage infrastructure and failing pipe systems create additional flooding problems. Urban flooding poses a distinctive kind of flood management problem for several reasons:

1. Flood problems reflect the history of a city and generally increase with urbanization. Many early U.S. cities were established along rivers and coasts to facilitate trade, support manufacturing, and transport people and goods. The growth of commerce and the availability of jobs and services drew people to these early settlements, a trend that accelerated with industrialization in the mid-19th century (Macionis and Parrillo, 2013). Hard street surfaces for motor-driven transportation became common, and city engineers began to design and construct sewer systems to carry human waste and storm water runoff away from homes and businesses (e.g., Figure 1.1).¹ Cities began to grow outward with the help of steam-powered trains, electric street trolleys, subways, and eventually cars.

FIGURE 1.1

Brick sewers built in Philadelphia, some of which are nearly 200 years old, are still in use today. SOURCE: Photograph by Kimberly Paynter.

As cities grow, so do flood problems arising from the increasing fraction of impervious surfaces, reliance on storm water systems built for yesterday's needs, development policies, and, often, proximity to water bodies. Land development alters natural drainage patterns that previously carried water away and creates hardened surfaces that inhibit infiltration of storm water (Appendix A). Storm water systems built to address these problems may be old (e.g., Figure 1.1), undersized for current needs, or poorly maintained (e.g., leaking pipes and clogged culverts and storm drains). In addition, permissive building practices or unenforced building or zoning ordinances have allowed dense land development in many flood prone areas (Appendix A).

2. A large and growing number of people are affected by urban flooding, and who is affected varies across the urban landscape. Approximately 86 percent of the U.S. population (277 million people) lives in metropolitan and micropolitan areas, defined as core areas containing a substantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core.² In addition, population density is expected to increase in many urban areas (Appendix A). Flood impacts across these urban areas can vary at small spatial scales, sometimes neighborhoods or even households. And the ability of people to cope with flooding is influenced by their access to resources, such as financial support or community networks (e.g., Figure 1.2).

FIGURE 1.2

A resident of the Chatham neighborhood of Chicago showing what she lost in the April 2013 floods. She was attending Gross Gathering, an event convened by the Center for Neighborhood Technology (CNT) to encourage flood survivors to speak out, share their (more...)

3. Roles and responsibilities for urban flooding are distributed among federal, state, and local entities. Depending on the metropolitan area, more than a dozen federal, state, and local government agency departments, nongovernmental organizations, and private entities may be involved in urban flood preparation, response, mitigation, and recovery. Which organizations are involved and their roles varies across metropolitan areas.

4. Tools to analyze and portray urban flood hazard in the United States are incomplete and inconsistent. In many cases, the default tools are FEMA maps and analyses, which were not developed to assess urban flood hazard. For example, FEMA maps and analyses do not include some aspects of pluvial flood hazard, flood hazard in small drainage areas (less than 1 square mile), or flood hazard created by drainage and other urban infrastructure (FEMA, 2003).

COMMITTEE CHARGE AND APPROACH

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 selected metropolitan areas, and to use that information to address three tasks (see Box 1.1). 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. As such, the report serves as a starting point for broader and deeper exploration.

BOX 1.1

Committee Charge.

Four metropolitan areas could be visited within the confines of the study, and the committee selected Baltimore, Chicago, Houston, and Phoenix. To address Task 1, the committee gathered information from stakeholders (government managers, academic scientists, nongovernmental organizations, community groups, and flood victims) through workshops and meetings, site visits, and telephone interviews in each metropolitan area. The discussions were organized around four dimensions of urban flooding: (1) physical—the built and natural environments, (2) social—impacts on people, (3) actions and decision making—steps and policies for managing flooding, and (4) information—data used to understand flood events (see Box 1.2; adapted from Linkov et al., 2013, and Fox-Lent et al., 2015).

BOX 1.2

Dimensions of Urban Flooding.

The workshop orientation of the study posed problems for addressing Tasks 2 and 3. In particular, the workshop discussions did not yield enough information to estimate the size or importance of flooding in the case study areas (Task 2). To address Task 2, the committee drew on published estimates of flood losses in the case study areas and also analyzed federal flood loss data. Task 3—relating urban flooding causes and actions to federal resources—is broad. 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.

ORGANIZATION OF THIS REPORT

This report explores the causes, impacts, and actions for managing urban flooding in four metropolitan areas: Baltimore, Houston, Chicago, and Phoenix. Chapter 2 summarizes stakeholder views of urban flooding in their metropolitan area, based on what the committee heard in workshops and field trips (Task 1). Chapter 3 estimates the magnitude of urban flooding in the four case study areas, based on committee analysis and published data (Task 2). Chapter 4 connects the causes and actions for managing urban flooding with federal resources (Task 3).

Supporting material for these chapters appears in appendixes. Appendix A summarizes trends in population, floodplain occupancy, land cover change, and climate change that influence the future magnitude of urban flooding. The participants and agendas for the stakeholder workshops, meetings, and site visits in the four case study areas are summarized in Appendixes B through E. Finally, a list of acronyms and abbreviations is given in Appendix F.