The Manitoba Infrastructure and Transportation Flood Forecast Centre’s first 2012 flood outlook shows spring flood potential remains low at this time for the Red and Souris rivers, and moderate on the Pembina and Assiniboine rivers, and in the southern Interlake.
Spring flooding is likely in portions of northern Manitoba including The Pas, where there are above average soil-moisture conditions and there has been significant snowfall this season.
The forecast notes that significant precipitation this spring could result in localized flooding including some portions of the upper Assiniboine and Souris river basins, and in The Pas area.
The spring flood potential is still very dependent on weather conditions from now until the spring melt. The amount of additional precipitation, the timing and rate of the spring thaw, and the timing of peak flows in Manitoba, the U.S. and other provinces will have a significant effect on flood potential.
There is a risk of significant precipitation leading to localized flooding, however, this is less likely to affect the main stems of the Red and Assiniboine rivers.
The chances of minor, localized flooding during the early part of the run-off period due to ice jams or snow blockages in drains, ditches and small streams is fairly low. Although major ice jams are unlikely to occur, the possibility cannot be ruled out, especially in the Interlake region.
Recovery efforts for the 2011 flood are still ongoing including financial compensation programs. To date, more than $500 million has been provided in flood recovery assistance through various programs such as disaster financial assistance, the Building and Recovery Action Plan, excess moisture insurance and AgriRecovery.
Climatic Conditions
Precipitation during the autumn of 2011 was well below normal in most of southern Manitoba, with the period between November and February being one of the mildest and driest on record in portions of the south.
November to February precipitation has been below normal, with several locations reporting the lowest precipitation in the last 50 years.
Precipitation in northern Manitoba has been near normal in most areas this winter.
The long-term March climatic outlook calls for above-normal temperatures to continue, especially in southern Manitoba, with precipitation amounts close to normal. Under these conditions, there is an increased risk for March precipitation to fall as rain or as rain mixed with snow and ice.
There is also a risk of above-normal precipitation into April and May.
Soil Conditions
An aerial soil-moisture survey conducted in early November 2011 shows that moisture in the top 20 centimetres of soil was generally below average in southern Manitoba and the U.S. portion of the Red River Basin.
An Agriculture and Agri-Food Canada survey shows general soil conditions for the prairies range from abnormally dry to exceptionally dry conditions.
Soil moisture at freeze-up was above average in northern Manitoba including the The Pas and northern Interlake areas. Some portions upstream of the Souris and Assiniboine river watersheds had near-normal to above-normal soil-moisture levels.
Major rivers have ice cover and flows either above or close to normal for this time of year and generally below those of February 2011.
Soil-frost information indicates the soil is frozen to a depth of at least 60 cm (two feet) in many areas. Wet, frozen soils impede infiltration of meltwater and increase spring run-off.
Snow Water Content
A recent survey showed snow-water content was non-existent or very patchy and well below average in southern Manitoba, the U.S. portion of the Red River and in downstream areas of the Assiniboine River from St-Lazare to Brandon.
Some areas upstream of the Saskatchewan-Manitoba boundary had normal to near-normal snow water content. Westman areas and The Pas region had near-normal to above-normal snow water equivalents.
Isolated areas on the upper Assiniboine River in Saskatchewan, northern Manitoba, Westman regions and the Interlake show near-normal to above-normal snow-water content.
Spring Run-off
The 2012 spring run-off is expected to be below normal in most parts of Manitoba and near normal to above normal in portions of the north Interlake, The Pas region and the Turtle Mountain area at the North Dakota-Manitoba boundary.
Spring run-off could change significantly if future precipitation and breakup conditions differ significantly from the average.
River Ice Conditions and Ice Jamming
Ice thickness varies according to the size of the river but ranges mostly between 0.56 to 0.71 metres (1.8 to 2.3 ft.) on major rivers like the Red and Assiniboine. Spring weather in the weeks preceding spring breakup affects deterioration of ice and will be a significant factor in determining ice strength at breakup.
