The 700-metre zig-zagging line along the deficient eastern edge of Mission City Dike which would need to be sandbagged in the event of extreme freshet. The endeavor would require a “major mobilization,” according to a city engineer.

The 700-metre zig-zagging line along the deficient eastern edge of Mission City Dike which would need to be sandbagged in the event of extreme freshet. The endeavor would require a “major mobilization,” according to a city engineer.

State of Mission’s diking systems detailed in report

Two major pump stations need replacement

Many are eyeing the Fraser River with concern as a cool Spring and rising snowmelt rates into June have forced evacuations in parts of B.C.’s interior.

The current risk to the Lower Fraser remains low, according to recent freshet forecasts, but the conditions highlight the vulnerability of Mission’s dikes in a major flood.

Public works staff presented a report to the council on June 6 on the state of their two dikes and three pump stations, a review of their performance in November’s atmospheric rivers, and future plans to start to address their major deficiencies.

“One gap is all it takes to flood an entire area, that’s an important thing to remember,” said Brendan Schneeberger, planning engineer with the city.

“You can have high dikes for 90 per cent, but if that 10 per cent is too low, then the water is going to come in and it’s going to flood the whole area.”

All of the city’s dikes were given “sub-standard” ratings in the provincial 2015 dike assessment, and portions were rated as unacceptable.

Both dikes protect sites of interest for future development in Mission, and no upgrades have taken place since 2011.

RELATED: Evacuations, local state of emergency in northwest B.C., as flood risk rises

The Silverdale dike runs 4.1 kilometres and protects 570 acres of mostly agricultural land; it has two pump stations. There are 15 points in the dike where it does not mean the flood construction level (FCL) of 10 metres above sea level.

The dike falls well below the current standards. It’s too narrow and the sides are too steep.

The city doesn’t know what materials were used to build the dike, and it does not have packed aggregates and an impermeable core. This increases the risk of collapse if water levels rise too high, and makes it susceptible to earthquakes.

Upgrades are further complicated, as the city doesn’t have legal right-of-way for 60 per cent of the length, and many of the major ditch culverts are owned by the province and railway company.

Even if the dike were fully raised, two gaps would still exist; one along the highway, which would need to be closed and sandbagged; and another area too close to the river with no erosion protection.

Infrastructure surrounding the Silverdale floodplain also exhibited defects during November’s flooding.

The largest, Chester Creek Pump Station on the southside of the Lougheed Highway, is well below the FCL and came close to failure, according to Schneeberger.

“The water got really close to the floor of the pump station. Some wires were actually already starting to be submerged,” he said. “If it fails … then it’s completely gone, and things go from bad to worse.”

The Mission City Dike is 3.5 kilmetres long and protects 370 acres of mostly industrial and commercial land; it has two pump stations, but only one is operated by the city. There are has six points in the dike below the FCL.

There have been numerous upgrades to the dike (2007, 2009, 2011), but the eastern edge has no defined structure, and there are several gaps which would need to be sandbagged in the event of a record flood.

Schneeberger said most of the gaps would be easy to fill in the event of a major freshet. But for the eastern gaps, a major mobilization would have to occur to temporarily dike approximately 700 metres in a zigzagging line to avoid private property.

“It would be quite an undertaking,” Schneeberger said.

The city has no right-of-way for 50 per cent of the length of Mission City Dike.

While the capacity issues of the city’s Lane Creek Pump Station are not as concerning as Chester Creek’s, it is also below the floodplain and in need of replacement.

Major development is planned for Mission’s waterfront area, which includes building a superdike to raise the entire flood plain to the FCL.

This will leave each individual landowner responsible for the expensive upgrade, and will likely require land assemblies by large developers along the eastern edge where parcels are fragmented.

RELATED: Building Mission ‘superdike’ likely requires developer land assembly

Though the current flood risk to the area protected by Silverdale Dike has been deemed of lower consequence, the area has been identified as a special study area for future employment lands by the city.

Schneeberger said potential updates to the developer cost charges bylaw are going to be critical for funding large upgrades for drainage, pump and diking systems.

Both the Lane Creek and Chester Creek stations will need to be either replaced or retrofitted, and low lying drainage improvements are also needed in Silverdale.

Consultants and engineers will need to be hired to come up with designs so the city can start to apply for provincial grants.

Further updates need to be made to the city’s Flood Response Plan, and new emergency supplies need to be purchased, such as tiger dams and lock blocks to replace sandbags.

Council passed a motion unanimously to set up a workshop with public works staff to develop priorities, and another to meet with provincial MLAs and the Fraser River Basin Council to strategize on provincial diking and drainage.


@portmoodypigeon
patrick.penner@missioncityrecord.com

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