Flood watch continues in Western Manitoba

Rainfall Amounts Could be Lower

 

·         The flood watch issued by Manitoba Water Stewardship for areas of western Manitoba is continued.

 

·         The prospect of heavy rain today and tomorrow has diminished but there is still considerable uncertainty in the rainfall forecast.  The heavy rain, if it develops, would likely be centered in the Duck Mountain area.

 

·         The flood watch includes the Assiniboine River upstream of Millwood and the Swan and Red Deer rivers watersheds.  It also includes streams in the Riding, Duck and Porcupine mountains regions and the Interlake region.

 

·         Run-off from the additional rain would be heavy due to saturated soil conditions.  Over-bank flows and extensive overland flooding could occur depending on rainfall amounts.

 

·         Calculations based on a general rain of 50 millimetres (two inches) in western Manitoba indicate that serious flooding would develop.  However, with rainfall likely being less than earlier predicted, serious flooding is less likely. 

 

·         Rain is also expected over the Manitoba portion of the Red River watershed and the Interlake.  While amounts should be generally less than 15 mm (0.6 in.), local thundershowers with heavy rain are possible and could cause renewed overland flooding in areas which experienced heavy rain in recent weeks.

 

Assiniboine River

 

Shellmouth Reservoir Operation

 

·         The Shellmouth Reservoir water level rose 0.15 metres (0.5 feet) since yesterday morning and stood at 429.02 m (1,407.56 ft.) this morning.  The computed inflow has risen to 6,300 cfs and the outflow was increased to 2,200 cfs at 3:50 p.m. yesterday.  The spillway will be overtopped by Saturday and the reservoir level is expected to rise to about 429.92 m (1,410.5 ft.) by June 24 to 27 even without additional rain. The spillway elevation is 429.3 m (1,408.5 feet). With the prospect of heavy additional rain diminished, it is possible that reservoir levels and outflows will not rise significantly higher than presently predicted.

 

·         Significant flooding of valley lands occurs in the Shellmouth to Millwood area with outflows of 2,200 cfs.  However, this is unavoidable since calculations show this is the minimum outflow possible.  The latest forecast suggests the total outflow (conduit plus spillway) will be 2,350 cfs.  The proposed operation scenario includes surcharging of the reservoir (a reduction in controlled conduit flows) commencing about June 20 to offset spillway flows and minimize the total outflow.  If rainfall later today and tomorrow exceeds 10 mm (0.4 in.), the forecast will need to be revised upward.

 

Assiniboine River from Shellmouth to Millwood 

 

·         River levels have risen above flood stage and a further rise of about 0.3 m (one ft.) is expected.  With little further rain, the crest is expected June 22 to 24, but levels will remain above flood stage until late June.

 

Assiniboine River from St. Lazare to Brandon

 

 ·         The river is expected to remain within its banks unless rainfall of more than 25 mm (one in.) falls over most of the Assiniboine River watershed upstream of Brandon.  Crests are expected about June 25 at St. Lazare, June 27 at Virden and June 30 at Brandon based on less than 10 mm (0.4 in.) additional rain during the next two days.

 

Assiniboine River from Spruce Woods to Winnipeg 

 

·         Flows will rise slowly next week as water arrives from the upper Assiniboine.  However, no difficulties are expected unless heavy rain develops. Flows in the Portage Diversion continue to be reduced and are expected to end this weekend. Flows down the Assiniboine River at Southport are being increased by 500 cubic feet per second per day.  As a result, river levels will rise and additional 0.45 to 0.6 m (1.5 to two ft.) from Baie St. Paul to Headingley during the next three to five days, still well within the banks.

 

Red River

 

·         Flows into the Red River Floodway ended overnight. The floodway control structure will continue to be operated to reduce levels at the floodway inlet by about 0.3 m (one ft.) per day. The flow at the floodway inlet this morning was 18,350 cfs.

 

·         The river level in downtown Winnipeg rose slightly since yesterday and stood at 2.79 m (9.17 ft.) this morning.  The natural level without operation of major flood control works would be 3.07 m (10.1 ft) this morning.  Increased flows on the Assiniboine River are stabilizing levels in the city.  River levels from Winnipeg to Lockport will begin to decline about 0.15 m (0.5 ft.) per day starting next week, unless heavy rain develops.  Levels from Emerson to the floodway inlet will continue to decline unless heavy rain develops.                                                           

Other Watersheds

 

·         The level of Red Deer Lake continues to rise slowly and stood at 263.365 m (864.06 ft.) this morning.  The record crest in 2006 was 264.56 m (868 ft).  Additional rises should be less than 0.15 m (0.5 ft.) unless rainfall in the watershed during the next few days exceeds 20 mm (0.8 in.).

 

·         Smaller streams are declining in most areas due to relatively little precipitation in most areas for the past five to seven days.  Lakes and streams in most areas of southern Manitoba remain unusually high for this time of year.