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Manitoba on alert as spring arrival means renewed flood risk
Thursday, April 20, 2017, 8:00 AM - Residents of southern Manitoba are being warned of the possibility of moderate to major flooding throughout the Red River Basin in the days and weeks to come. Here are current conditions and the factors that are determining just how bad the flooding may be.
The current situation:
As of April 26, 2017, Manitoba Infrastructure’s Hydrologic Forecast Centre has issued the following alerts (newest updates are italicized):
High Water Advisories: water levels are expected to be elevated, but not to flood levels.
Flood Watches: water levels are expected to reach flood levels, sometime beyond the next 24 hours.
• Lower Assiniboine River, from Portage la Prairie to Headingley
Flood Warnings: water levels are expected to reach flood levels, sometime within the next 24 hours.
• Carrot River
• Upper Assiniboine River, between Virden and Holland
• Pelican Lake
Flood Risk Forecast
According to the Spring Forecast report from Manitoba Infrastructure, the best practice for flood forecasting is to plan for the worst-case scenario, for both the timing of the melt and the amount of spring rainfall.
Based on this, since their March 24 update, forecasters have been calling for the following flood risks, for the various watersheds:
• Watershed in the South Western region: major risk
• Pembina River: major risk
• Roseau river: major risk
• Assiniboine River: major risk
• Eastern Region and the Winnipeg River: major risk
• South Western Region: major risk
• Interlake Region and the Fisher River: moderate risk
• Northern Manitoba/The Pas Regions and the Saskatchewan, Carrot and Swan rivers: moderate risk
In addition, the agency provided some context to their forecast, based on previous flooding the region has seen:
• Red River: from Emerson to the Floodway Inlet with normal weather levels would be similar to 2013, with unfavourable weather observed levels would be similar to 2010
• With the benefit of floodway operations James Avenue 4.6m (15 feet) for favourable weather or 5.5m (18 feet) for unfavourable weather conditions
• Pembina River: with normal weather river levels would be similar to 1998, with unfavourable weather levels would be lower than 2011
• Roseau River: with normal weather river levels would be similar to 2013, with unfavourable weather levels would be similar to 2011
• Assiniboine River: moderate to major risk; with normal weather river levels from Shellmouth to Brandon would be similar to 2009 but lower than 2005, levels from Brandon to Portage would be similar to 1995 levels, with unfavourable weather levels from Shellmouth to Brandon would be slightly lower than 1976 or 1995, levels from Brandon to Portage would be less than 2011 or the summer of 2014
• Souris River: with normal weather river levels at Wawanesa would be similar to 2014 summer levels, with unfavourable weather levels would be lower than 1976 and 2011 levels at Wawanesa
• Fisher River: with normal weather river levels would be similar to 2006, with unfavourable weather levels would be lower than 2008
One added factor that the report mentioned is potential for ice jams along these rivers, which is a concentration of ice in the river flow that blocks the flow of water. This happens most often in places where the river narrows or where it bends, the thickness of the ice changes, the speed of the flow changes, and if broken ice encounters a solid section of ice downstream. Ice thickness is a major contributor to this factor, since thicker ice is more difficult to break up into smaller chunks, and raises the potential for jamming.
What is contributing to this situation?
Several factors determine if an area is vulnerable to flooding, including:
• how much water flow the local system of waterways can handle,
• the shape of the local terrain, since that will determine where all the water in the system comes from,
• the type of soil that is most common, since the soil acts to slow the progress of water as it moves from where it fell to where it enters the system of waterways, and
• how much water actually enters the system at any one time.
Any bad combination of these factors will result in local to widespread flooding, and in the spring, we need to add a few more considerations, as snow, ice and frozen soil get added to the equation.
Water levels in rivers and lakes before the spring melt
The first thing to look at when considering the potential for spring flooding is the amount of water that is flowing through waterways, even before the seasonal thaw begins.
It's simply a matter of mathematics. Only so much water can flow through these waterways at any one time before the river overflows its banks. Therefore, the more water there is in the river to start off the melt season, the less extra water it can take on before it results in flooding.
Currently, water levels and flow rates in the Red River are above normal for this time of year. In the Assiniboine River, which flows into the Red River at Winnipeg, water levels and flow rates are not only above normal, but are record- breaking in some places.
