Flood is not over, nor is the fight

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One senses that a general malaise has set in about the Lake Manitoba flood, that it is over and that the construction of the emergency channel has solved the problem. But it is far from over and the construction of the emergency channel has not solved the problem. It is only the beginning of a solution to the problem.

Premier Greg Selinger and his government should be commended for the rapid and decisive action they took when they built the new outlet channel from Lake St. Martin last fall at a cost of $100 million. This provides a long-term solution to future flooding for those who were among the worst affected -- the residents of Lake St. Martin. But for those around Lake Manitoba the story is very different: for them there are no guarantees this won't happen again.

The greatest "natural" disaster in Manitoba's modern history occurred because up to 60 per cent of the flow of the Assiniboine River was artificially diverted into Lake Manitoba through the Portage Diversion between April and August last year.

A primary function of the Portage Diversion is to prevent flooding on the Assiniboine downstream of Portage la Prairie. Between Portage and Winnipeg the channel capacity of the Assiniboine falls to just 18,000 cubic feet per second. In the past year, the Assiniboine flow has reached 52,000 cfs and the excess -- up to 34,000 cfs -- was dumped into Lake Manitoba.

This was not the first time. In 1976, Assiniboine flows reached 50,000 cfs, but then the downstream channel capacity on the Assiniboine was much greater at nearly 24,000 cfs. Since the Assiniboine could carry more water, less needed to be diverted into Lake Manitoba.

Since the 1970s, however, we have failed to maintain the Assiniboine channel capacity between Portage and Winnipeg and are now forced to open the diversion at lower and lower Assiniboine flows. And since we can just dump the excess water into Lake Manitoba, why bother maintaining the dikes on the lower Assiniboine?

That question was answered last year.

The use of the Portage Diversion has steadily increased since it was first opened, and it has been open eight of the last 10 years. In seven of those eight years, it was open when Assiniboine flows were well below the 18,000 cfs flood level on the lower Assiniboine.

The diversion is also used to move water away from Winnipeg when there is high water on the Red River. But it now appears to be used more or less routinely "just in case" there might be high water levels on the Red.

The diversion is now used so often that it has effectively re-established a connection between the Assiniboine River and Lake Manitoba that has not existed for 3,000 years. Lake Manitoba is now a storage reservoir.

Overuse of the diversion since 2004 was an important cause of the flooding of 2011/12. Since 2004, Lake Manitoba has sat in the upper reaches of its 810.5- to 812.5-foot operating range. In five of these years, including the fall of 2010, it rose above its normal operating level. With little ability to dump water from Lake Manitoba, continued use of the diversion ensures that lake levels are chronically high.

It was high water in the fall of 2010 that led to very high water in April 2011, before the spring freshet and when the lake naturally rises. With the level at 813 feet at the beginning of April last year, a flood on Lake Manitoba became a virtual certainty.

Operations policy on the Portage Diversion needs to be changed so that it is used only when there is a clear and present danger downstream. Until Lake Manitoba reaches safe water levels at the end of this year, perhaps next year, there needs to be a moratorium on its use except in the case of genuine emergency.

But if overuse of the diversion helped cause the flood, it persists because the flood waters trickle out of an exit far too small. Even with the Fairford Water Control Structure wide open this winter, the net outflow from the lake is less than 5,000 cfs, enough to drop the lake three inches a month.

The Fairford Water Control Structure was completed in 1961 in the aftermath of flooding on Lake Manitoba in the 1950s. It was built to allow better water level regulation and in large part to avoid future floods.

That work was undone with the completion of the Portage Diversion. Finished in 1970, the diversion was built recklessly, making no provision for the exit of the water artificially dumped into Lake Manitoba.

It is long past time to address this problem: the emergency channel alone does not do that. The emergency channel flows out of Lake St. Martin, not Lake Manitoba as many mistakenly believe.

It does serve a useful purpose. Extra water can now be emptied over winter (but only over winter) from Lake Manitoba into Lake St. Martin without flooding Lake St. Martin. A bottleneck has been removed.

The emergency channel provides no help, however, during the open-water season between April and November. And that is when flooding actually occurs.

If we are again caught with our pants down by unexpected flooding this spring, as we were last year, it will again take nearly a full year before anything can be done.

The real bottleneck is the Fairford Water Control Structure. It still has the same limited outflow capacity as it had in 1961 before the diversion was built.

The KGS/AECOM engineering report from last summer detailed a second channel to be built after the emergency channel, a 2,500-cfs outlet directly from Lake Manitoba: this is the Fairford Bypass. Though its capacity is not enough (the inflow capacity of the Portage Diversion is more than 10 times as much), it is still useful and badly needed.

There is a big difference between the 5,000 cfs that is leaving the lake now and the 7,500 cfs that could leave the lake if the bypass was present. (I note that the current outflow through Fairford is just under 14,000 cfs, but since 9,000 cfs is still entering the lake through the Waterhen River, the net outflow is about 5,000 cfs.)

Used over the full year, the Fairford Bypass would allow water managers to drop lake levels an extra 18 inches and would be an important tool in new water management policy on Lake Manitoba, one that actually creates room in the reservoir before flood waters are poured in.

But our provincial government has not yet committed to building the Fairford Bypass, and it is looking increasingly unlikely that it will.

This is an opportunity missed. It is human nature to wait until a disaster occurs before taking needed action, but it is human folly to experience a disaster and not take the obvious steps to prevent it from happening again.

Scott Forbes is an ecologist at the University of Winnipeg with property along Lake Manitoba. In reviewing various designs for arks last summer, he was surprised to learn there are so many different kinds of cubits.

Republished from the Winnipeg Free Press print edition January 28, 2012 J11