Manitoba Water Stewardship’s final spring flood outlook shows that the spring flood potential in 2011 remains high for much of Manitoba. The outlook includesthe Red, Souris, Pembina, Assiniboine, Winnipeg, Saskatchewan and Fisher rivers as well as the Interlake region.
The flood potential is high in these areas due to above-normal winter precipitation, high river flows, very-high soil-moisture conditions at freeze-up, above-normal snow water content in the snowpack and an expected wetter spring.
Additional precipitation experienced in the southern areas of the Red River basin is expected to cause a slight increase in the river levels compared to predictions in the February flood outlook.
With average weather conditions along other rivers and streams in Manitoba, flooding is still anticipated and, with unfavourable weather, significant flooding could occur.
Average weather conditions along the Red River south of Winnipeg could result in a flood slightly higher than 2009 while unfavourable weather conditions could result in Red River water levels lower than 1997.
Open water levels on the Red River north of Lockport could be slightly higher than 2009 for the normal weather scenario and 0.61 metres (two feet) higher than 2009 for unfavourable weather conditions.
The spring flood potential is still dependent on weather conditions in the next few weeks until the spring melt begins. The amount of additional snow and rain, the timing and rate of the spring thaw and the timing of peak flows in the U.S., Manitoba and other provinces will have a significant effect on flood potential.
Localized overland flooding is expected in most of central and southern Manitoba and could occur during the early part of the run-off period due to ice jams, snow blockages or frozen culverts in river channels, drains and ditches.
Climatic Conditions
Precipitation during the autumn of 2010 was well-above normal in most of Manitoba, central and southern Saskatchewan, North Dakota and Minnesota.
Cold temperatures since mid-November 2010 have resulted in deeply frozen soil conditions in most areas.
Widespread and heavy snow occurred from late November through early February in Manitoba, central and southern Saskatchewan, North Dakota and Minnesota. Many areas generally experienced below-normal to normal snowfall in February, but the overall snow precipitation ranges from normal to above normal. Based on recent measurements, the ice thickness in the Assiniboine main stem ranges from 10 centimetres (four inches) to one m (3.3 ft.), while in the Red River the ice ranges from thin to 0.9 m (2.9 ft.).
Significant meltwater is still trapped within the snowpack on fields, in depressions and ditches.
Recent mild weather has slightly decreased the depth of the snowpack but the overall water content over the watersheds remains relatively unchanged. Most of the snow water is contained in the snowpack or near the surface layer and is expected to remain in place until the start of the spring thaw.
Both the Assiniboine and Red River main stem channels have risen slightly, by less than three cm (1.2 in.), indicating minor flow from the recent snowmelt run-off.
The onset of colder weather has reduced or completely stopped the spring melt.
Currently, a moderate La Niña in the equatorial Pacific is expected to gradually weaken through the spring months and may bring below-normal temperatures and normal to above-normal snowfall in the Red River Basin.
El Niño conditions may also occur, bringing normal spring rain. Alternatively, a cooler-than-average spring might result in a staggered, slowmelt.
Temperatures are expected to remain below the freezing point in most areas of Manitoba, Saskatchewan and North Dakota until the end of March, followed by a very gradual rise in the early part of April.
Soil Conditions
The soil moisture at freeze-up was well-above normal across most of central and southern Manitoba, central and southern Saskatchewan, North Dakota and Minnesota.
Soil-frost information is sparse, but the data available up to mid-March for southern Manitoba and North Dakota suggests the soil is frozen to a depth of 10 cm to one m (3.6 in. to 3.3 ft.). Wet and frozen soils do not allow meltwater to soak in and, as a result, increase spring run-off potential.
Snow Cover and Winter Precipitation
A ground snow survey was conducted over the Red, Assiniboine and Souris rivers watersheds from March 4 to 9. The survey indicated that, snow-water content ranged from four to 15 cm (1.6 to 5.8 in.) or an average of 9.6 cm (three in.) over most of southern Manitoba. This is approximately five to 10 per cent above-normal snow-water content for the region.
Spring Run-off
Based on soil moisture and snowpack conditions as of March 20, the 2011 spring run-off is expected to be average to above average over most of central and southern Manitoba.
Well-above-average run-off potential is expected in the following locations:
• along the Saskatchewan-Manitoba border between The Pas area and Roblin;
• in the upper Assiniboine Watershed; and
• the southwestern Manitoba-North Dakota border.
The expected run-off is based on 2010 summer and fall soil moisture, snowpack conditions as of late March and average future weather conditions for precipitation.
Spring run-off could change significantly if future precipitation and breakup conditions differ significantly from the average.
River Forecasts
The spring flood outlook is classified into three future weather scenarios related to additional snow, melt rates and spring rain. These scenarios are based on available climate statistics for the past 30 to 40 years.
The three future weather scenarios are referred to as favourable, average and unfavourable.
