By definition, inland flooding doesn't occur on the coast, yet hazard specialists consider it with other coastal issues because it is often the result of landfalling coastal storms. Inland floods can also result after rain falls for many days in a row, following brief periods of intense precipitation, when snowpack melts quickly, or when dams or levees fail. Whenever the volume of water on land overcomes the capacity of natural and built drainage systems to carry it away, inland flooding can result.
After landfalling storms wreak havoc along the coast, they can still produce widespread, torrential rains and floods as they move inland. As demonstrated by the widespread flooding impacts of Hurricane Harvey in 2017, inland flooding is a major threat that tropical and extratropical cyclones pose for people and property inland from the coast. Over the coming decades, even if the frequency or intensity of storms such as hurricanes and nor'easters doesn't change as global average temperature increases, sea level rise will increase the frequency and extent of extreme flooding associated with coastal storms.1
Back-to-back coastal storms also pose risks for inland flooding. For example, Tropical Storm Dennis soaked eastern North Carolina in 1999. Right after that, Hurricane Floyd hit the same area. Floyd’s storm surge over saturated soils and already swollen rivers from Dennis led to epic (500-year) flooding in places like the Tar/Pamlico River basin. The severity of compound events—the coupling of surge, discharge from rivers, and heavy precipitation—has increased in many coastal cities.1
Across the nation, increases in heavy precipitation over the last three to five decades have increased the likelihood of inland flooding.2 The heaviest rainfall events have become heavier and more frequent, and since 1991, the amount of rain falling in very heavy precipitation events has been significantly above average.2
On average, the annual cost of damage from inland floods is higher than any other severe weather event—averaging $6.9 billion per year for the period 1976–2006. More than 60 percent of U.S. hurricane-caused deaths from 1970 to 1999 occurred in inland counties, with more than half of those deaths related to freshwater flooding.3
- 1. a. b. Sweet, W.V., R. Horton, R.E. Kopp, A.N. LeGrande, and A. Romanou, 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I[Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2.
- 2. a. b. Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, S. Doney, R. Feely, P. Hennon, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Ch. 2: Our Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 19-67. doi:10.7930/J0KW5CXT.
- 3. Moser, S.C., M. A. Davidson, P. Kirshen, P. Mulvaney, J. F. Murley, J. E. Neumann, L. Petes, and D. Reed, 2014: Ch. 25: Coastal Zone Development and Ecosystems. Climate Change Impacts in the United States: The Third National Climate Assessment. J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 579-618. doi:10.7930/J0MS3QNW.