Engineer explains flood modelling tools, as Assiniboine River in Manitoba is breached

On Sunday, a river dyke along the Assiniboine River in Manitoba was deliberately breached, leaving people to watch anxiously as hundreds of hectares of farmland and as many as 300 houses were expected to be inundated.

Record high water levels along the Assiniboine forced the provincial government to act and remove a section of roadway that doubles as a dike at Hoop and Holler close to Portage La Prairie. By releasing water at that location the authorities expected to flood an area of 225 square kilometres, thereby aiming to prevent an even bigger flooding of over 500 square kilometres downstream.

So far the released water has been seeping out over the adjacent lands, at a lower speed than was expected. The river is expected to crest in the area this week.

Consulting engineers are helping the Manitoba Ministry of Infrastructure and Transportation cope with the situation, doing geotechnical, hydraulic and structural work, according to one source, who preferred to remain unnamed. However, in the midst of the crisis there was no-one available  to give more details from the Ministry.

CCE did manage to talk to Tamsin Lyle, P.Eng. of Northwest Hydraulic Consultants of Vancouver. She recently returned from Manitoba where she has just begun a flood modeling study of the Red River. (Northwest Hydraulics recently won an award of excellence from Consulting Engineers of British Columbia for a study they did on managing flood risks in Prince George.) 

We asked Lyle how engineers’ modeling studies can help to predict how flood water will behave and how different areas will be affected. She explained that while one-dimensional models work well for flood forecasting in a confined topography such as B.C.’s Fraser River, for wide, flat land with just a few undulations like that of southwest Manitoba, two-dimensional models provide more accuracy.

With one-dimensional modeling, the engineers create cross sections of ariver. But “in a two-dimensional model, you basically cover a river flood plain with a mesh,” says Lyle. “You create cells and in each cell the model will calculate the water level, the velocity and direction of flow at the cell.”

Cells could be made as small as 1 m x 1 m, but in wide open areas they are more like 800 m x 800 m — “the size of the average field in Manitoba,” Lyle explains. Using GIS and LIDAR data, the engineers build up a representation of the land, “and then,” says Lyle, “you add on top of that a hydraulic mesh to calculate where the water is going to go.”

Northwest Hydraulic has done two-dimensional flood models for several rivers in B.C., the U.S., Bangladesh and Brazil, “with fairly good success” says Lyle.

However, she says, flood modeling is a “very complex” problem that can be difficult to convey to the public. To that end, Northwest Hydraulics has developed a flood modeling tool that is being used for a community along Cowichan River on Vancouver Island. The model is dynamic, she explains, “so you can see the water washing over the ground,” and it can be viewed on Goole Earth, “so a person can zoom into their house and watch flood events, such as a recent event they have experienced or a more severe event like a 200-year flow, to see where the water gets to in relation to features they know. They can see how deep the water would be in terms of their own backyard. This has been a fabulous tool in terms of educating the public and policy makers.”

The Assiniboine River basin is 163,000 square kilometres, and part of the Red River watershed which runs north from North and South Dakota and Minnesota into Lake Winnipeg. According to Natural Resources Canada, the river system has an average valley gradient of only 0.0001 in Manitoba, creating frequent flooding problems in spring.