The 100-year flood: Building ‘RiverSmart’ communities for flood resiliency in the Connecticut River Valley

Rivers are constantly moving. They shift position over time, meander and twist into S-shaped curves. Sometimes they flood or break their banks, depositing sediments and forming beaches while simultaneously eroding others.

While rivers moving and flooding is a natural, the process can threaten homes, essential infrastructure, roadways and farmland when it crowds too close.

“If we let a river have enough space to be river and do the processes they would naturally do, then that keeps us and our infrastructure out of harm’s way,” said Christine Hatch, an extension associate professor of geosciences at the University of Massachusetts Amherst.

Hatch studies fluvial geomorphology, or how rivers change over time, move sediments and interact with groundwater. She is the principal investigator for river science and research for the UMass RiverSmart Communities program, which she co-founded with her colleague, geology professor Eve Vogel, in 2012.

Inspired by tropical storm Irene’s devastating flooding, the RiverSmart program aims to promote ideas and tools for policy makers and riverside communities to reduce flood severity, risk and cost in the face of climate change. Their top priorities are educating communities and policy makers about flooding and river dynamics, and finding ways to minimize stream bank failures, bridge and road collapse, and farm field destruction.

“Most flood-preparedness information is based on large population centers and inundation (imagine a tranquil bathtub filling), but not this kind of powerful erosion (think high-pressure fire hose coupled with sandblaster filled with large rocks),” Hatch wrote in a guest column to the Gazette. “So many places, even some on relatively high ground considered safe, were very vulnerable.”

According to Hatch, scientists don’t know if rivers ever stop moving, but understanding the ways rivers move can offer important insights on how to mitigate flood risk and build safer infrastructure.

“I think we need to recognize flooding is natural and rivers naturally move over the course of time,” said Andrea Donlon, a river steward for the Connecticut River Conservancy.

The Connecticut River Conservancy is a nonprofit environmental advocacy network for the protection, restoration, and sustainable use of the Connecticut River watershed. They too support the smart development of riverside communities, infrastructure investment, proper zoning and wastewater management strategies with flood risk in mind.

While scientists hesitate to attribute any singular event to climate change, they acknowledge that extreme or abnormal weather events happening with more frequency are a telltale sign of changing atmospheric levels. According to a RiverSmart study, precipitation in New England is expected to increase by 10 to 30 percent (depending on the season) by 2100, brought by potentially stronger and more frequent storms.

Ice jams

Last month, large chunks of ice released by heavy rain and snowmelt floated down the Connecticut River, causing flooding and damage to some parts of the county.

According to the National Oceanic and Atmospheric Administration, ice jams are common in late winter when snowmelt or an early rain flush into the river, breaking up the frozen surface. Ice jams happens when these broken pieces pile up on the riverbank or under low-lying structures like bridges, forming a temporary dam. When an ice jam breaks suddenly, everything downstream is vulnerable to a flash flood.

“This year has been interesting, because we’ve had snow and then these midwinter, high-flow events because of rain on top of it and a warming trend,” Donlon said.

Edward Capone, a service coordination hydrologist for the National Weather Service’s Northeast River Forecast Center, said there have been 40 ice jams in the northeastern U.S. this year. A 25-year veteran of the NWS, Capone said this is an early start to the flood season with an already high number of ice jams.

“Each event to us is a meteorological event,” Capone said. “One event alone does not indicate a changing climate.”

In Amherst, a footbridge along the Robert Frost Trail over Amethyst Brook was destroyed by what was likely an ice jam. Dave Ziomek, Amherst’s director of conservation and development, said the town plans to rebuild the bridge keeping in mind this risk.

“We will work with the Commission to design (and) construct a new bridge so the abutments are farther away from the brook offering more long-term stability,” Ziomek said in an email.

Floodplain forests

In Easthampton, an ice jam under the Pearl Harbor Veterans Memorial Bridge caused an extension of the Connecticut River, the Oxbow, to flood in January. However, thanks to over 700 acres of preserved wetlands, forest and meadows in the Arcadia Wildlife Sanctuary, the river had room to swell. These habitats are known as a floodplain forests.

“Floodplain forests occur along edges of rivers, which are prime areas for agriculture and development,” said Jonah Keane, sanctuary director of Arcadia. “At Arcadia, we have some great examples of floodplain forest which is a rare natural community.”

Paradoxically, a river’s volatile floodplains are the reason the region has such fertile farmland. When rivers are allowed to flood they deposit fertile nutrients in the soil, making the riverbanks an attractive place for the Connecticut River Valley’s first farmer-settlers.

Currently, a group of scientists are working to restore about 15 acres of abandoned farmland on in the Arcadia sanctuary back into floodplain forest. Mass Audubon’s regional scientist, Tom Lautzenheiser; the Nature Conservancy’s floodplain forest ecologist, Christian Marks; and Eric Ford from the DER are looking for researchers to assist their work.

