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Case Studies: Flood Control on the Red, Assiniboine and Fraser Rivers

Most flood control structures across the country are smaller versions of the structures used on the Red or Fraser rivers. For this reason, we offer detailed information regarding flood control on the Red and Assiniboine River in Manitoba and the Fraser River in British Columbia.

Flood Control on the Red and Assiniboine Rivers in Manitoba

The earliest flood control structures built in the Red River basin were a series of dykes along the Assiniboine River between Portage la Prairie and Winnipeg. Deposition of stream sediment along parts of this stretch of the Assiniboine River has resulted in the river bottom being higher than the surrounding land surface. Dyking began along the Assiniboine before the 1900s but did not become more widespread until after the floods of 1922 and 1923. In addition to dyking, cutoffs were made through sharp river bends. The dykes were generally low and could not contain a major flood.

Prior to the construction of the dykes, flooding was a severe problem. When riverbanks were overtopped, many square kilometres of agricultural land would be flooded, often without the possibility of the water returning to the river when floodwaters receded.

Subsequent to the Red River flood in 1950, a large scale structural approach was planned and implemented for protection against future flood damages by the Red and Assiniboine rivers. The Greater Winnipeg Dyking Board was established to construct a system of dykes to provide a degree of protection for the city against floods. In low areas, boulevard type dykes were constructed and pumping stations installed to lift storm and sanitary sewage into the rivers. In addition, "borrow" areas were established from which material could be obtained to raise dykes during flood emergencies. Approximately 110 kilometres of dykes and 31 pumping stations were built.

In 1958, a Royal Commission on Flood Cost Benefit reported on a major study to investigate structural solutions to the flooding problems on the Red and Assiniboine rivers. As a result, three projects were constructed between 1962 and 1972:

  • a floodway to divert 1700 cubic metres per second from the Red River south of Winnipeg to the east of the city, discharging back into the Red River at Lockport (completed in 1968);
  • a 700 cubic metre per second capacity diversion channel to convey floodwaters from the Assiniboine River just upstream from Portage la Prairie, northward to Lake Manitoba (completed in 1970);
  • the Shellmouth Dam in the upper reaches of the Assiniboine River just north of Russell (completed in 1972) to store flood waters and reduce peaks downstream.

The Red River Floodway

Construction of the Red River Floodway was started in 1962 and completed in 1968. The floodway consists of four components: the floodway channel, the inlet control structure, the dykes, and the outlet structure.

The floodway channel is approximately 48 kilometres long, with a drop of 5 metres between the inlet and outlet and a designed depth of flow in the channel of 8 metres. The width at the top of the channel varies from 213 metres to 305 metres. Material excavated from the channel was deposited along the side of the channel to form an embankment 6 metres high.

Photo - Red River floodway
Red River floodway

The entrance to the floodway is located in the eastern bank of the Red River near St. Norbert. An earth-fill weir at the entrance ensures that flows below flood level continue down the Red River.

The inlet control structure is located on the Red River just downstream from the floodway inlet. The purpose of the control structure is to regulate the flow between the natural channel of the Red River and the floodway channel, during the period of high water levels. The gates of the control structure are normally in a submerged position with about 1.8 metres of water over them in the summer months.

To prevent floodwaters from bypassing the inlet control structure, dykes have been constructed on either side. On the east side of the Red River, the dyke is incorporated into the floodway embankment. To the west of the Red River, the dyke extends for 34 kilometres from the inlet control structure to a point where the natural ground is above the design flood elevation. During the 1997 flood, the dyke was raised and extended an additional 25 km to prevent flood waters from bypassing the structure and entering the city.

The drop over the entire reach of the floodway is 5 metres, but the corresponding drop of the Red River is 10 metres. The purpose of the outlet structure is to dissipate the energy in the water at its point of re-entry to the Red River near Lockport, thereby preventing damage and erosion in the river.

The Portage Diversion

The Portage Diversion is a 29-kilometre channel designed to carry up to 700 cubic metres per second of flood flow from the Assiniboine River just upstream from Portage la Prairie northward to Lake Manitoba. Construction of the project began in 1965 and was completed in 1970.

A dam was constructed across the Assiniboine River, creating a reservoir covering 652 hectares with a storage capacity of 18 000 000 cubic metres. The dam allows flow down the river under normal conditions and is used to help regulate flow down the diversion channel, particularly during high river levels. An inlet control structure regulates the flow to the diversion channel itself. The diversion channel enters Lake Manitoba through the Delta Marsh.

