[B]ENNINGTON — Streambed work done along the Roaring Branch before Tropical Storm Irene struck in 2011 is being recognized by the state as an example of how to work with a river’s natural flow, not against it.
In town as the anniversary of the storm neared, Josh Carvajal and Todd Menees, engineers with the state Watershed Management Division’s Rivers Program, pointed out locations along the river that illustrate Vermont’s favored approach to flood and erosion control.
The volatile Roaring Branch flows rapidly downslope from the Green Mountains in Woodford, headed toward flatter sections of Bennington and the Walloomsac River. During the Aug. 28, 2011, storm, raging water ripped up berm work, swept away or destabilized streambank structures and collapsed a major bridge on Route 9 in Woodford, cutting off the main route toward Wilmington and Brattleboro.
After the rains stopped, riverbed in sections of Bennington was stripped of vegetation and packed with rocks, sediment and debris from upstream, and lower-lying areas around the confluence with the Walloomsac River were inundated.
But it could have been worse, Carvajal said, if not for the town’s participation, beginning in 2008, in a state Department of Environmental Conservation initiative to discourage streambank development and allow rivers wider channels in which to roam.
Effects of berming
“Historically, the Roaring Branch has had berming along it,” Carvajal said, installed on each side by property owners or the town to protect land and structures from flooding.
But each major storm, typically arriving every 10 to 15 years, caused flooding, filled the riverbed with sediment and sometimes threatened or ripped away berms, levees or buildings.
Dredging out the excess sediment after a storm — the traditional flood-control response — tended to merely increase flow rates within the artificially narrowed riverbed and likely contributed to damage and erosion during the next storm.
The critical areas of the Roaring Branch, as with other Vermont rivers, Carvajal said, are where the slope begins to flatten into an alluvial fan. That’s where sediment is deposited and builds up, forcing the water into multiple, meandering channels. The effects are intensified during major storms.
Sediment “creates multiple channels coming down, and it is very hard to have infrastructure, homes adjacent to that, because these are such dynamic areas of streams,” Carvajal said. “That’s usually when we run into problems.”
He and Menees said that 250 years ago, before the town was settled, annual deposits of river sediment had formed an alluvial mound across the north side of present-day Bennington, which can be envisioned as a rounded shape leading up to both sides of the Brooklyn Bridge on North Branch Street.
Today, a cement flood-control wall near the bridge and numerous sections of earthen or riprap berms — one piece protecting Mount Anthony Union High School off Park Street — keeps the river out of much of the town center, sometimes with difficulty, as during Irene.
But berm work also edged inward over the years from the opposite bank, marching over time toward the center of the river channel, the engineers said. That created a narrow, relatively straight channel and boosted stream flow, leading to a cycle of storm damage and rebuilding with each storm.
A narrow channel, without natural meandering, “shoots the river down quicker,” Carvajal said, resulting in overflowed banks and flooding downstream — such as where the Roaring Branch enters the Walloomsac River.
New approaches
The idea of allowing rivers to meander and slow from their spill down from the mountains has led to new approaches to stream management over the past two decades.
“It’s a change of mindset for people if they have not seen a natural stream,” Menees said of a river without a straight, narrow channel.
“It looks messy,” he said, causing some people to wonder, “why isn’t this straight?”
In highlighting river projects like those on the Roaring Branch, the engineers hope to spread the message that these methods are better at controlling flooding and erosion and reducing the costly damage than traditional berming and dredging.
In 2008, Bennington and the DEC formed a partnership to try to reduce flooding and erosion on the Roaring Branch. That began with a process to adopt what are now called River Corridor zoning bylaws to restrict development along the riverbanks.
The town then became eligible for funding toward the design and construction of a floodplain restoration project, including the removal of 3,500 linear feet of earthen berms and excavation of more than 35,000 cubic yards of earth and rock and construction of a new berm well back from the river channel.
In all, about 12 acres of floodplain area was restored or created, providing more room for the river in key sections near the town center. In particular, that pre-Irene project is credited with reducing overall storm damage, particularly for businesses along Kocher Drive.
“This was a very good project for the whole state to see,” Carvajal said.
The pre-Irene floodplain project cost an estimated $750,000 but was estimated to have prevented tens of millions of dollars in damage to public and private property and infrastructure.
After Irene, the town spent more than $6 million on repair or restoration work, all but about $1.5 million covered by federal disaster relief funding or insurance payments.
Town Planning Director Dan Monks said that this much larger floodplain project, designed by Roy Schiff of Milone & MacBroom, who also designed the pre-Irene project, expanded and continued the process of recreating a more natural flow patterns for the Roaring Branch.
He said the engineering firm also now periodically measures sediment levels, particularly around bridge abutments, to point out areas that might need more work prior to the next big storm.