“In using that as a learning experience and as an educational tool, we can help with more projects around the state,” Keane said. “There’s not a lot of this type of work happening, so this just seems like a good opportunity to learn.”

Their goal is to restore the floodplain area faster than can be done naturally. Fighting invasive species and planting wetland trees like cottonwood, silver maple and shagbark hickory are some methods they plan to attempt.

After farmers clear-cut and cultivated the floodplain forests in the 18th century, the Industrial Revolution followed, further constraining riverbanks with more bridges, infrastructure and development.

“Down by some of the urbanized parts of the watershed, that’s where rain events get trickier,” Donlon said. “When you have a rain event over land you don’t have as much absorption into the ground.”

Today rivers have much less room to flood, shift and move debris. Without floodplain forests as a buffer, water moves faster and more violently during a flood event, accelerating erosion and making for more dangerous flood conditions.

“All these cities and towns have roads called River Road,” Capone said. “Don’t buy a house on River Road.”

According to Keane, ice jams in Arcadia and flooding scraped the bark on some trees, but didn’t cause any damage. In the springtime, he said, kayakers often paddle between the submerged trees.

“It’s really an important natural process that we try to control because we’ve built so much infrastructure close to rivers in flood plains,” Keane said. “But really, natural systems can handle floods just fine, we just get it in way.”

Decades of data

With data provided by about 7,400 stream gauges in the country, the U.S. Geological Survey helps the National Weather Service forecast floods and warn citizens of any impending danger.

“The USGS has their feet on the ground collecting gauge data,” Capone said. “These models are constantly being updated with new data.”

Richard Verdi is the chief of hydrologic surveillance for the USGS Water Science Center, overseeing a team of 15 hydrologists who periodically check 126 stream gauges in Massachusetts, in addition to more in Rhode Island. Their data is widely used by USGS, the Federal Emergency Management Agency, the Army Corps of Engineers and the Department of Homeland Security to determine flood risk, inform dam operations, and building and bridge design, according to Capone.

“The more data you have the more accurate you will be in statistical analysis,” Verdi said. “It’s good to have at least 10 years of continuous data, but when you have 100 years, it’s even better.”

In 1904, the USGS installed the first steam gauge along the Connecticut River in Montague. The average flow rate for the Connecticut River in the last century has been 14,280 cubic feet per second, according to data provided by Verdi, with dramatic fluctuations seasonally. In early spring, The USGS also monitors snow depth and water content to prepare for peak flood season due to snowmelt.

“Flows are highest basically in the month of April,” Donlon said. “It’s a huge watershed and the northern portions get a fair amount of snow so every spring we have higher flows as the snow melts.

Using historical data from USGS combined with other weather forecast metrics, the National Weather Service is able to create flood forecasts and warn residents of impending danger. They categorize floods as minor, moderate, or major, with minor involving road flooding and inconvenient commutes, and major involving major evacuations and infrastructure damage.

The flow gauge on the Mill River, installed in 1939, is used to a much smaller average flow rate at 101 cubic feet per second, according to Verdi. The historic peak for the Mill River, 7,020 cubic feet per second, came just after tropical storm Irene hit on Aug. 28, 2011, dumping over seven inches of rain on the region in 12 hours.

Irene caused some of the worst flooding the region had seen in decades, due to an already saturated landscape and unanticipated severe flooding. After the storm, FEMA provided Massachusetts more than $29.7 for public recovery assistance, and over $5.5 million in assistance to individuals, according to a study by USGS.

However, the flood of 1936 still remains the flood of record for much of the Connecticut River Valley, when a heavy rain melted a thick layer of snow that washed into the watershed. According to USGS data, the flow rate on the Connecticut River peaked at around 236,000 cubic feet per second, seven times the average flow rate.

“There were boats out in downtown Northampton,” Capone said.

A dam-building boom in the 1950s has since prevented flood events of the same scale, Verdi and Capone said, but they introduced another hazard. Of the 1,454 state-regulated dams in Massachusetts, 290 are considered “high hazard” potential, meaning failure would result in loss of life, according to Capone. It would take between 15 to 20 inches of rain in a single day to breach a high hazard dam, said Capone.

“The hazard classification is not classifying the integrity of the dam, just what may happen in the unlikely case of a catastrophic failure,” Capone said.

Flood frequency is usually talked about in terms of the likelihood they will occur within a certain number of years. Floods are referred to as 10-year, 50-year, or 100-year floods. However, scientists say this is a misleading measure. With the potential for more powerful and frequent storms due to climate change, there can be more than one “100-year flood” in a decade. Data is limited and the climate is changing, scientists say, so our approach to river landscape management must as well.

Sarah Robertson can be reached at srobertson@gazettenet.com

Author: Going Green

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