An Inland Hurricane Scenario
One flooding scenario in the U.S., especially, the eastern U.S., is when a hurricane or tropical storm moves overland and brings tropical moisture and rain to the interior of the country.
A famous event is Hurricane Agnes in 1972. Agnes flooded the Mid-Atlantic, especially Virginia, Maryland, Pennsylvania, and New York.
In Virginia, in southern Albemarle County, south of Charlottesville, I have driven many times by the painted marks on the rocks on the side of Plank Road. The records of the floods of the tiny Hardware River are high above my head. I think this was from Agnes, but it might have been from Camille, in 1969.
In 1996, Hurricane Fran caused huge wind damage in my hometown, Cary, North Carolina, 150 miles from the coast. In 1954, again in Cary, Hurricane Hazel toppled a several hundred foot tall tower at WPTF radio station. The eye was still together, and it stayed together into southern Virginia.
The inland hurricane is a recurring type of weather event.
Hurricanes and tropical storms causing flooding and wind damage 100s of miles from the coast are frequent enough and cause enough damage, that they should be part of scenario planning for disaster preparedness in most of the eastern half of the U.S. The consequences of these events are enormous. As with all weather events, they now exist in a warmer and warming world, and that increases the imperative to walk through the plans for preparedness and response.
This scenario article is motivated by the immense flood caused by Hurricane Helene in September 2024.
Helene came ashore in the Big Bend of Florida, maintained its hurricane characteristics inland into Georgia, and ultimately led to flooding in the mountains of South Carolina, North Carolina, and Virginia. It changed the landscape. 1Helene occurred in a contentious political environment, with some claiming it was engineered by some mysterious power broker and others maintaining that it did not even occur.
From a weather-science point of view, historical hurricane-related floods are not all alike. Some of these floods are associated with the inland storm being very slow moving. In many cases, the tropical storm gets incorporated into a weather front or already existing storm and intensifies. Hurricane Sandy has become an iconic example of this. The 1969 Camille flood in Virginia came after a long path over land followed by growth associated with a front in the interior of the U.S. As with most extreme weather and climate events, there is no one cause of the extreme behavior.
From a flood point of view, the first rule is to look at the surface around you. For Helene, as well as Agnes and Camille in Virginia, the terrain is hilly, with narrow gaps and small canyons. It is largely forested and there are structures of many types. There are urban, agricultural, and wilderness landscapes. There is water and storm management infrastructure. The terrain makes flooding likely in the presence of heavy rain.
The hilly terrain also potentially enhances the precipitation when saturated air flows from lower to higher elevation. The precipitation is likely most extreme at higher elevation, with little area and extreme slopes.
The state of the ground is also important. Is it paved, impervious, plowed, natural, wooded, grassland, etc.? That is, what are the runoff characteristics? Very important, is the ground already wet and saturated from previous storms? Are the reservoirs full? Is the ground exceedingly dry and, itself, initially impervious. In rare conditions the ground might be optimal for infiltration of water, which can greatly reduce the amount of flood for a given amount of rain.
It is, also, necessary to consider the interface between the built environment and the details of the surface. Have we built in flood plains, and if so, have we considered the risk profile for different flooding scenarios. What is the state of the infrastructure? What are the design specifications and is it maintained? Is there green infrastructure, such as wetlands and parklands designed to accommodate and absorb excess water?