It is virtually impossible to predict the occurrence and extent of ice jamming. However, with the relative weakness of the ice in the Selkirk area and the extensive ice cutting and breaking that is underway ice jamming and related flooding should be limited. Brief flooding can occur in locations where ice jams develop, even with below-average river flows.
River Forecasts
Spring flood outlooks are based on three weather scenarios that look at additional snow, melt rates and spring rain for the past 30 to 40 years. The three future weather scenarios are referred to as favourable, average and unfavourable.
Red River Main Stem
The potential for spring flooding is low due to low soil-moisture conditions in most of the watershed and below-average snow cover in the whole watershed.
With both favourable and average weather conditions, no flooding is expected. For median conditions, levels would be close to those of 2008.
With unfavourable weather conditions, minor, localized flooding could occur in small tributaries. Levels in the main stem would be close to those seen in 2005. There should be sufficient protection to prevent any over-bank flooding.
Levels at James Avenue in Winnipeg are forecast to be 4.84, 9.18 and 14.75 ft. for favourable, average and unfavourable weather conditions, respectively, and based on these potential levels, floodway operations would not be required.
Pembina River
The flood potential is very low on the Pembina River due to below-average soil-moisture and snow cover conditions. The average weather scenario would produce no flooding. The unfavourable weather scenario would produce levels close to those of 1992.
Assiniboine River Main Stem
With favourable and average weather from now on, no flooding along the Assiniboine River is expected this spring.
The unfavourable weather scenario would result in localized minor to moderate flooding of the Assiniboine River Valley in small tributaries from Shellmouth to St-Lazare, similar to 2007.
Souris River
The flood potential is low due to below-average winter precipitation and below- to near-normal soil moisture in the North Dakota, Saskatchewan and southern Manitoba portions of the watershed. The unfavourable weather scenario would result in localized minor flooding with peak stages lower than those of 2005.
Interlake Region
Soil moisture is below normal in southern portions of the Interlake region but the subsoil is still wet based on a November survey. With favourable to average weather from now on, no flooding is expected. With unfavourable weather, localized minor to moderate flooding could occur.
With median conditions, Lake Manitoba is expected to drop from its current level of 814.5 ft. to 813.4 ft. by the end of winter, rise slightly with the spring run-off and then continue to decline.
Fairford River flows will remain high through the rest of winter. Lake levels will continue to drop unless heavy spring storms develop. No Portage Diversion flows are expected this spring.
Eastern Region
Both the soil-moisture and snow-water equivalent are below average at this time.
The favourable and normal weather scenario would not produce flooding.
The unfavourable weather scenario could produce minor flooding in the low areas along the Whitemouth and Brokenhead rivers and undesirably high levels on many lakes.
Westman and The Pas Regions
Soil moisture and snow cover is below normal in most areas apart from areas around the The Pas region. Flooding is unlikely with normal weather conditions from now on.
The unfavourable weather scenario could produce localized flooding, especially if there is a rapid melt in the higher terrain, with levels similar to those of 2005 for the The Pas region.
Northern Manitoba
Run-off in most of far northern Manitoba (north of latitude 54) is expected to be below average to near normal.
Minor, localized flooding is expected.
Flood Preparations
The Manitoba government and municipalities are continuing to prepare for spring flooding. This includes work with municipal emergency-management teams to review existing plans, gather information through conference calls and conduct flood-preparedness meetings, disaster financial assistance sessions and other related activities.
The ice-jam mitigation program north of Winnipeg is underway with ice cutters and Amphibex machines working along the Red River. Approximately 10 km of ice-cutting is complete. The Amphibex AE400s have broken a channel approximately three km long.
Provincial flood-control works such as the Red River Floodway, Portage Diversion and Red River Valley community dikes will be operational if the flood potential changes.
Forecast Updates
This forecast will be updated in late March when further precipitation and weather details are available.
Detailed forecasts (text and charts) are available at www.manitoba.ca/flooding.
