Flooding events in Canada: British Columbia

Introduction

The mountainous terrain of British Columbia is a strong influence on climate and flood characteristics. Moist air from the Pacific Ocean is forced by the prevailing westerly winds up over coastal mountain ranges where it drops its moisture load as rain or snow. By the time the air masses reach the Interior of the province, most of the moisture has been lost. In addition, as the air descends from the high altitude of the mountains to the lower Interior plateau it heats up and dries even more by the process of adiabatic warming. As a result, the Interior plains are semi-arid. In the coastal regions, the most common cause of flooding is rainfall, sometimes in association with melting snow. In the drier Interior, flooding generally results from melting snow, and may be intensified by river ice jams.

Streams in most regions of the province have high flows from seasonal snowmelt, with the magnitude of the peak flow dependent on the depth of the snowpack and the weather conditions during the melt period. Peak flows from snowmelt typically occur from May to mid-July.

The major floods of 1948 in the Fraser and Columbia basins resulted from snowmelt. Snow continued to accumulate in the basins until mid-May due to cool weather. A sudden change to very warm weather over large parts of the province led to high snowmelt rates and resultant record flood conditions on many rivers.

Heavy rainfalls occasionally produce high peak flows in the summer period from June through September. The rainfall floods usually occur in the smaller drainage basins and normally there is little effect on major rivers.

For streams draining the coastal slopes of British Columbia, peak flows may also occur during the fall and early winter as a result of storm rainfall. These floods can potentially be greater than the springmelt peaks. The most severe events happen during a cool early winter with a shallow snowcover down to low elevations. With the onset of warm temperatures from a major storm system, snowmelt from higher elevations along with the rainfall contributes to substantial flood runoff. Once the snowcover has reached a sufficient depth, considerable rainfall may be absorbed without appreciable increases in streamflows. On occasion, these storms have enough intensity to carry over the Coast Mountains and produce flooding in the basins on the east side of the mountains.

In mountainous regions with steep stream channel slopes, high amounts of precipitation or rates may trigger debris torrents. Such torrents take the form of a fast-moving surge containing boulders and large plant debris followed by a liquefied mass of sediment and organic debris. The streambed and banks may be scoured to bedrock, resulting in a large volume of debris carried by the torrent.

Debris torrents are of particular concern in the areas of steep mountain streams on Vancouver Island and the west coast of the province. There is extensive evidence of past debris torrents near some of the municipalities at the base of the mountains on the north shore of Vancouver's harbour (the Lower Mainland) The hazard area also extends north along Howe Sound.

Present-day debris torrents are often due to poor logging practices: log jams are created on mountainous streams and then burst during a storm. Many of the recent debris torrents on the creeks of the east shore of Howe Sound have this origin. As well, landslides caused by saturation of unstable slopes are also a major cause.

A less common but intriguing phenomenon is that of floods due to the outburst of glacier-dammed lakes, or jökulhlaups. The lakes may drain through or underneath the ice, or over the ice. Glacier-dammed lakes may remain stable for some time and then release suddenly, or they may slowly fill and then release on a periodic basis.

Fraser River Floods

The Fraser River drains a basin of 233 000 square kilometres along a distance of 1370 kilometres from its source in Mount Robson Provincial Park to its mouth at the Strait of Georgia. This drainage basin is essentially a plateau comprising most of south-central British Columbia, with extensive mountain ranges forming its eastern and western limits. The river is the largest in the province, with a long-term mean annual flow of 2720 cubic metres per second (for the period 1912-1988 inclusive) recorded at Hope, where it emerges from its deep gorge in the Coast Mountains and flows its final 160 kilometres through the lower Fraser Valley to the sea.

The annual spring snowmelt freshets of the Fraser River system pose the principal flood hazard to those occupying its floodplain areas. Autumn or winter rainfall flooding occurs on some tributaries, but its effects are localized and not generally disruptive. The same is true of occasional winter inundation resulting from channel obstruction due to ice jam formation. Extremely high tides sometimes cause or aggravate flood conditions near the mouth of the Fraser, especially if they occur in association with storms.

In the Fraser River drainage basin, the floodplain areas prone to inundation by spring snowmelt freshets occupy only about one half of one percent of the total basin, but they include most of the lower Fraser Valley, parts of the key Interior communities of Kamloops (on the Thompson River), Prince George and Quesnel, and a portion of the Pemberton Valley (on the Lillooet River). Segments of the principal railway and highway facilities serving the province are located on these floodplains, as are two of the most important airports. The Lower Mainland economic region in and adjacent to the lower Fraser Valley contains more than half of the population and much of the commercial and industrial development of the province, as well as the principal port facilities serving Western Canada.

