UPDATED 2006 SPRING FLOODING OUTLOOK FOR MANITOBA

The spring flood potential has increased in most areas of southern Manitoba since late February when the previous flood outlook was issued. This is mainly due to heavy snowfall in early March. There has been little change in the flood potential for northern Manitoba.  

A potential for spring flooding is highest in western Manitoba from the Riding Mountain area to The Pas where snowcover is at near record levels. There is also a potential for significant flooding in the Red River watershed, portions of the Assiniboine River watershed and the Interlake.

Spring weather conditions will be highly significant in determining the extent and severity of flooding. A gradual melt with little precipitation would result in only minor flooding whereas a quick melt with above average precipitation could result in serious flooding in many areas of Manitoba.

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Manitoba Water Stewardship conducted an airborne snow survey (under contract with NOAA in the U.S.A.) and a conventional surface based survey in mid March, 2006. These surveys showed that the water content of the snowpack ranged generally from 75-100 mm in southeastern Manitoba and the Interlake to as much as 175 mm in the Duck Mountains in western Manitoba. The airborne Snow Water Equivalent map (NOHRSC logo) shows that the snowpack is less than 25 mm in southwestern Manitoba. The surface-based survey by Manitoba Water Stewardship (map attached) shows that snowcover exceeds 200 % of normal in many areas from Neepawa north to The Pas.  

Snowcover is also available from a satellite survey prepared by Environment Canada on March 22, 2006. The snow water content may not be reliable in many areas of the southern prairies this year due to the many episodes of thawing and freezing this winter. The satellite estimates are of particular interest in northern Manitoba where other surveys are not available. The estimated snow water content in the north ranges from 75 to 125 mm, which appears to be close to average.

Cumulative winter precipitation can be used as an estimate of snowcover. It may over-estimate the snowcover in southern regions this year due to periodic melting and some sublimation losses. A map showing the percent of normal precipitation from November 1, 2005 to March 20, 2006 is attached. Snowfall is difficult to measure accurately in gauges due to wind effects.

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A map showing the spring runoff potential, as a percent of normal, based on present basin conditions and average future weather is available under "Attachments" at the end of this report -  (2006 spring runoff potential map). 

Spring runoff is expected to be above average in most of southeastern Manitoba, the southern Interlake and in western Manitoba from the Riding Mountain to The Pas. Southern Manitoba should experience below average spring runoff based on average spring weather. Runoff in northern Manitoba is expected to be somewhat above average in most areas due to high soil moisture and high pothole storage remaining from the extremely wet late-summer and autumn conditions in 2005.

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The spring flood outlook is based on three future weather scenarios that take into account probabilities of additional snow, melt rates and spring rain based on statistics for the past 40 years. These have been superimposed on the present basin conditions for input to river forecasting procedures. In terms of future weather, the terms ‘favourable’, ‘average’ and ‘unfavourable’ hereafter refer to the lower decile, median and upper decile conditions respectively. There is a one-in-ten chance of the weather  being ‘favourable’ or better and there is a one-in-ten chance of it being ‘unfavourable’ or worse. Additional precipitation ranges from about 10 mm for the lower decile, 20-25 mm for the median and 40-50 mm for the upper decile condition. The lower decile and upper decile conditions also assume that snowmelt will be slow and fast respectively while an average melt rate is assumed for the median.

Peak discharges and water levels have been computed for the Red, Assiniboine and Souris rivers based on tributary forecasts and flood routing. Probabilistic information on expected peak flows for the Red River at Pembina N.D. and for the Souris River at Westhope N.D. was obtained from the U.S. National Weather Service. Information on probable runoff in eastern Saskatchewan was obtained from the Saskatchewan Water Authority.

Predicted peak stages for the Red River, Assiniboine River and Souris River are shown in Table 1, along with bankfull capacities, dike elevations and recorded peaks for past years.

Predicted peak discharges for smaller watersheds are shown in Table 2 together with bankfull capacities and peak discharges for past years.

Numerical peak stage forecasts will be issued for streams with a high flood potential once spring runoff commences.

The following sub-sections describe the forecast and flood potential for specific watersheds.

There is a potential for significant flooding due to high soil moisture and above average snowcover in most of the watershed including the U.S. portion.

With favourable weather (1:10 chance), crests from Emerson to the Floodway Inlet would be similar to spring crests of 2005 with minimal flooding. The scenario would depend on a gradual melt, little further precipitation and runoff from Manitoba portions preceding the crest coming from the United States.

With average weather, peak levels would be similar to those of April, 2001. Flooding would occur within several miles of the river from Letellier to Morris, somewhat greater than that of July, 2005. Flooding of low lying areas from Morris to the Floodway Inlet would be similar to that of July, 2005. Floodway operation would keep the crest in downtown Winnipeg from exceeding 19.0 feet.

The unfavourable weather condition could lead to a serious flood similar in magnitude to that of 1996. This scenario would depend on a rapid melt, additional precipitation of at least 50 mm during the snowmelt runoff period and on unfavourable north-south timing of runoff. The 1996 flood was a few feet lower than the 1979 flood and much lower than the 1997 flood. The 1996 flood is the fourth highest flood this century after the 1997, 1950 and 1979 floods. The extent of flooding in the Red River Valley for these floods is shown in this map. This scenario would produce serious flooding along the Red River from Emerson to the Floodway Inlet, with the extent of flooding similar to that for 1996. The peak stage in downtown Winnipeg would be held to 19.5 feet by operation of flood control works. Levels above 18 feet at James Avenue require rapidly increasing flood protection activities by the City.

Red River Floodway construction activities related to expansion of the Floodway Channel will not interfere with this spring’s floodway operation.

