Expanding Winnipeg’s Floodway

The slow and easy meandering of the Red River towards Winnipeg belies the dangers lurking in its waters. Six years ago, in the spring of 1997, the river’s flow north from North Dakota and Minnesota flooded a vast area of southern Manitoba, creating the “Red Sea.” The world watched as Winnipegers frantically heaved sandbags into piles in what seemed a futile attempt to shore up the city and protect their homes and businesses against the waters. Their valiant effort succeeded, though many communities around Winnipeg were not so lucky and saw their farms and homes engulfed.

Winnipeg’s narrow escape in 1997 was only one in a series of disasters that have bedeviled the city. It was probably not a good idea to build a city on such a flat flood plain in the first place, and as Norman Ball reports in Mind, Heart and Vision, a history of Canadian engineering, back in the 19th century Sir Sandford Fleming had suggested Winnipeg be located elsewhere. However, Fleming’s advice was ignored and the city grew despite the periodic danger of being overwhelmed by melting spring ice.

After the last “Great Flood” in 1950, the government of Manitoba constructed the great Red River Floodway, a massive channel and earthworks to divert water around the city. Completed in 1968, the Floodway has protected Winnipeg over 20 times in the decades since it was completed, but the 1997 “flood of the century” was a wake-up call, and prompted the Manitoba government to look for ways to reduce the risks.

Expansion or new dam?

KGS Group of Winnipeg has led the studies to discover what should be done. After considering over 15 options, the consulting engineering firm was hired in 2000 under the Canada-Manitoba Flood Reduction Agreement to do the “SAFE” (Ste. Agathe Floodway Expansion) study that compared the most promising two alternatives. They looked at either expanding the existing Floodway around the city, or building a new dam to hold back the waters near Ste. Agathe, a village on the Red River about 30 kilometres south of Winnipeg.

To compare the two schemes, KGS carried out engineering, environmental and socio-economic studies, and used computer hydrogeology models for the very complex problem of predicting the impact on groundwater resources in the area. Public meetings were held to help local people make sense of the issues.

The proposed Ste. Agathe scheme included a dam 40 kilometres long across the Red River and adjacent floodplain, and gated control structures on the Red River and two of its tributaries — the Rat and Marsh Rivers. It cost less than the other option — $500 million vs. $600 million — and it could withstand the kind of extreme flood that occurs once in every 1,000 years. At the same time, however, the system would only kick in for floods measured at a 1-in-90 year level of danger, (about the same as the 1997 flood). Also, in order to protect Winnipeg, the Ste. Agathe structure would produce water levels in the Red River Valley upstream that would be higher than in the state of nature (i.e. if the structure didn’t exist). The result would be more problems for residents and businesses in that upstream area to the south.

The other option, expanding the existing Floodway, will protect against a 1-in-700 year flood, has a benefit/cost ratio of 2.5 and reduces the risk of major flood damage in Winnipeg by a factor of five. It will also cause less disruption to the environment than the Ste. Agathe scheme.

An international panel of 28 engineers, scientists, local engineering companies and government agencies endorsed KGS’s conceptual design of the Floodway expansion, and the provincial government has decided to go ahead with it. In April, it joined the federal government in announcing $160 million in funding. The rest of the money still has to be found even though the project is planned for completion in 2009. Winnipeg itself will contribute some of the cost. To compare the expansion project to another famous landmark project, it will involve moving 35 million cubic metres of earth — about the same amount excavated for the Suez Canal, and just less than half the volume of earth moved in the 1968 Floodway project.

Features of the expansion

The existing Floodway system starts with a massive Inlet Structure located at St. Norbert, a few miles south of Winnipeg. When operating, the structure diverts overflow from the Red River into the Floodway channel, a 9-metre deep earth trench that runs for 42 kilometres east and north around the city. Running southwest from the Inlet Structure is the West Dike. It extends for about 45 kilometres across country until it reaches higher ground. Rick Carson, P.Eng., manager of water resources services with KGS, says the purpose of the raised dike is “to stop the river doing an ‘end run’ around the inlet structure into the west side of Winnipeg.”

To increase the capacity of the Floodway, the channel will be made deeper and much wider. The present width of 300 metres will be increased by an average 40 metres, but up to 100 metres in some places. The depth of 8.5 metres will be increased by another 1 to 2 metres. After the changes the discharge capacity during emergency conditions will be 13,000 cubic metres per second compared to about 8,350 cubic metres feet per second now.

Instead of expanding the cross-section of the channel uniformly, the engineers saved about $75 million by adjusting the cross-section at critical points along the way where the channel meets transmission lines, underground aqueducts, pipelines and bridges. Carson calls their approach “threading the needle” to describe how the channel adjusts its path through 13 bridges, two aqueducts, the Seine River inverted siphon, local drainage inlet structures, and 15 transmission lines. The narrower crossing points constrict the water somewhat, but KGS has ensured that the flow conveying capability is made up in other locations.

The channel is to be made deeper partly by excavating, but also by allowing the water to flow at higher levels than were considered in the design of the channel in the 1950s. The result is that water could overtop several existing bridge that were not designed to be totally submerged. Instead of replacing the bridges — they still have some 40 years of service life — KGS found ways to retrofit eight of the 13. Depending on the circumstances, they may add more spans to a bridge, or they may make the channel sides steeper to increase the cross-sectional area and keep the bridge span the same. In some cases they will deepen the channel and strengthen or replace the bridge pier foundations. They’ll also have to reinforce the deck structures of some bridges to resist the drag and impact forces of flood waters.

The Floodway Inlet Control Structure has a pair of massive steel gates shaped like a pie, each 11 metres high and 34 metres long. Once the gates are raised, the water flows over top, taking any debris such as trees down into the Red River below. Because the gates are so critical, the new plan calls for a detailed assessment for adding a set of eight back-up barriers. They would be lowered down from the top of the existing structure if the main service gates fail to operate. Based on KGS’s preliminary designs, the back-up gates would cost around $30 million.

The West Dyke, which ranges from between 8 metres high at the Inlet Control Structure to less than 1 metre at its far end, will be raised by another 1 to 2.5 metres. KGS designed the new freeboard allowance based on numerical models prepared by the Canadian Hydraulics Centre. They also had to consider erosion from wave action. During flooding the area resembles a sea, and winds from the south can whip up waves that batter against the Dyke. Carson says they are studying a variety of mechanisms to protect the dyke. One option is to use riprap, though this would be expensive. Other options include creating a very low upstream slope to the bank to dissipate the wave energy, or growing a natural breakwater of trees and vegetation.

Both the Dyke and the Floodway are left to nature’s devices when floods aren’t rampagi
ng over the land. The structures grow wild with grasses and vegetation, and are home to small animals and birds. Winnipegers already head out to the landmarks for recreational activities like hiking, and skiing in winter. There are now plans for improving the existing ski hill on the dyke, and talk of building a hang-gliding centre. Also on the table for discussion is a whitewater rafting park near the Floodway’s outlet north of Winnipeg, near the town of Selkirk.

Selkirk is one of the most vocal of the communities around Winnipeg where people have expressed concerns over the Floodway expansion. They worry that the changes will increase the dangers to their property. Since the 1997 flood, the provincial government has spent $130 million on mitigating flood dangers in the surrounding regions, and by law it will have to pay financial compensation to those who suffer damages from artificial flooding as a result of the Floodway expansion. There is also the question of whether deepening the Floodway could affect the groundwater conditions in the area.

KGS has been studying these and the myriad other possible conditions that could be changed by an engineering work of such gigantic proportions.