The province has defined rules by which it is able to operate the Red River Floodway. ((John Woods/Canadian Press))

It is one of the world’s engineering marvels, and envied by flood-prone communities in both Canada and the U.S.

But to Winnipeggers, the Red River Floodway is salvation from the "Red Sea" that awakens in the spring.

The floodway opened in 1968 after the devastation of 1950. In May of that year, 100,000 people fled their homes as one-eighth of Winnipeg went under water.

Since then, the floodway has been called to action more than 30 times. The Manitoba government credits the diversion with preventing $30 billion in damages.

Operating the floodway however, is a delicate balance between two imperatives. Manitoba’s largest city and economic heartbeat must be protected from major flooding. At the same time, communities outside the diversion should not pay the price by being subjected to un-natural water levels.

This precarious compromise is accomplished through four rules, dictated by the Water Resources Administration Act and enforced by Manitoba Water Stewardship.

Rule 1: Normal operation

Steve Topping, the Executive Director of Regional and Operational Services at Water Stewardship, says most floods are governed by this rule.

The devastating flood of 1950 triggered the mass evacuation of 100,000 Manitobans. ((CBC)) Although the profile of the Red River changes as it flows northward through Winnipeg, flood officials use the James Street pumping station as a benchmark.

Under Rule 1 the floodway can be used to hold the river to 24.5 feet (7.47 metres) at the James station, and levels south of the city, or upstream, are spared un-natural levels.

Topping says Rule 1 saves Winnipeg from significant flooding.

"The objective is to control a maximum of 80,000 cfs (cubic feet per second) through the city. I call it the safe capacity that ensures that the freeboard on the primary dikes is not compromised."

Rule 2: Major flood

It’s a controversial rule that provides some protection to Winnipeg’s infrastructure, but allows levels south of the city to exceed natural levels.

Topping describes natural levels as "very much a theoretical calculation." It estimates what river levels would be if Manitoba had no flood protection whatsoever.

A new rule for the floodway? During public consultations on the rules of the floodway, in 2010 Topping says Winnipeg residents had a "loud and clear" message. They believe the floodway should be used to regulate river levels – outside of flood season or summer storms. Topping says a Rule 5 could be in the future for Winnipeg’s floodway. "It would have boating benefits, benefits to the walkway (at the Forks), and all the businesses associated with the walkway. The river taxi. The city’s parks would be more accessible to the public. The river, in general would be more accessible". That walkway at the Forks goes under water at about 9 feet James Avenue. To protect it, and other recreation and tourism locations in Winnipeg, the floodway would have to operate at levels several feet below current regulations. And upstream communities would again be on the receiving end of Winnipeg’s unwanted water. Topping’s Department is now gathering research on what the impact would be on fish habitat and the environment along the Red River. It’s also considering what compensation would be required for upstream property owners.

"That includes all flood control works. The Portage Diversion, the Shellmouth Reservoir, the 100 kilometres of dikes along both sides of the Assiniboine River west of Winnipeg, and the Red River floodway".

In 1997, during the "flood of the century" – and for the only time since it opened – the floodway was operated under Rule 2.

"Once we reach 24.5 at James Avenue and the crest is still not here at Winnipeg, we’re anticipating more water to come. We put the city on notification that you need to raise the primary dikes because we’re going into an emergency flood event".

Topping says upstream communities were also warned, and the floodway began operating under Rule two. The result was 2 feet of artificial flooding in those communities.

The 1997 flood forced the evacuation of thousands of people.

Damage was estimated at $450 million.

As a result of the 1997 experience, close to $1 billion was spent on ring dikes, raising homes, buy-outs of some properties, and the expansion of the floodway.

It can now accommodate a one in 700 year flood.

Rule 3: Extreme flood

Topping says it’s a rule he hopes is never used.

"It’s beyond a 700 year event. In essence, the floodway is maxxed out in capacity. The city is maxed out in terms of the amount of water they can handle even after they’ve raised their primary dikes. It’s an extremely rare event. We cannot do anything more to control the water".

In this scenario, the floodway is diverting 140,000 cfs around the city. Its sole purpose is to mitigate damage to Winnipeg, as much as possible.

Rule 4: Summer flooding

Topping says in 1993 two intense summer rainstorms swept through Winnipeg. River levels rose rapidly and drainage and sewage systems were overwhelmed. The result was $140 million in damages as basements flooded and city infrastructure was compromised.

At the time, many saw it as a rare event.

But there were major summer rainstorms again in 2002, 2004, 2005 and 2010.

So, although the initial purpose of the floodway was to regulate spring flooding – Topping says that changed in 2005 when Rule 4 was created.

"This must be a function of climate change because the propensity of high water levels in the summer has increased over what we’ve seen in the past. Rule 4 allows us to operate the floodway to reduce the risk of basement flooding, which has health risks and causes damage to the city of Winnipeg."

The effectiveness of Winnipeg’s sewage and storm water system declines as river levels rise. By the time it reaches approximately 18 feet James Avenue, manholes must be sealed and pumps are activated to keep wastewater moving.

Topping says under Rule 4, the floodway can be activated at 14 feet James if heavy rain is forecast.

Red River Road, which is just south of the city of Winnipeg, was closed in April 2009. Local residents claimed the water level increased at least a metre since the raising of the Winnipeg floodway gates and caused this flooding. ((John Woods/Canadian Press))

"The benefit is if we can lower the water levels in the city it gives pump stations greater capacity. The end result is the city is able to lower the pressure on the system such that it’s below the basement levels of homes."

Under this rule, upstream communities are on the receiving end of water beyond natural levels.

Topping says levels south of the city can run 6 feet above normal, and the impact can extend as far as Morris.

The floodway has been operated four times under this rule since 2002.

About 120 residences, farmers and market gardeners have received compensation for damage to property or lost business.

Last line of defence

The Red River Floodway has evolved since 1968.

It began as the last line of defence against the unpredictable Red Sea in the springtime.

It matured into the city’s protector during unprecedented summer storms.

In the future it’s likely to take on a new role — as defender of Winnipeg’s economic health and quality of life.

Julie Bell is CBC Manitoba's City Hall reporter in Winnipeg.