WESTBROOK — A large landslide completely blocked the Presumpscot River in Westbrook on Wednesday afternoon, threatening nearby water and gas mains and triggering a flash-flood watch and an emergency order from the city.

The landslide was reported Wednesday morning after people saw trees moving on the river’s forested bank. The landslide, which happened directly behind Les Wilson & Sons excavators at 161 Warren Ave., also took a large area of soil and pile of fill.

The landslide was so large it initially blocked the river just downstream from the Sappi Mill, Westbrook Fire Chief Andrew Turcotte said Wednesday afternoon.

The National Weather Service issued a flash flood watch that was in effect until nearly 10 p.m.

A landslide in Westbrook on Wednesday morning temporarily blocked the Presumpscot River, causing the water to back up.  Roger McCord for the Portland Press Herald

“River levels behind the slide are rapidly rising. Expect water levels upstream of the landslide to continue to rise. Downstream of the slide, sudden changes in river level are possible if the slide breaks,” the weather service said.

But by Wednesday night, Turcotte said the immediate risk of flooding had passed.

“Sappi dam engineers were able to significantly lower the water to reduce the flow,” he said. “It appears that there’s a breach somewhere in that artificial dam that was created from the landslide so there’s water flow now.”

It wasn’t clear how much the river rose behind the landslide before the it appeared to begin flowing at least partially through the area around 2:30 p.m..

The weather service updated its assessment of the conditions in a tweet around 10 p.m.

Mayor Michael Foley declared a state of emergency in the city following the landslide. He said the city was working with the Maine Department of Environmental Protection, the U.S. Army Corps of Engineers, Sappi and other agencies to address the situation. An emergency operations center was established, he said.

Foley asked people not to attempt to go to the area to view the damage.

Turcotte said crews on the scene Wednesday afternoon assessing the potential flood impact also were assessing the possible impact to a nearby 60-inch water main and a 16-inch gas main.

There were no reported injuries and there are no impacts to roads in the area, Turcotte said.

The cause of the landslide wasn’t immediately clear. The area is known to have a layer of soft clay deep underground that was linked to a historic landslide across the river.

Lindsay Spigel, senior geologist at the Maine Geological Survey, said she visited the site Wednesday but didn’t get too close because the nearby land was still unstable. She cited that soft clay layer – called the Presumpscot Formation – as the cause of this landslide but couldn’t say for sure what triggered its movement.

She said it could have been the weight of the fill on top of that clay layer or the shaking of heavy equipment. Or, she said, it could have been a fluke.

“It’s a really interesting spot for a landslide,” Spigel said. “It’s not one I would have picked for the next landslide. It’s also a very unusual time of year. Usually, our landslides happen in the spring when the earth is very wet.”

Chris Wilson, owner of Les Wilson and Sons, an excavating contractor, said the slide began around 9 or 9:30 a.m. and consumed piles of aggregate he was keeping on his lot.

Wilson said his property extends back toward the river from Warren Avenue, but the property line stops before the river. Sappi and the Portland Water District own the land closest to the river, he said.

“After their land failed, my land and the neighbor’s land followed,” Wilson said. “A lot of money went over the edge,” he said, referring to the wasted material.

The sliding earth eventually consumed one pile of dirt and half of a second pile, and dragged a metal building and some equipment with it. Cracks in the earth were visible in Wilson’s equipment yard near the edge of the slide.

A viscous substance oozes from the ground after a landslide occurred behind Les Wilson & Sons excavators at 161 Warren Ave. in Westbrook on Wednesday morning. Derek Davis/Staff Photographer

Mike Maples, who rents a garage nearby and went to the site of the slide Wednesday afternoon, said that decades ago the area that is occupied by Les Wilson and Sons used to be a large gully that led to the river. But over the years, Maples said, Wilson dumped fill into the ravine, reclaiming more space.

“That used to be a big ditch and they filled it in,” Maples said.

Another witness, Jeremy Tardif, who was working on a job site finishing a concrete foundation that overlooks the area, said a bucket loader on the Wilson property was dumping buckets of earth on top of a large pile before the slide.

“The next thing you know, you see everything moving and it kept going,” Tardif said. “The bank kept moving and moving.”

A shed was destroyed when the landslide occurred behind Les Wilson & Sons in Westbrook on Wednesday morning. Derek Davis/Staff Photographer

Wilson dismissed the idea that his company’s piles of fill and aggregate and the daily operations with heavy equipment might have contributed to the unstable soil conditions.

Wilson said the piles of road-building material and clean fill have been there on and off for years, growing and shrinking with the various construction projects that require it.

Wilson also pointed out that his property is about 200 feet from the river’s edge, and that nothing has changed recently in how they do business. The gully that existed near his property was filled in at least a decade earlier, he said.

“Nothing’s changed with our operations or footprint or anything,” Wilson said.

The landslide destroyed a metal building at 161 Warren Avenue in Westbrook on Wednesday morning. Staff photo by Derek Davis

Wilson also pointed out that Wednesday’s slide is across the river from the site of the state’s largest landslide, which occurred in 1868, when about 20 acres shifted, eventually covering about 38 acres with displaced soil, according to materials published by the Maine Geological Survey.

A geologic investigation of the area in 1990 found that a layer of soft clay lies beneath the area and extends deep underground. The softness of the clay means that the material behaves like a viscous fluid that can “flow” rather than break into discrete pieces.

This layer is referred to as the Presumpscot Formation, and a later 2008 geological survey found that the formation surrounds the river outside of its immediate banks.

“It’s really something that people need to be aware of as things become more developed in southern Maine,” said Spigel, the senior geologist. “There is this clay and it’s hazardous, and we’ve been lucky that no one has been hurt.”

Staff Writer Megan Gray contributed to this report.

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