Dredging, not bridge, threatens salinity levels in lower Fraser
The provincial government’s plan to replace the George Massey Tunnel with a new bridge near Ladner will have no significant impact on salinity levels in the lower Fraser River, a water and agriculture conference in Richmond heard last Thursday.
However, increased dredging of the Fraser River to accommodate bigger ships once the tunnel is removed stands to have a greater impact in the near term on salinity than rising sea levels, said Albert Leung, a consulting engineer with Tetra Tech, which conducted a study for the Delta Farmers Institute.
Leung said the tunnel, opened in 1959, represents about a one-metre hump on the bottom of the Fraser and compares with sediments in the river up to three metres high.
“When you look at a scale of the river, it’s a little feature,” Leung told Postmedia News.
Leung said the foundations of the planned $3.5-billion 10-lane toll bridge would also not impact salinity in the river. Removing the tunnel “opens the door” to port- related dredging to accommodate larger ships headed upriver, allowing a “salt wedge” to creep up the Fraser, he said.
Currently, ships with drafts up to 11.5 metres can navigate upriver, he said. He modelled numerous scenarios at various locations in the river in the event dredging allows ships up to 13.5 metres over the next 10 to 25 years.
“That would allow the next generation of Panamax ships to come in,” he said.
“The one thing that jumps out to me is the effect of the deepening of the channel. When you deepen it, you give the salt wedge more opportunity to creep up the river. That is very significant … (and) the impact could be very imminent.”
The impact of dredging would be exacerbated by the effects of climate change — not just rising sea levels, but also wetter winters and drier summers, which would affect the ability of farmers to extract water for irrigation.
The wedge is influenced by factors such as sea levels, tides and flows down the Fraser River, with fresh water located above salt water.
When the river becomes too salty, it cannot be used by farmers for irrigation. During dry summers or early freshets, the window for Delta farmers to draw upon the municipal irrigation system narrows.
Hugh Fraser, deputy director of engineering for the municipality of Delta, said the Fraser River becomes unusable to farmers once salinity levels reach an average of about .35 parts per thousand. One of the municipal water intakes is located at 80th Street, where Fraser River water is drawn in when condition permits and shipped through an extensive ditching system. Once river flows at Hope dip below about 2,000 cubic metres per second, irrigation becomes marginal near the river mouth.
The municipality encourages farmers to do their own monitoring since salinity levels can vary throughout the system. Farms on Westham Island closest to the ocean have greater salinity problems.
The province hopes to begin construction of the bridge next year, with completion by 2022.