June 16, 2010
Flood Watch Continued for Western Manitoba
– – –Increased Outflows from Shellmouth Reservoir
Manitoba Water Stewardship is continuing the flood watch issued yesterday for areas of western Manitoba.
· The weather forecast calls for 50 to 75 millimetres (two to three inches) of rain in areas of western Manitoba and eastern Saskatchewan tomorrow through early Friday due to a slow-moving storm system developing over Montana.
· The heaviest rain is likely to fall in the Duck Mountain area but there is a chance of heavy rain in most areas from Brandon to Mafeking.
· The flood watch includes the Assiniboine, Swan and Red Deer river watersheds. It also includes streams in the Riding, Duck and Porcupine mountain regions and the Interlake.
· Run-off from the additional rain will be heavy due to saturated soil conditions. Over-bank flows and extensive overland flooding are likely to occur if the rain falls as predicted.
· Rain is also expected over the Manitoba portion of the Red River watershed. While amounts should be generally less than 25 mm (one in.), some renewed overland flooding could occur in areas which experienced heavy rain in recent weeks.
Red River Floodway Operation
· Operation of the Red River Floodway continues to reduce levels at the floodway inlet by about 0.3 metre (one foot) per day. Flows into the floodway are expected to end tonight, but operation of the control gates will continue for about another two weeks to gradually reduce levels upstream of the floodway inlet.
· The river level in downtown Winnipeg was unchanged from yesterday and stood at 2.8 m (9.1 ft.) this morning. The natural level without operation of major flood control works was 3.2 m (10.5 ft.) this morning. Increased flows on the Assiniboine River are stabilizing levels in the city.
Red River
· Levels of the Red River declined 15 centimetres (six in.) at Emerson and 21.3 cm (8.4 in.) at St. Adolphe during the 24-hour period ending this morning. Levels are unlikely to rise significantly due to rainfall later this week.
· The flow in the Red River Floodway this morning was 520 cubic feet per second and the total flow at the floodway inlet was 18,930 cfs.
Assiniboine River, Shellmouth Reservoir Operation
· The Shellmouth Reservoir water level rose 15 cm (six in.) since yesterday morning and stood at 428.9 m (1,407.05 ft.) this morning. The computed inflow was 5,200 cfs based on the change in the reservoir level. The reservoir level is expected to rise to about 429.92 m (1,410.5 ft) by June 23 even without additional precipitation, well above the spillway elevation of 429.3 m (1,408.5 ft.).
· The outflow was increased to 1,800 cfs yesterday afternoon and a further increase to 2,200 cfs may occur today. 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 eventual peak outflow would be greater if there is no increase made today. The proposed operation scenario includes surcharging of the reservoir (a reduction in controlled conduit flows) commencing about June 20 to offset spillway flows to minimize the total outflow. The effect of expected heavy rain on reservoir operations is being reviewed.
Assiniboine River from Russell to Headingley
· Over-bank flows could occur from Russell to Brandon if widespread heavy rain falls as presently predicted. Portions of the Assiniboine River downstream of Brandon to Headingley are unlikely to experience over-bank flows even with the predicted rain. Computations are underway to determine what peak river levels might result from 50 mm (two in.) of rain over the Assiniboine River watershed upstream of Brandon.
· Flows in the Portage Diversion are being reduced now that the river levels in Winnipeg are below the range where sewer backup would be affected by the river level. Flows down the Assiniboine River at Southport have been increased daily by 500 cfs since Saturday morning. River levels from Southport to west Winnipeg will gradually increase by about an additional 0.6 m (two ft.) or so during the rest of this week.
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