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Otter Creek wetlands seen as shielding Middlebury

WATER INUNDATES A wetland area in Salisbury, a fairly common sight in these parts in the wake of an unrelenting period of rainfall. Independent photo/Steve James

MIDDLEBURY — Since last week’s heavy rainfall and disastrous flooding across the state, the bridges above the Otter Creek Falls have been occupied by walkers pausing to peer over the edge and snap a few photos of the raucous rapids. 

But last Saturday, the bridge had a few visitors with more scientific interests: United States Geological Survey NH-VT Hydrologic Monitoring Section Chief Richard Kiah and United States Geological Survey Hydrologic Technician Sara Weaver peered over the Cross Street Bridge with interest.

“We were there specifically to calibrate the stream gage during higher streamflow conditions… measuring the depth and velocity of the river,” said Kiah. The site is valuable because of the robust uses of the data gathered. 

“Streamflow information from this site is used for the protection of life and property, for the assessment, allocation, and management of water resources; for the design of roads, bridges, dams, and water works; for the delineation of flood plains; assessment of habitat; understanding the effects of land-use, water-use, and climate changes; evaluating water quality and for recreational purposes,” he said. 

One of the most obvious things they could determine is that the area’s wetlands are to thank for the less damaging outcome to the Middlebury area than other Vermont communities faced. 

“There are a considerable amount of wetlands along Otter Creek between Rutland and Middlebury that attenuate the streamflow,” he said, meaning that the wetlands absorb much of the creek’s energy and spread it out before it reaches Middlebury’s downtown. 

Vermont Agency of Natural Resources Wetlands Program Manager Laura Lapierre corroborated this explanation for limited flood damage in the area, as did Sewanne Assistant Professor of Environmental Studies and former University of Vermont Gund Institute PhD student Keri Watson, 33. 

Watson further explained what a wetland is and the characteristics that help it mitigate flooding. 

“In this context, a wetland is an ecosystem that is regularly inundated with water, and you find different forms of vegetation, but also different soils,” she said. “In this context, what’s important about there being wetlands upstream of Middlebury is that those soils have an incredible capacity to kind of soak up water like a sponge.”

Research Associate Professor in the University of Vermont Department of Civil and Environmental Engineering Kristen Underwood explained the process: “These extensive wetlands are able to receive and store large volumes of floodwater, and slowly release them back to the river over several days following a storm.”

When it comes to water storage, these natural structures are incredibly effective. “It is estimated that a one-acre wetland, one foot deep, can hold between 330,000 to 1.5 million gallons of water,” Lapierre said. 

The wetland’s mechanisms first drew Watson’s attention following Hurricane Irene. She conducted a study in 2016 that found the Otter Creek wetlands were integral in preventing extreme damage from flooding. 

“Even in this event, which feels like massive flooding across most of the state, these wetlands and floodplains were able to reduce the felt impact for areas downstream,” she said. 

“Looking kind of upstream from Middlebury, there’s a wide floodplain before you see elevation kind of rise more steeply, whereas a lot of Vermont has narrower valleys,” Watson said. “So what that wide floodplain means is that it takes a really large volume of water to raise the height of the water level by even a small amount. Whereas if you imagine a really narrow floodplain, changes in water volume would quickly result in increases in flood height.” 

The county’s geographic assets in its wide valley floor were also aided by a lower amount of rainfall. “In part, this is because Addison County, and the Champlain Valley more generally, received less total rainfall in the 48-hour period ending on July 11 as compared to other parts of the state, particularly east of the Green Mountains.”

According to Underwood, the current flooding in Addison County is comparable to what Hurricane Irene produced in August of 2011.

But she says other parts of Vermont are experiencing worse flooding than following the storm: “For example, Winooski River floodwaters in Montpelier reached a higher elevation, for a longer period of time, than they did during Tropical Storm Irene. Similarly, the Lamoille River at Johnson exceeded flood stage by greater amounts and for a longer duration than during Tropical Storm Irene.”  

Underwood explained the remaining concerns regarding flooding: “The concern in recent days, raised by some including the Middlebury fire chief, is that these wetlands are now mostly full to capacity with floodwaters from earlier this week, and may have a diminished ability to dampen near-future flooding if we continue to receive high amounts of rainfall in the coming days.” 

She says it can take days to weeks for the wetlands to become effective again.

Watson said that the relevance of her study remains high: “We’re going to expect more extreme precipitation under changing climate. We know that this kind of flooding is now something that we’re going to have to adapt to and become resilient to. And one of the things that can help us become more resilient to extreme weather events and extreme rain is green infrastructure, like wetlands and floodplains.”

Underwood said Addison County has been preparing for an episode like last week’s: “Impacts from this recent flooding have been lessened as a result of flood mitigation efforts taken on by many Addison County communities in recent years.

“Actions have included conserving floodplains and wetlands, removing vulnerable buildings and infrastructure from the river corridor, and replacing culverts and bridges with new more flood resilient structures that more easily pass debris and floodwaters,” she said. 

Lapierre explained the benefits of investment in wetlands range from minimal maintenance to their resilience. 

“Wetlands are more resilient and have natural processes to help them absorb the effects and continue to function. This is why wetland restoration is such a great nature-based solution to flooding and clean water concerns — once a natural wetland is restored, it needs minimal maintenance to continue to provide benefits,” she said.  

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