The Shellmouth Dam

The third major structure in the flood control system is the Shellmouth Dam, located about 19 kilometres northwest of Russell in an area where the valley of the Assiniboine is wide and deep. Construction was initiated in 1964 and completed in 1972. The dam is an earth-fill structure with gravel and rock riprap for slope protection and a reinforced concrete spillway. The reservoir created by the dam is approximately 56 kilometres long with a gross storage capacity of nearly 500 000 000 cubic metres. The protection afforded by the reservoir extends along the Assiniboine from the dam site to its confluence with the Red River. Cities benefiting from its flood protection and low flow augmentation include Brandon, Portage la Prairie, and Winnipeg.

Other Manitoba flood mitigation projects

Subsequent to the 1966 flooding, the federal and Manitoba governments agreed to provide protection to communities along the Red River south of Winnipeg. Permanent dykes were constructed around seven communities. In addition, farmsteads were protected either by dykes or by raising the foundations of the farm buildings. The dyking projects, constructed to the 1950 flood levels, were built between 1967 and 1972. Following a major flood in 1979, ring dykes around the seven communities were raised to the 100-year level and one additional community was dyked. Following the 1997 flood, these dykes were further raised to the 1997 level plus freeboard, and additional communities are being considered for dyke protection.

Performance of flood control works

Flood control works on the Red and Assiniboine rivers have been extensively used since their completion. Floods occurred on the Red River in 1969, 1970, 1974, 1979, 1987 and 1997. The 1979 flood was similar in magnitude to the devastating flood of 1950, which was the catalyst for the construction of these works. In 1997, a flood surpassed only by the 1826 flood of record occurred on the Red River. This event served to demonstrate yet again the enormous value of the flood control works to the City of Winnipeg and, at the same time, to emphasize the likelihood that a flood of even greater magnitude could occur. A record flood occurred on the Assiniboine River in 1976.

An example of the benefits from such flood control works has been the City of Winnipeg, where serious flooding has been avoided since control works went into operation. Potential damages prevented during the 1969 to 1991 period are in excess of $1.1 billion (in 1998 dollars) in Winnipeg alone. The damages prevented in 1997 are significant. In the absence of these works, a large area of the City would have been inundated, some 150 000 people or more would have been evacuated, and the economic centre of Manitoba would have been completely paralyzed. The cost of the Red River Floodway, Portage Diversion and Shellmouth Reservoir totalled $94 million when constructed, or about $416.6 million in 1998 dollars. It is clear that these flood control works have been extremely cost effective.

Flood Control on the Fraser in British Columbia

In British Columbia, the structural approach is relied upon to protect Fraser River floodplain development from flood damages, particularly in the lower Fraser Valley. Dyke construction together with associated works has been the historic method of containing floods. Also, the Nechako River diversion assists flood control. The storage capacity of reservoirs for hydroelectric developments on the Bridge and Stave rivers provides further limited protection when operated for flood control purposes during the onset and occurrence of extremely high spring freshet.

Initial flood prevention efforts in the lower Fraser Valley protected agricultural lands. Early settlers constructed dykes in 1864 on Lulu Island, and by 1878, dykes were found in the Chilliwack, Surnas and Matsqui prairie areas.

By 1960, these first efforts had grown to some 375 kilometres of river and sea dykes from Agassiz to Steveston, protecting nearly 65 000 hectares of a total of 75 000 hectares of valley floodplains and tide-lands. Along with the increase in area came a change in the type of land protected. A major shift had occurred to prevent flood damage to the continually encroaching residential, industrial and commercial properties.

Photo - Fraser River dyking
Fraser River dyking

Following the disastrous flood of 1948, more than 260 kilometres of the dykes were repaired or rebuilt by the Fraser River Dyking Board. Most of the work was completed prior to the 1950 spring freshet, when the peak water level at Mission was less than 0.3 metres below the 1948 level. However, because of the haste required to restore the dykes, time did not permit the design and development of more permanent structures, nor the use of higher quality materials other than those immediately available. As a result, dykes deteriorated during subsequent high river stages. More emergency repairs allowed the dykes to withstand the high freshet levels of 1964 and 1967.

On May 24, 1968, the federal and provincial governments entered into an agreement to undertake a massive program of dyke extension and improvement. The agreement provides for the Fraser River Flood Control Program to rehabilitate and improve river and sea dykes, and to provide bank protection to ensure the safety of the dykes against erosion. Over the past 24 years, the Program has established a system of dykes that protects most of the Fraser Valley floodplain. In some areas, the Program has also provided pumping capacity and main drains to prevent flooding behind dykes as a result of local runoff. This system will protect against the highest floods of record resulting from spring snowmelt runoff. The river dykes completed under the Program have successfully withstood the 1972 freshet (which exceeded the levels of the 1964 and 1967) and the sea dykes have endured the record high sea levels of 1982.

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