There is a one-in-10 chance of the weather being favourable and there is a one-in-10 chance of it being unfavourable. The favourable and unfavourable scenarios assume that breakup will be slow and relatively fast, respectively. Average weather and breakup conditions are assumed for the average scenario.
Forecasts do not include the effects of possible ice jams which are generally unpredictable. Localized and brief flooding can occur in locations where ice jams develop, even with below-average river flows.
Red River Main Stem
The soil moisture index at 2010 freeze-up was the second highest recorded since 1948 and well above the level before the 1997 flood. However, it was slightly lower than the record high measured prior to the 2009 flood.
Snow cover is generally above average, especially south of Grand Forks, but still less than in mid-March 1997.
With little additional precipitation and a gradual snowmelt, a flood close to that of 1979 is expected. This would cause significant over-bank flows from St. Jean Baptiste to Morris, with bank-full conditions elsewhere. With average weather from now until April, flood levels are expected to exceed those of 2009 by 0.4 to 0.7 m (1.3 to 2.3 ft.).
Flood protection works will prevent flooding of communities and most homes in the Red River Valley. However, transportation may be significantly disrupted with the closure of PTH 75 near Morris and the closure of many smaller roads in flooded areas.
Under this scenario, operation of the Red River Floodway would keep levels in downtown Winnipeg in a range of 6.1 to 6.6 m (20 to 23 ft.), depending on the flows from the Assiniboine River. In 1997, water levels in the Red River at James Avenue reached 7.47 m (24.5 ft.).
There is a one-in-10 chance of unfavourable weather. With unfavourable conditions, peak stages would be similar to or slightly lower than 1997 levels at most locations.
At most locations south of Winnipeg, Red River levels would not exceed those of 1997. As a result of flood protection measures, it is unlikely that homes would be flooded even under unfavourable weather conditions. However, transportation and access to properties in the flood plain would be reduced for up to three weeks. The crest in downtown Winnipeg would be slightly lower than 1997, with a maximum of about 7.47 m (24.1 ft.).
Peak stages from Selkirk to Breezy Point would depend on whether ice jams develop. Ice-cutting and breaking operations will reduce the chance of significant ice jams. Currently, ice buildup is slightly less than the winter average due to high river levels and substantial groundwater flows which typically add warmer water to the Red River. With unfavourable weather, significant ice jamming will be reduced by ice cutting and icebreaking.
Flooding of low-lying properties in the Breezy Point area is likely, even if no significant ice jams form. Elsewhere, flooding is not expected unless significant ice jams develop.
Red River Tributaries
With favourable weather from now through to spring, flooding is expected to be fairly localized. If average weather conditions occur, significant over-bank flows could develop on portions of most streams and there would be overland flooding in low-lying areas. With unfavourable weather, extensive over-bank flows would occur on all streams, bringing them close to 1997 levels.
Pembina River
With favourable weather conditions, there is the potential for flooding higher than 1969, but lower than 1974. With average weather conditions, peak water levels along the Pembina River and tributaries would be slightly higher than 1997. With unfavourable weather there is a one-in-10-chance that flooding could exceed that of 2009.
Roseau River
With favourable weather conditions, minor flooding is expected. With average weather conditions, flooding would occur at close to 1986 levels. With unfavourable weather conditions, flooding would occur at close to 2009 levels.
Assiniboine River Main Stem
With favourable weather conditions, there would be minor flooding along the Assiniboine River. With average weather conditions, there would be flooding along the Assiniboine River. The Shellmouth Reservoir is now at the target level following the drawdown of the reservoir. This is expected to minimize the risk of flooding while providing sufficient water supplies to meet downstream needs in 2011 and beyond. Average weather conditions may also result in flooding from Millwood to Brandon, with levels generally below those of 1995, but higher than 1979. Unfavourable weather would result in flooding of valley lands from Shellmouth to Brandon with levels likely higher than those of 1976 and 1995 by about 0.2 to 0.8 m (0.5 to 2.7 ft.).
The Shellmouth Reservoir is being operated this winter to provide maximum storage space for spring flood control along the Assiniboine River. This will reduce spring peak stages along the river, especially for the portion between the dam and St. Lazare, where flooding will be avoided only if favourable weather conditions occur.
The Portage Diversion will be operated to prevent ice jams and flooding from Portage la Prairie to Winnipeg. Flooding may occur in this region with unfavourable weather conditions and, as a result, protective dikes along parts of the Assiniboine River are being strengthened.
Assiniboine River Tributaries
With favourable weather, localized flooding is expected. With average weather conditions, flooding is likely. Unfavourable weather conditions would result in significant flooding on most tributaries leading to over-bank flows.
Souris River
The flood potential is high for the Souris River due to high soil moisture and higher-than-normal snow cover in the North Dakota portion of the watershed. Significant flooding of agricultural lands adjacent to the river is likely.
A favourable weather scenario would produce a flood similar to that of 1969. With average weather conditions, a flood slightly less than that of 1974 is likely. Low-lying portions on the south edge of Melita would require diking. Unfavourable weather conditions would result in water levels higher than those of 1974 but lower than the peak stages of the 1976 record flood. Low-lying homes near Souris would require diking in addition to the area next to PTH 3 at Melita.