The proximity of the ocean, the prevailing westerIy winds which move the maritime air masses over the basin, and the north-south trending mountain ranges are the major features that control the climate of the Fraser River basin, define the areas of high and low precipitation, and bring about the variations in mean annual temperature throughout the province. Precipitation and temperature are the dominant factors influencing the hydrology of the Fraser River system. Each spring, increasing temperatures melt the winter snow and initiate the freshets in tributary streams that, in turn, merge to form the annual high flow event.

The basin's temperature is largely influenced by the incursions of the maritime and continental air masses. Cold arctic air masses can drop winter temperatures to extreme lows of -46°C in the northern part of the basin and -34°C in some parts of the southern half. The January mean temperature in most areas is below -29°C. Summer temperatures rise to monthly means in the order of 22°C, with daily maxima at some valley stations exceeding 38°C.

Altitude is a major factor influencing temperatures. In general, the annual increase in temperatures above 0°C occurs initially in March in the southern portion of the basin and moves northward and to higher elevations as the warmer maritime air masses displace the colder arctic air.

Movement of moist maritime air masses into the Fraser River basin creates substantial quantities of precipitation on the westward slopes of the Coast Mountains due to orographic uplift. Once into the basin, the eastern slopes of the Coast Mountains are in the rain shadow. As the air moves east over the Columbia Mountains precipitation increases significantly again.

Most precipitation in the basin is in the form of rain. In the lower Fraser Valley, 95% of the precipitation is rain, while at higher elevations in the northern part of the basin, 65% is rain. However, it must be noted that the meteorological stations tend to be in the valley floors, and the levels of precipitation and the percentage that falls as snow change significantly with increased elevation.

Some 75 000 hectares of floodplain borders the Fraser between Hope and the river mouth, of which 65 000 hectares lies outside a series of individual dyking systems stretching along both sides of the river from Agassiz to the Strait of Georgia. Most of these dykes protect agricultural land, but some stand between the river and more densely populated areas. Chilliwack, Harrison Hot Springs, and Agassiz all lie behind the dykes, as do parts of Mission and New Westminster. At the river mouth, both Lulu Island and Sea Island, as well as a number of smaller delta islands, are ringed with dykes protecting rural, urban and, in some cases, industrial areas south of Vancouver.

The duration of high water levels contributes to the flood hazard in a dyked area. Dykes tend to deteriorate when subjected to high river levels for extended periods, and so become a less reliable means of flood protection. The floodplain of the lower Fraser Valley is dependent on dykes for protection when the water elevation at Mission exceeds 5.5 metres, and this condition usually prevails for a month or more during high freshets. River velocities increase with rising water levels, augmenting the occurrence of bank erosion and scour which tend to weaken dykes. When high freshets occur, the rise in the water elevation at Mission from 5.5 metres to the peak usually extends over two or three weeks. The duration of the peak or near-peak levels intensifies the tendency of dykes to deteriorate. In high freshet years, the water elevation at Mission has remained within 0.15 metres of the Peak from four days to two weeks.

1894

The greatest Fraser River flood in the past century occurred in 1894, when the floodplain areas were in the very early stages of settlement and development. The lower Fraser Valley was sparsely populated, and Kamloops, Prince George, and Quesnel were essentially frontier settlements. This flood forewarned the hazard of occupying the Fraser River floodplain in the years to come.

1948

In 1948, Fraser River flooding was the greatest since that of 1894. The passage of five decades had witnessed the transformation of the lower Fraser Valley into a highly developed agricultural area, with commercial and industrial development becoming appreciable and suburban residential areas beginning to appear. Two transcontinental rail lines and the Trans-Canada Highway had been built through the valley, and the largest airport in the province had been established on Sea Island.

Photo - Flood of the Fraser River at Mission, 1948

Flood of the Fraser River at Mission, 1948

On June 10, 1948, the Fraser reached a peak elevation of 7.6 metres at Mission. Before the waters receded, over a dozen dyking systems had been breached and more than 22 000 hectares, nearly one third of the entire lower Fraser Valley floodplain area, had been flooded to this depth. The floodwaters severed the two transcontinental rail lines; inundated the Trans-Canada Highway; flooded urban areas such as Agassiz, Rosedale, and parts of Mission, forcing many industries to close or reduce production; and deposited a layer of silt, driftwood and other debris over the entire area.

