Not all floods are deadly disasters. As anyone who has ever waded through a flooded Washington, D. C., subway station can attest, many floods are merely bothersome, tangling public transportation and causing other daily hassles. As sea levels rise and populations grow, such “nuisance floods” are growing more frequent. A new study proposes a global definition for such floods, in the hope of helping governments, cities, and insurance companies plan for them and cope with rising costs.
Scientists predict that humans will experience a twofold to threefold increase in exposure to river and coastal flooding between 2010 and 2050. Many factors contribute to this rise, including urbanization, aging infrastructure, and climate change, which is altering where snow and rain fall and how fast snow and ice melt, resulting in sea level rise.
Some characteristics of nuisance floods are widely accepted. By definition, these floods don’t cause major property damage or seriously threaten public safety (e.g., by sweeping away people and cars). Yet nuisance floods can strain infrastructure like roadways and sewers, block transportation, threaten water quality, provide habitats for mosquitoes and bacteria, affect property values, and discourage tourism. In places where such floods happen all the time, the cumulative costs may be comparable to one extreme event, like Hurricane Sandy. Beyond such generalities, however, there is no precise definition of what constitutes a nuisance flood. Some countries rely on impact reports to determine whether a flood was severe or just a nuisance, but such reports are often hard to come by.
To arrive at a clearer definition, Moftakhari et al. did a massive literature review on nuisance flooding. They looked at a range of different factors of nuisance flooding, including pedestrian safety, property damage, hydrology, and transportation impacts. On the basis of this research, the team honed the following definition, which they argue can be applied around the world: A nuisance flood is a layer of water between 3 and 10 centimeters high, traveling at a speed of less than 3 meters per second.
Roughly 13 centimeters of water will reach the undercarriage of most passenger cars, and street curbs typically rise 10–20 centimeters above the crown of roadways. Therefore, the team concluded, once water rises beyond 10 centimeters, more serious damage ensues, and the flood is more severe than a “nuisance.” Defining nuisance floods more clearly could help the federal government better determine which events are eligible for disaster assistance.
Between 2004 and 2011, the number of flood-related disaster declarations in the United States totaled 539, with aid obligations reaching over $90 billion. At present, the threshold for public assistance after a flood occurs is an estimated loss of $1.46 per capita statewide and $3.68 per capita countywide, rates that have not been adjusted for increased incomes or inflation since 1986.
The new definition could also help cities measure the cumulative impacts of nuisance floods and encourage flood-resilient construction, the authors write. Building new developments higher, including with water-resistant building materials in the nuisance flood zone, and placing electrical components higher are examples of how communities can increase their resilience. (Water Resources Research, https://doi.org/10.1029/2018WR022828, 2018)
—Emily Underwood, Freelance Writer