Serious ice jamming from Selkirk to Breezy Point appears unlikely this spring due to below average ice thicknesses and the ice breaking activities of the Amphibex being conducted by municipalities in the region. An ice survey conducted in mid March indicated close to half a metre (1.6 feet) of ice in the portion from Lockport to Breezy Point. The normal ice thickness in the late winter is 0.8 m (2.6 ft.). Flooding is unlikely in this portion of the Red River, even with the unfavourable weather scenario.

Flooding along the Assiniboine River should be narrowly avoided with favourable weather from now on. 

With median weather, flooding of valley lands would occur from Millwood to Brandon. Operation of the Shellmouth Reservoir and the Portage Diversion should prevent flooding in the Shellmouth area and from Portage la Prairie to Winnipeg.

An unfavourable weather scenario (1:10 chance) would result in significant flooding along the Assiniboine River with crests similar or somewhat greater than those of April, 2005.  Operation of the Portage Diversion would prevent serious flooding from Portage la Prairie to Winnipeg, although brief flooding due to ice jams could occur. 

Shellmouth Reservoir is being drawn down to near 1391 feet to provide for additional spring storage. Forecasts indicate that this drawdown will minimize the risk for both flooding and water supply concerns this spring and summer.

With favourable weather flooding should be limited to the Shell River and the headwater areas of the Little Saskatchewan River and Birdtail Creek. With average spring weather flooding on these streams could become more significant and some flooding could also occur on tributaries from Shellmouth to St. Lazare.  The unfavourable weather scenario (1:10 chance) would result in serious flooding on the Shell River and the Little Saskatchewan River and would require some diking of low-lying buildings at Minnedosa. Flooding would also occur on many other tributaries except western and southern tributaries from St. Lazare to Brandon. Sturgeon Creek could overtop Ness Avenue in Winnipeg under this scenario.

Flooding is not expected along the Souris River with favourable weather.        

Median weather could produce minor flooding of agricultural lands in the Coulter area.

The unfavourable weather condition (1:10 chance) could produce minor flooding from the U.S. boundary to Hartney, but still somewhat less than that of last spring.

Eastern tributaries from Waskada Creek to Elgin Creek could reach near bankfull levels under the unfavourable weather scenario. Western tributaries would remain well within their banks.

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Minor localized flooding is expected with average future weather conditions.

The unfavourable weather scenario could considerable overland as well as flooding of low areas along the Brokenhead River, Whitemouth River, Birch River and streams in Whiteshell Provincial Park.

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Snowcover is up to 200 percent of normal on the Riding, Duck and Porcupine mountains. Snow on the high ground often melts slowly over a period of several weeks to a month most years. Occasionally it melts very quickly in a few days. The flood threat for streams flowing off the high terrain is highly related to the melt rate as well as to additional precipitation.

Soil moisture is below average in the Dauphin Lake watershed and the Westlake region but somewhat above average in the Swan River watershed.

With favourable weather flooding would be relatively minor.

Average weather conditions could produce considerable flooding on streams flowing off the high ground on the Riding, Duck and Porcupine mountains. This flooding would occur mainly in low lying portions of the sub-escarpment area. Below average soil moisture in the Westlake and Dauphin areas should absorb a lot of the snowpack and prevent serious overland flooding or overbank flows. However overbank flows are likely in the Dead Lake Drain area south of Gladstone, in portions of the Turtle River watershed, and on some streams flowing eastward off the Duck Mountain. Levels on the Whitemud River at Westbourne should be similar to those or 2001 but could be lower if ice breaking with the Amphibex is possible.

The adverse weather scenario (1:10 chance) could produce serious flooding on most streams in the region. This scenario pre-supposes a rapid melt in the highland areas with daily high temperatures of +15 C or higher for at least 5 consecutive days. Flooding could also develop on the Swan River but it is unlikely that the Town of Swan River would not be affected.  Flooding in the Whitemud River Watershed including the Big Grass River and Big Grass Marsh could reach near 1979 proportions. The villages of MacGregor could sustain flooding and some diking might be required at Gladstone and Westbourne. There is a possibility that a partial dike closure may be required on the Turtle River at Ste. Rose under this scenario.

The Pas Region:

Soil moisture and snowcover are unusually high in the lower Saskatchewan, Carrot and Red Deer River Watersheds. The portion of the Saskatchewan River watershed lying in east-central Saskatchewan often produces the main spring crest at The Pas. This area has very high soil moisture from last autumn’s heavy rains and has 150%-200 % of normal snowcover. The presence of river ice during the snowmelt crest raises river levels and increases the chance of flooding.

With favourable weather, unusually high river levels would occur but flooding would be relatively minor and diking would not be required.

With average weather conditions, including a near normal breakup date, above bankfull flows would occur in the Pasquia Valley and some flooding would occur along the Carrot River and the Red Deer River. Flooding at Rall’s Island could occur if river ice is still in place when the flow crest arrives. The possibility of breaking the ice downstream of Rall’s Island with the Amphibex is being explored. Some diking of properties at OCN First Nation and the Bracken area may be necessary.

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Runoff in northern Manitoba (north of latitude 54) is likely to be somewhat above average based on normal weather conditions from now on. While snowcover appears to be close to average, soil moisture and depression storage is well above average due to heavy rain last summer and autumn. Localized flooding of low-lying areas is likely to occur.

Flooding is not expected along the Churchill River and the Nelson River this spring and early summer under the average weather scenario as both rivers would experience relatively minor rises. The unfavourable weather scenario (1:10 chance) could produce significant flooding of low lying areas and lake shores in northern Manitoba, although not likely as great as that of the late summer and autumn of 2005. Levels of the Churchill River and the Nelson River would still be lower than those of 2005. Manitoba Hydro may have more specific information for certain areas in northern Manitoba.

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