Water levels in waterways throughout southern Manitoba and southeastern Saskatchewan. Credit: watermonitor.gov
How much water was in the soil when it froze
For the second indicator, one has to look farther back, to when the ground froze in late fall or early winter.
Just as the waterways only have a limited capacity to move water, soil has a limited capacity to absorb water. If the soil was relatively dry when freezing temperatures set in, ice crystals will set up between the soil grains, but the ground will still have a large capacity to absorb water from melting snow and rainfall when spring finally returns.
If, however, the soil was saturated when it froze, it freezes into a solid sheet of dirt-filled ice, and its capacity to absorb extra water is greatly reduced. Picture the difference between pouring a bucket of water into a box of cold sand, as opposed to pouring that same bucket of water onto a block of ice. Whereas the sand will absorb the water, the water will run across the surface of the block of ice. This is the same thing that will happen with saturated frozen soil, causing overland runoff and increasing the risk of widespread flooding.
According to a report from the Hydrologic Forecast Centre of Manitoba Infrastructure, the soil moisture was normal to well above normal in the Red River basin when the soil froze.
Credit: Manitoba Infrastructure
How deeply the ground is frozen
The colder it is during winter, and the longer freezing temperatures persist, the deeper the ground will freeze, which is especially important when the ground is saturated at freeze-up.
During a relatively mild winter, only a thin layer of ice sets up in the top soil, so it will not take long for that ice to melt when it warms up. Rain and melt water will gain access to the full column of top soil fairly quickly once the spring thaw begins, providing a better buffer to keep too much of that water from entering the waterways at any time. Thus, there is a lower chance of flooding.
The thicker that sheet of ice is, though, the longer it takes for the soil to thaw come spring. The top-most layer of soil may thaw and become available to absorb water, but it will take longer for the full column of soil to become available for all of the rain and melt water. This will result in less of a buffer for the water, and more of it will flow over-land and into the waterways, increasing the chance of flooding.
Based on Manitoba Infrastructure's report, although data on frost depth is sparse, from the information they do have, it is likely normal to below normal across much of Manitoba.
How much snow and rain falls during the winter
The more snow and rain there is during the winter, the greater the load that waterways will need to carry when the spring thaw begins, and this isn't just local snow and rain, either.
In the case of the Red River, flood forecasters need to consider the winter precipitation over a very large area, that stretches both south and west of southern Manitoba.
This is because the Red River starts off in southeastern North Dakota, at the junction of two other rivers, the Bois de Sioux River, which runs along the South Dakota/Minnesota border, and the Otter Tail River, which flows through west-central Minnesota, and then it flows north, downslope along the North Dakota/Minnesota border, taking on the flow from the Sheyenne River along the way, then running straight north through southern Manitoba, to drain into Lake Winnipeg. Along the way, it is also joined by the Assiniboine River at Winnipeg, and the Assiniboine River flows out of Saskatchewan, picking up the flow from two other rivers in the process - the Qu'Appelle River, which flows all the way from southwestern Saskatchewan, and the Souris River, from southern Saskatchewan and northern North Dakota.
Add to this a multitude of minor tributaries along the lengths of these rivers, and the Red River ends up taking on the collective snow and ice melt from over thousands of square kilometres of the countryside.
Flood forecasters examining the amount of precipitation so far, from late fall and winter, are showing below normal snow and rainfall to the west of the Red River basin. Inside the river basin, however, and extending all the way into North Dakota and Minnesota, precipitation has been anywhere from above normal to extremely above normal.
Credit: Manitoba Infrastructure
How quickly the snow will melt
Rather than simply keeping track of how much rain fell before the soil froze, tallying the rain and snowfall during the winter, and monitoring the water levels in the river as we approach spring, figuring out how fast the river will be taking on melt water takes a bit more finesse.
This is where long-range and even seasonal weather forecasting comes into play, to give us an idea of how the temperatures will change as the season progresses.
How much rain falls in mid-Spring
How quickly the snow and ice melts is one thing, but at the same time that all of the runoff from that melting is entering the waterways, rain will also be falling over these regions. This directly adds water to the system, from the rainfall itself, but it also accelerates the melting of the snowpack. This increases the amount of water the system is forced to deal with all at once, and with the ground potentially still be partially frozen, this can result in overland flow, as well as swollen rivers and streams.
These last two factors depend on what happens for the rest of the winter and into spring, and involves long range and seasonal forecasting.
Stay tuned for updates on the situation, as the season progresses.