The estimated damage amounted to $20 million ($146.9 million in 1998 dollars), most of which occurred in the lower Fraser Valley. Minor damage occurred in the Kamloops, Prince George and Quesnel areas, owing to the limited development of their floodplains at that time.

1972

The next major flood year was 1972. The first sign of potential spring flooding was predicted from snow surveys in February when above-average snowfalls were recorded. Subsequent surveys also indicated heavy mountain snowpacks. In mid-March, the provincial government alerted personnel and civil defence units of the possibility of flooding along the Fraser. In mid-April, meetings were held to coordinate and plan the activities of agencies that would be involved in the response to the flooding.

High temperatures in the Interior valleys toward the end of May caused rapid snowmelt, and many of the Interior rivers peaked at record levels in the latter part of May and in the first week of June. To protect low-lying lands, sandbagging of Prince George and Kamloops was undertaken. A number of subdivisions were inundated on June 2 in Kamloops, and in one area, 150 homes and 52 mobile homes were flooded due to dyke failure.

Following a cooling trend beginning on May 30, substantial snowmelt again occurred with a sudden rise in temperature, accompanied by heavy rains. Subsequently, the upper Fraser reached another peak between June 11 and 14. On June 16, the lower Fraser peaked at Hope, with a maximum instantaneous flow of 3400 cubic metres per second and a maximum elevation of 7.1 metres, well above the danger level of 6.1 metres.

This second and last major peak of the season, which was higher than the first, increased the extent of flooding in already flooded areas. On the Fraser River, the areas most affected were Prince George in the upper Fraser and downstream from Hope in the lower Fraser Valley. The dyking systems were generally effective in preventing large-scale damage.

Damage on the Fraser in 1972 amounted to $10 million ($36.9 million in 1998 dollars) and occurred mainly in the upstream communities of Prince George and Kamloops, and in the Surrey area of the lower Fraser Valley.

Tsunami Strikes West Coast

On March 27, 1964, North America suffered its strongest earthquake this century. The epicentre of the quake was located 1300 kilometres north of Prince Rupert, British Columbia, off the coast of Anchorage, Alaska. The quake, which registered 8.5 on the Richter scale, heaved up a section of the ocean floor 15 metres. The energy released by the earthquake was estimated to be equivalent to the explosion of 32 million tonnes of TNT.

The resulting wave generated by the earthquake travelled across the Gulf of Alaska at speeds of up to 720 kilometres per hour. The tsunami struck the Canadian West Coast near the time of high tide. The highest wave was at Shields Bay on the west coast of Graham Island where the crest was reported to be more than 5 metres above the highest normal tide. Although damage along the coast was widespread, most of it occurred at Port Alberni.

To reach Port Alberni, the tsunami had to travel 40 kilometres up Alberni Inlet. The inlet served to concentrate the energy of the wave. The first wave arrived approximately 4.5 hours after the earthquake. The wave rose 2.1 metres above the normal tide level, causing some flooding. While the wave receded the water from the harbour also drained, leaving fishing boats on the bottom. Two hours later, the second wave arrived, raising water levels 4 metres above the normal tide level. It was this second wave that caused the damage.

As the waters rose, boats were torn from their moorings and piled logs were picked up and added to the destructive force of the gigantic wave. Telephone poles were snapped and homes floated 1 kilometre up the Somass River. People reported simply opening the doors and letting the water wash through their homes. Silt was deposited in many of the flooded homes. Cars and boats were strewn about and 69 homes were damaged. Approximately $2.5 to $3 million ($11.4 to $13.6 million in 1998 dollars) in damages were inflicted on homes, businesses, and industries.

In Hot Springs Cove near Pacific Rim National Park, 18 of 20 homes were knocked off their foundations. In Bamfield, on Barkley Sound, basements were flooded.

No one in Canada was killed. However, 11 people in Crescent City, California, were drowned when they went down to the beaches to watch the waves come ashore.

Kicking Horse Pass - 1978

In 1978, debris flows triggered by a jökulhlaup from Cathedral Glacier destroyed three levels of Canadian Pacific railway track, derailing a freight train. Sections of the Trans-Canada Highway were also buried. Debris flows at the same site in previous years have also been linked to jökulhlaups.

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