When Hurricane Katrina struck New Orleans in 2005, the city experienced one of the most severe traffic jams in U.S. history because of insufficient evacuation planning and limited coordination among city agencies, disaster relief organizations, and residents. An estimated 25% of the city's population chose to remain in their homes, but the remainder - as many as 1 million people - clogged routes away from Katrina's wake on their way out of the city or to designated shelters. Emergency management professionals often attribute part of Katrina's historic death toll to the city's failure to plan for a worst-case evacuation scenario.

Perhaps the worst-affected demographic during Katrina's cautionary tale was the elderly. Approximately 71% of the storm's fatalities were people over 60 years old, and 47% were older than 75. With limited mobility that often makes fleeing cities unfeasible, elderly residents require realistic evacuation plans that include not only sufficient shelter capacity in safe areas, but also robust preplanning to ensure they can reach these shelters quickly during the chaos of a mass evacuation.

An international team of researchers representing the U.S., U.K., China, Switzerland, and the Netherlands is studying evacuations from storms such as Katrina to help cities maximize coastal-flooding protections for their most vulnerable residents. Their results appear in the journal Nature Water.

Different Responses to Similar Threats

To study which approaches resulted in the most efficient evacuation times for all residents, including the most vulnerable, researchers focused on two cities with similar coastal-flooding stressors but different emergency strategies: New York City and Shanghai.

The team's analysis shows that New York City excels in its contingency planning ahead of major storm events, undertaking comprehensive coastal flood modeling and zoning practices that identify six at-risk areas where evacuation protocols are particularly robust. Together, these hurricane evacuation zones cover about 49% of the city's total area and are home to about 3 million residents, including about 41% of New York City's elderly population. A system of 60 evacuation shelters with a combined capacity of approximately 600,000 people stands ready to accept evacuees ahead of major storms. Although this may seem a small number compared to the city's total population, evidence from Hurricane Sandy in 2012 shows that only 2% of evacuees - many of them elderly - sought refuge in public shelters while most evacuees fled the city.

[Link]When residents of New Orleans evacuated the city before Hurricane Katrina in 2005, insufficient planning exacerbated deaths and damages. Katrina, in which 71% of the fatalities were people older than age 60, serves as a stark reminder that cities specifically must consider the elderly when conceiving pre-storm evacuation plans. Image courtesy of Mark Moran/U.S. National Oceanic and Atmospheric Administration

While researchers suggest existing shelter capacity is ample in the case of another major coastal-flooding event, they suggest that the city's reliance on personal responsibility is its weakness. Researchers contend that current protocols to coordinate transportation to these shelters are lacking, requiring individuals to find their own way.

Shanghai has nearly three times the population of New York City, with a considerable portion of its residents distributed across the offshore Chongming and Hengsha islands. Unlike in New York City, Shanghai's municipal government coordinates well-organized evacuation procedures that transport residents to the city's 74 indoor shelters. However, Shanghai's lack of designated hurricane evacuation zones creates confusion regarding specific evacuation routes as well as which areas are expected to suffer the worst effects from flood events of a given severity.

Following New York City's methodology, researchers created a series of six hypothetical evacuation zones for Shanghai, finding that 25 of the city's indoor shelters are located in areas likely to experience severe flooding during major storms. The remaining 49 shelters have a combined capacity of about 230,000 people - representing less than 3% of the population living in expected evacuation zones, which also includes roughly 520,000 elderly residents. Because no shelters currently exist on the Chongming and Hensha islands, residents must relocate to the mainland during emergencies.

Researchers estimate that most evacuations to existing shelters within New York City's vulnerable areas will take less than 20 minutes from point to point. By contrast, evacuations may take up to 4 hours in Shanghai due to insufficient shelter capacity and coverage.

A Global Blueprint for Flood Evacuations

Study authors sought to use examples from New York City's evacuation plans to bolster those of Shanghai and develop a framework to strengthen evacuation procedures for coastal cities worldwide.

Part of this effort included examining the distribution of existing buildings in Shanghai that easily could become temporary shelters during emergencies, such as schools, stadiums, and convention centers, to recommend ways to increase overall shelter capacity. Researchers also deployed algorithms to incorporate such factors as speed limits, road hierarchy, and traffic patterns to identify critical evacuation arteries, using this information to suggest optimal locations for new shelters that would benefit the largest amount of people likely to require them.

Their results led researchers to recommend two distinct scenarios for Shanghai, each of which can ensure elderly residents have options during severe flooding events. The first and less costly scenario would involve constructing 22 additional shelters in strategic locations. While this would provide sufficient shelter capacity to accommodate Shanghai's full elderly population, evacuation times would remain lengthy, particularly for residents not on the mainland. The second option would raise the city's total number of shelters to 128, including the construction of six high-capacity "megashelters" on the Chongming and Hengsha islands. This would ensure that at least 90% of Shanghai's total evacuees, including elderly residents, could reach shelters within 23 to 60 minutes, according to the study.

Nigel Wright, study co-author and University of Birmingham (Edgbaston, England) Civil Engineering Professor, described how cities can follow the research team's methodology to plan the number and location of new shelters as well as how these new shelters can improve evacuation efforts. This is increasingly important as the world's 136 largest coastal cities are expected to experience population booms in the coming decades. By the 2070s, more than 150 million people are projected to live in the most flood-prone parts of these cities, which would make proactive evacuation planning critical to protect lives and property.

"Differences in evacuation patterns for elderly residents in Shanghai and New York City demonstrate the value of risk-informed, strategic evacuation planning for storm flooding," Wright said in a release. "Our work provides new insights into operational emergency evacuation decisions and could provide a blueprint for flood management policy development in major coastal cities globally."

Read the study, "Strategic Storm Flood Evacuation Planning for Large Coastal Cities Enables More Effective Transfer of Elderly Populations," in Nature Water.

Top image courtesy of Stefan Wiegand/Pixabay

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