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Report Concludes Winslow Dam Removal Didn't Trigger Landslide

April 22, 2010 - WINSLOW -- The summer 2008 removal of the Fort Halifax Dam "did not play a significant role" in a recent landslide that occurred on a slope with a historic cemetery on top, according to a report released late Wednesday.


Instead, the report cites as top factors the slope's steep angle, unstable soil conditions, recent heavy rain and a small earthquake that was recorded 41 miles away.


Ever since the landslide was discovered March 31, Winslow officials and others have been awaiting the assessment's findings anxiously to decide what should be done and who's responsible for taking action.


The 58-page report, which also includes several photos and charts, was completed independently by Dr. Craig Findlay of Findlay Engineering Inc. in Yarmouth and commissioned by the former dam's owner, NextEra Energy, also called FPL Energy Maine Hydro LLC. It was ordered by the Maine Department of Environmental Protection.


NextEra spokesman Steve Stengel said Wednesday that a copy of the report had been sent to Winslow town officials and they were reviewing it.


"Once they have completed their review, we will discuss next steps with them," Stengel said.


Both Town Manager Michael Heavener and Code Enforcement Officer Frank Stankevitz were out of the Town Office this week and unavailable for comment.


Fire Chief David LaFountain, who has been monitoring the slope regularly since the slide, said the findings surprised him, though he noted that the science involved in the report "is out of my league." Still, he said town officials were considering commissioning their own study for comparison.


"Last I knew, the town was leaning toward having their own study done, but that's going to be determined by the town manager," LaFountain said.


Officials with NextEra Energy Resources discovered the landslide on March 31, noticing a large chunk of top-layer soil had gone missing and several trees had fallen into the Sebasticook River. The slope is between the former dam and Dallaire Street, where homes were removed last month after long-standing concerns about stability there, and it is known as Cemetery Hill because of the Fort Hill Cemetery that's on top.


The landslide has raised fears about the slope and cemetery and re-ignited criticism from people who opposed the dam's removal. It has also left open the question of who -- the town, the state or the company -- should be responsible for the results. FPLE was required under its dam-removal town permit to "closely monitor the slope adjacent to the cemetery and promptly remediate any slumping or erosion" in order "to protect the Fort Hill Cemetery from irreparable harm."


The Findlay report appears to exonerate dam removal, meaning FPLE/ NextEra Energy would not be required to do anything.


The report concludes that the dam removal and subsequent drawdown of water level in the reservoir didn't create enough pressure that could have contributed to the slide, and that any such pressure "would be substantially dissipated" because "it had been on the order of a year and a half since drawdown."


Findlay believes that the landslide, which nobody apparently saw happen, occurred either the night of March 30 or morning of March 31. Evidence suggests that when the slide happened, it "caused a wave that over-topped the opposite river bank," the report says.


In listing the most likely contributing factors, the report points to "record-making wet weather," with the National Weather Service reporting March as the ninth-wettest on record in Portland. During the month of March, Winslow received 6.63 inches of rain, compared with an average 3.63 inches, the report states, and more than half of that rainfall occurred just days before the landslide.


Also, the slope, consisting of a "sandy superficial ground" on top of stiff clay, already had "marginal" stability because of its steep angle, the report says. The result of unusually heavy rain and an already unsteady slope means the occurrence of a slide "is not surprising."


"As the sand becomes saturated, its weight sensed by the underlying clay increases significantly, possibly on the order of 20 percent or more," the report states. Thus, "the weight of the water within the sand and other superficial soils and brown desiccated clay increased the overall weight of these materials to trigger the slope failure."


Furthermore, although perhaps not as significantly, the report notes that the U.S. Geological Survey recorded a 3.0-magnitude earthquake at 4 p.m. March 30 in the town of Bucksport, which is near Bangor and about 41 miles away from Winslow. A smaller 1.1-magnitude "event" occurred shortly afterward, too.


"The timing of this event and the occurrence of the slide cannot be discounted, although it is noted that the ground motion would likely have been small," the report states. "However, in combination with the above-normal rain experienced in the area that resulted in saturated ground condition, the combination of the increased pore pressures and slight seismic ground motion could contribute to the triggering of the slide."


By Scott Monroe, April 22, 2010, Kennebec Journal

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