Bridge over troubling waters
Officials say motorists need not be worried about safety of crossing as lake level rises
Okanagan Lake is projected to crest 60 centimetres below the point at which operating changes would be required to the William R. Bennett Bridge, parts of which float on the water.
The lake is forecast to rise to 343 metres above sea level, based on factors such as predicted weather conditions and the rate of snowmelt in the mountains.
The bridge, opened in 2008, was designed to accommodate lake levels as they rise and fall throughout the year.
“Under the current configuration, it is capable of handling water levels up to 343.6 metres above sea level,” Sonia Lowe, a B.C. Ministry of Transportation spokeswoman, wrote Tuesday in an email.
“Beyond that point, engineers will monitor and make modifications to the bridge as necessary to safely accommodate changes in water levels,” Lowe wrote. “Engineers are continuing to monitor the bridge to ensure that it is safe for the travelling public.”
The five-lane crossing is one of only eight such floating bridges in the world, and the only one of its kind in Canada, according to a 2009 article in Canadian Consulting Engineer magazine.
“The floating bridge type was chosen partly because the depth of Okanagan Lake is up to 260 metres, and the soil conditions are problematic for heavy foundations,” the article said.
Just over one kilometre in length, the bridge includes a 690-metre section that floats on pontoons.
“Floating bridges can pitch, roll, yaw, move up and down, and respond dynamically to waves and wind,” the engineering article says.
Since the Canadian Highway Bridge Design Code is written only for fixed structures, other building codes, particularly one from Norway, were used in the design of the William R. Bennett Bridge for issues such as pontoon stability, vessel collision and allowable movements.
“World specialists in floating bridge design, Aas-Jakobsen and Johs Holt of Denmark, were subconsultants” on the William R. Bennett Bridge, the engineering article says.
Earlier this month, in an email to transportation officials, Kelowna resident Donna Crofford wondered about the ability of the bridge’s floating components to rise high enough to accommodate the elevated lake level.
She received an email from Jeff Wiseman, the ministry’s operations manager for the Okanagan Shuswap District, which read in part: “As of May 17, 2017, lake water levels do not pose a threat to the structure and closure of the bridge is not expected.
“The bridge joints, bearings and shorelines are continuously monitored by the concessionaire (bridge builder) SNC Lavalin,” Wiseman wrote. “SNC design engineers are on standby to adjust anchor tension, assess structural components and maintain or repair the bridge as needed during high lake levels.”
Okanagan Lake is only nine centimetres below this year’s projected flood level, and it is expected to continue rising as the threat of flooding remains high in the region.
The lake has been rising three to four centimetres a day for the past few days.
As of Tuesday morning, it was at 342.91 metres above sea level, surpassing the 1990 flood level of 342.87 metres and approaching the 1948 flood level of 343 metres.
The dam on the channel in Penticton was designed to release only 1.5 centimetres’ worth of water out of Okanagan Lake a day.
“We are letting out as much as we can at the moment through Penticton and down the system through Osoyoos Lake and into the U.S.,” said Brian Symonds, director of the B.C. Ministry of Lands, Forests and Natural Resource Operations. “We have the ability to make some adjustments. However, we don’t have total control on the lake. We are, to some degree, at the mercy of what the weather does.”
Even when flows into the lake start to drop, Okanagan Lake will continue to rise until an equilibrium is reached, he said.
“We are going to see high lake levels for some time to come throughout the summer.”
A cooler and wetter April and May led to increased snowpack in the mountains, said Matt MacDonald, meteorologist with Environment Canada.
Hot weather over the May long weekend then accelerated snowmelt, he said.
The Okanagan was expected to see winds up to 70 kilometres an hour Tuesday night, putting already vulnerable shores at risk of erosion and flooding.
Environment Canada is predicting a high temperature of 16 C today with a 40 per cent chance of showers in the evening.
Temperatures are expected to rise to the mid-20s starting Thursday, with above-seasonal temperatures lasting until the end of the month.
As of May 15, the Okanagan snowpack level was at 151 per cent of normal, the River Forecast Centre reported in its May 23 snowpack report.
On May 1, the Okanagan snowpack was at 147 per cent of normal.
The majority of the snow at mid-elevations has melted, but higher elevations still have 80 per cent to 90 per cent of their total accumulated snow from this season, said Dave Campbell, head of the River Forecast Centre.
Over the May long weekend, there were rapid rises in river flows, and that risk will continue for another week or two, said Campbell.
As of Tuesday afternoon, 85 homes were on evacuation order, 195 homes were on evacuation alert and 500 people had registered with emergency services.
There have also been 1.7 million sandbags shipped so far, and another million sandbags available for use.
“There’s still significant threats from rising waters, whether that’s slow-rising lake levels or fast-rising rivers,” said Chris Duffy, executive director with Emergency Management BC. “We need to maintain vigilance.”