Underwater surveyors had detected some erosion under a transmission tower by the Fraser River a day or two before it collapsed on Monday, but the erosion was not significant enough to be of concern, BC Hydro says.
Nonetheless, the tower fell into the Fraser River around 9 p.m. Monday, cutting power to 25,000 customers in Surrey, and in a written statement sent out the day after, the utility blamed the incident on "sudden and unexpected erosion." The tower, which supports a 230-kilovolt line, had collapsed soon after crews, in a separate operation, had inspected the footing and deemed it secure.
"We had an ongoing survey, looking at any movement or tilt in the tower and the last reading was taken half an hour before [the tower fell] and it showed no movement at all," said Julius Pataky, vice-president of asset investment management, transmission and distribution with BC Hydro. "There [was] some evidence [from an underwater inspection] of some erosion, but it was not deemed to be severe."
A thorough analysis of the soil support structure upon which the 230-kilovolt tower stood was last conducted in 2010. But recent inspections of the tower were done as a precaution because an adjacent 500-kilovolt tower had been damaged and the area around that tower showed signs of erosion. The neighbouring tower was reinforced with support lines and rocks, called rip-rap.
Mr. Pataky said the tower failure was "quite unexpected," particularly when an annual flood review conducted six weeks ago for transmission towers that are in or near B.C.'s rivers had concluded that, despite higher river levels this summer, nothing unusual was anticipated.
"Rivers are very unpredictable," he said. "Debris in the river that comes down may get lodged, water-logged trees that get caught up and capture sand can changes flow patterns, and a number of things quite external to our business may have had an impact on the erosion, like other construction activity in the vicinity."
Since Monday's incident, crews have re-inspected a dozen towers that cross the Fraser River by measuring any tilt or movement, and have deemed them all structurally sound, Mr. Pataky said.
High snowmelt and a rainy spring have contributed to elevated river levels across the province, but on Monday, the Fraser River peaked at almost six metres in Mission – about two metres higher than normal, according to B.C.'s river forecast centre.
"It is a significant increase, definitely quite a bit higher than what would be typically seen in an average year," forecast hydrologist Luanne Chew said. "Typically, if the water level remains elevated for a higher period of time, the soils along the banks of a stream or river could become more saturated, which could increase their instability."
Simon Fraser University geographer Jeremy Venditti said sudden erosion could appear to occur if one was not paying attention to movement at the river bottom during high water levels. Shifting sand could direct flows to the bank and scour it.
"There may not have been any erosion directly at the tower when [BC Hydro] last checked it," he said. "The river bed is obviously moving around a lot right now because of the high flows … it may not have been obvious right there at the bank that something was going on, but it would have been obvious further on in the channel where flows are being directed toward the bank."
Mr. Venditti said repeat mapping of the river bed and estimating the volume of sand moving through the river can help predict flow trends, as well as erosion and deposition patterns, which in turn would indicate areas that are potentially hazardous.
BC Hydro is planning to remove the nearly 60-year-old fallen tower from the Fraser River and to reconstruct the 230-kilovolt circuit. It will also conduct a full review of the incident and report findings publicly to the British Columbia Utilities Commission within 30 days.