Souris River Tributaries
With average weather conditions, flooding is likely on Souris River tributaries. Under unfavourable weather conditions, Souris River tributaries would experience significant flooding.
Interlake Region
Soil moisture is well-above normal and most small depressions are relatively full from heavy rains last summer and fall. Snow cover is currently average to above average.
With favourable weather, flooding would be limited to low agricultural lands and streams would remain within their banks. With average weather conditions, flooding would occur and the Fisher River would see flood levels close to those of 1996 but lower than 2006. Overland flooding would also occur. With unfavourable weather, flooding similar to 1986 (but lower than 2009) could occur. There could also be difficulties due to a high water table, resulting in seepage into basements in rural areas. Record high levels are likely on the Shoal Lakes, even with average weather conditions.
Eastern Region
Soil moisture is currently above average and snow cover overall is above average in the region. With favourable weather conditions, localized flooding could occur on the Brokenhead, Whitemouth, Whiteshell and Winnipeg rivers. If average weather conditions develop, flooding is likely to occur along streams and lake shores. The Winnipeg River could experience levels similar to those of 1994. Unfavourable weather conditions could result in flooding of agricultural lands close to previous record floods. In unfavourable weather conditions, water levels in the Winnipeg River would be similar to 2009 flood levels.
Westlake, Dauphin and The Pas Regions
Soil moisture is above normal to well-above normal in most areas. Snow cover remains generally near normal to above normal.
With average weather conditions, flooding is possible on the Whitemud, Turtle, Swan and Carrot rivers. With average conditions, the Saskatchewan River would be expected to see flood levels similar to the 1983 flood at The Pas. With unfavourable weather conditions, general flooding could occur. Brief localized flooding is also possible due to snow-blocked or ice-filled streams, drains or ditches. If unfavourable weather conditions develop, flood levels similar to 2005 are expected on the Saskatchewan River at The Pas.
Northern Manitoba
Run-off in most of Manitoba’s far north (54 degrees north) is likely to be average. Run-off is expected to be more substantial in northeast Manitoba.
Flood Preparedness
The $1.1 million ice jam mitigation program has been very successful. So far 693 km (430.6 miles) of ice has been cut along the Red River and about 30.5 km (19 miles) has been broken and cut between Netley Creek to the Selkirk area. Cutting the ice weakens it and makes it easier to break up without jamming, and breaking the ice with the Amphibexes further reduces the chances of jamming.
Ice cutting crews were able to cut the ice on the Red River as planned and have been operating along the Whitemud River and the Icelandic River at Riverton. An Amphibex has been breaking ice at the Brokenhead River and the Portage Diversion. Work is also expected to proceed at Fisher River and the Assiniboine River near Glenboro.
Over 230 pieces of equipment are working to reinforce and raise 70 km (43.5 miles) of earthen dikes along the Assiniboine River between Portage la Prairie and the Baie St. Paul Bridge, north of Elie. Work has progressed well and the dikes will be completed before the spring flood crest.
Approximately five km (3.1 miles) of dikes have been raised in Brandon and super sandbags have been installed along First and 18th streets.
Maintenance work along the Portage Diversion is continuing and currently heavy rock, also known as rip-rap, is being placed along the channel to protect against stream bed erosion during spring flows. Traffic going east bound to Portage la Prairie is being redirected for the next ten days to allow for safe movement of heavy equipment. Detours are signed and marked.
The Manitoba government is continuing to assess short- and long-term solutions to control Shoal Lake flooding north of the City of Winnipeg.
Steaming of provincial drains and culverts is expected to begin next week providing the weather is warm enough.
The following equipment is currently in place in Manitoba:
• three Amphibex icebreakers
• seven ice-cutting machines
• three Argo amphibious ATVs and six trailers
• three million sandbags
• six provincial sandbagging machines, operating in various locations around the province
• 30,000 super sandbags
• 43 km (26 miles) of rapid-deployment cage barriers
• 24 additional heavy-duty steamers for a total of 61
• 21 new mobile pumps, bringing the provincial total to 36
• 72 km (44 miles) of water-filled barriers of which 30 km (18 miles) are in rapid-response trailers
Three municipal flood seminars have been held and two seminars on the Disaster Financial Assistance program have also been held.
Municipalities in partnership with the Manitoba government, are finalizing flood response plans and undertaking preparation work as needed.
Volunteers interested in helping municipalities with sandbagging efforts outside of the city of Winnipeg can link up with communities that need volunteers through the Volunteer Manitoba website at www.mbvolunteer.ca. The City of Winnipeg is co-ordinating its volunteers through its 311 call centre.
The Canadian Red Cross is also recruiting volunteers at www.redcross.ca/manitoba to assist with their flood response and recovery operations.
Information is also available at www.manitoba.ca and www.gov.mb.ca/waterstewardship/floodinfo
