How Vermont’s physical and human geography help explain recent flooding
by Ethan Weinstein July 31, 2023, 5:29 pm Vt digger
Vermont’s physical features affect its weather, its rivers and how humans have come to inhabit it.
When flooding hit Vermont this month, destruction spanned the length of the state, from Windham County up through Northeast Kingdom.
But the series of storms did not damage regions equally, with many mountain towns and riverside communities bearing the brunt of the impact.
The precipitation itself, as well as the flood-prone nature of many Vermont towns, can be explained by the state’s physical features, experts say. The Green Mountains along the state’s spine determined where Vermonters settled, where they worked and the weather they endured.
“Mountains play a big role in Vermont’s weather in general,” explained Pete Banacos, science and operations officer at the National Weather Service’s Burlington office.
As moving air hits the Green Mountains, it rapidly cools as it rises, often triggering precipitation, Banacos said, in a phenomenon called “orographic lift.”
That precipitation typically occurs on the “upwind” side, he explained, or the direction from which the wind originated.
Unlike Tropical Storm Irene, which brought strong winds from the east — causing the most extreme rain on the eastern slope — July’s storms included lighter winds moving in multiple directions, which produced orographic lift on both sides of the Green Mountains.
“We saw maybe a little bit of a tendency for it to be on the east side of the mountains, but the west side got hit as well,” Banacos said. Rainfall totals indicate many eastern slope towns received the greatest precipitation totals, but Mount Holly, situated on the western edge of the mountain spine, received more than 8.5 inches of rain.
Top 10 locations with the highest rainfall totals for the first 38 hours of July storm:
Unofficial rainfall reports from government agencies, media, and local observer partners sent to the National Weather Service as of 2 p.m. on Tuesday, July 11, 2023.
Map: Erin PetenkoSource: National Weather Service, Burlington and Albany
All that rain on steep terrain causes many of Vermont’s watersheds to be “flashy,” Banacos said. They rise quickly and fall quickly.
“Your likelihood to get flash flooding is much greater in the steep terrain,” he said, as water rushes downhill at a fast pace.
“Flash flooding is the nature of our risk,” said Rob Evans, river programs manager for the Vermont Department of Environmental Conservation. Whereas in places such as the Midwest, rivers enter and exit flood stage gradually, in Vermont that process happens rapidly, in large part due to the mountainous topography.
According to Evans, Vermont’s mountains don’t just increase flood risk by making streams susceptible to flash flooding. The state’s steep land has also caused people to settle in more flood-prone areas, what he called “our historic settlement patterns in floodplains and along rivers.”
Vermonters settled along rivers for a number of reasons, including mill power, transportation and waste disposal, Evans said. Village centers, which tend to be located in river valleys, have also come to house many of Vermont’s lower-income residents, placing them at a greater risk of flood damage.
Along the way, humans have made those rivers even more dangerous.
“The history of straightening our rivers, dredging our rivers, (has caused) problems,” he said. “What was thought to be a flood recovery, flood mitigation effort” — digging rivers deeper — “that creates additional danger.”
Vermont Public dug into the history in a recent episode of “Brave Little State,” exploring how years of human intervention have made Vermont’s streams more powerful and replaced wetlands that serve as a protective barrier for flooding with human infrastructure.
State officials have recently implored Vermonters to resist the urge following the July flooding to remove logs and debris from rivers, calling on them to keep waterways “beautifully messy.”
Evans pointed to the Otter Creek between Rutland and Middlebury as a uniquely healthy river with its floodplain intact, posing a lower flood risk than rivers straightened over time. Plus, the areas with more clay and sandy soils — compared to rocky soil in other parts of the state — “can really make a difference” with regard to flooding, Evans said.
The Otter Creek at Center Rutland. National Weather Service photo
While the specific geographic and human-centric features of Vermont’s towns make them more or less prone to flooding, Evans said he expects disasters like this month’s to “become more normal.”
“Each time we come out, I just hope we’ve mitigated.”
