Urban Sprawl
A view of a sprawling subdivision from above, showing single-family homes on residential lots.
As disasters become increasingly more destructive and more expensive, understanding the relationship between development and disaster can help us make smart building decisions to minimize the risks, and subsequent costs, of future disasters. This article is the first in our series, "Development and Disaster," in which we examine the role that development plays in disaster.
Over the last 40 years, the number of weather-related disasters in the US with damages exceeding one billion dollars has significantly increased, growing nearly ten-fold, from a total of three in 1980 to 27 in 2024. What was once a semi-rare occurrence in the 1980s and 1990s has become commonplace in the 21st century. Even when adjusting for inflation, the evidence is clear that disasters are becoming more frequent and more expensive.
Climate change is often cited as a key contributing factor to this phenomenon, but less widely discussed is the impact urban sprawl has on disasters. As the human population spreads, we create larger targets for weather-related disasters; covering the landscape with asphalt and concrete makes an area more susceptible to flooding, and the attraction of waterfront property creates foreseeable risks that further compound the costs and damages from disasters.
What is urban sprawl?
As cities expand, they tend to grow in one of two ways: up or out. New York City is a great example of a city growing up. With its iconic skyline and high-density urban center, New York City was compelled to develop upward with multi-story buildings and skyscrapers because of its island location.
More often, cities grow out, slowly swelling to envelope the suburbs and surrounding countryside. This is urban sprawl, and it can be seen in metropolitan areas throughout the US.
For a more precise definition, urban sprawl is “characterized by low-density residential housing, single-use zoning, and increased reliance on the private automobile for transportation.” The above description depicts a hallmark of urban sprawl: large residential subdivisions that are disconnected from commercial areas, usually separated by several miles. This sprawl development has many costs – increased utilities, transportation, and energy costs, for instance – but it also makes citizens more vulnerable to disasters.
A view of a single-family home on a quiet residential street
How does urban sprawl exacerbate disasters?
In an urban environment, there are more impervious surfaces (i.e., paved roads, parking lots, and rooftops), which impacts water absorption and results in more runoff when it rains. During a normal rainstorm, this runoff can often be handled by the city’s sewer/stormwater management system. However, during a tropical storm or hurricane with an accompanying storm surge, these systems can quickly become overwhelmed. As development continues across the US and suitable real estate becomes scarcer, city centers and subdivisions are being built in more exposed areas.
The biggest problem posed by urban sprawl lies in its inefficient use of impervious surfaces. More reliance on personal vehicles for transportation (rather than public transport like buses and trains), greater distances between residential and commercial areas, more parking lots, less green spaces, and haphazard development create conditions that make citizens more vulnerable when disaster strikes.
Furthermore, EMS response times in urban sprawl areas can be nearly double other settings because of decreased travel efficiency. Less dense development also requires more police and fire services to ensure adequate coverage, so as urban sprawl increases, its impacts grow exponentially, and because of the traffic congestion that nearly always accompanies urban sprawl, evacuations during disasters become yet another hazard.
What can be done to reduce our vulnerability?
The best antidote to urban sprawl is smart growth. Urbanization is increasing around the world to support overall population growth. In 1990, 43% of the world’s population lived in urban areas. That number grew to 55% as of 2018, and by 2050, it’s expected to rise above 65%.
Smart growth can entail many things, but there are a few tactics that can reduce an areas vulnerability:
Zoning Reform: Updating regulations can help decrease sprawl and control impervious surface coverage. Zoning updates can be used to incentivize development in less exposed areas and/or require additional steps to build in flood-prone areas.
Green Infrastructure: In areas with large swaths of impervious surfaces, incorporating green infrastructure can help balance the scales and reduce runoff. Bioswales, rain gardens, and green roofs are all strategies that improve flood control while also beautifying the surrounding neighborhood and improving quality of life for residents.
Building Code Enhancements: Strengthening building codes can help as part of a larger strategy (especially when paired with zoning reform). However, this approach tends to be more expensive than others and, in some cases, can have unintended consequences. For instance, elevating a building to keep it safe from flood waters could end up making a neighboring structure more vulnerable.
Evacuation Routes/Resources: No matter how much work is done to mitigate hazards or how much pre-disaster planning takes place, every community must have a viable evacuation plan. When urban sprawl is concerned, the over-reliance on personal vehicles for transportation is inherently problematic, and special care must be taken to map out evacuation routes and create additional strategies to ease congestion so that citizens are able to get out of harm’s way.
An example of a green roof in an urban setting
Building more cities, towns, and neighborhoods is unavoidable. What is avoidable, however, is repeating the same mistakes our predecessors made. Many major cities around the world were constructed near coasts or rivers, which makes them susceptible to flooding. This approach to location selection exposes residents to danger during disaster. As new development continues, standards for managing flood risk must be updated and disaster mitigation must be a fundamental part of urban planning.