103 13.5 Flooding — Physical Geology – 2nd Edition

Figure 13.5.1 Variations in discharge of the Stikine River during 2013. [Image Description]

Streams in coastal areas of southern British Columbia show a very different pattern from those in most of the rest of the country because their drainage basins do not remain entirely frozen and because they receive a lot of rain (rather than snow) during the winter. The Qualicum River on Vancouver Island typically has its highest discharge levels in January or February and its lowest levels in late summer (Figure 13.5.2). In 2013, the minimum discharge was 1.6 m3/s, in August, and the maximum was 34 times higher, 53 m3/s, in March.

Figure 13.5.2 Variations in discharge of the Qualicum River during 2013. [Image Description]

When a stream’s discharge increases, both the water level (stage) and the velocity increase as well. Rapidly flowing streams become muddy and large volumes of sediment are transported both in suspension and along the stream bed. In extreme situations, the water level reaches the top of the stream’s banks (the bank-full stage, see Figure 13.3.4), and if it rises any more, it floods the surrounding terrain. In the case of mature or old-age streams, this could include a vast area of relatively flat ground known as a flood plain, which is the area that is typically covered with water during a major flood. Because fine river sediments are deposited on flood plains, they are ideally suited for agriculture, and thus are typically occupied by farms and residences, and in many cases, by towns or cities. Such infrastructure is highly vulnerable to damage from flooding, and the people that live and work there are at risk.

Most streams in Canada have the greatest risk of flooding in the late spring and early summer when stream discharges rise in response to melting snow. In some cases, this is exacerbated by spring storms. In years when melting is especially fast and/or spring storms are particularly intense, flooding can be very severe.

One of the worst floods in Canadian history took place in the Fraser Valley in late May and early June of 1948. The early spring of that year had been cold, and a large snow pack in the interior was slow to melt. In mid-May, temperatures rose quickly and melting was accelerated by rainfall. Fraser River discharge levels rose rapidly over several days during late May, and the dykes built to protect the valley were breached in a dozen places. Approximately one-third of the flood plain was inundated and many homes and other buildings were destroyed, but there were no deaths. The Fraser River flood of 1948, which was the highest in the past century, was followed by very high river levels in 1950 and 1972 and by relatively high levels several times since then, the most recent being 2007 (Table 13.1). In the years following 1948, millions of dollars were spent repairing and raising the existing dykes and building new ones; since then damage from flooding in the Fraser Valley has been relatively limited.

Table 13.1 Ranking of the maximum stage and discharge values for the Fraser River at Hope between 1948 and 2008. Typical discharge levels are around 1,000 m3/s.[1]
Rank Year Month Date Stage (metres) Discharge (cubic metres per second)
1 1948 May 31 11.0 15,200
2 1972 June 16 10.1 12,900
3 1950 June 20 9.9 12,500
4 1964 June 21 9.6 11,600
5 1997 June 5 9.5 11,300
6 1955 June 29 9.4 11,300
7 1999 June 22 9.4 11,000
8 2007 June 10 9.3 10,850
9 1974 June 22 9.3 10,800
10 2002 June 21 9.2 10,600

Just a year after the Saguenay floods, the Red River in Minnesota, North Dakota, and Manitoba reached its highest level since 1826. As is typical for the Red River, the 1997 flooding was due to rapid snowmelt. Because of the south to north flow of the river, the flooding starts in Minnesota and North Dakota, where melting starts earlier, and builds toward the north. The residents of Manitoba had plenty of warning that the 1997 flood was coming because there was severe flooding at several locations on the U.S. side of the border.

After the 1950 Red River flood, the Manitoba government built a channel around the city of Winnipeg to reduce the potential of flooding in the city (Figure 13.5.3). Known as the Red River Floodway, the channel was completed in 1964 at a cost of $63 million. Since then it has been used many times to alleviate flooding in Winnipeg, and is estimated to have saved many billions of dollars in flood damage. The massive 1997 flood was almost too much for the floodway; in fact the amount of water diverted was greater than the designed capacity. The floodway has recently been expanded so that it can be used to divert more of the Red River’s flow away from Winnipeg.

Figure 13.5.3 Map of the Red River floodway around Winnipeg, MB (left), and aerial view of the southern (inlet) end of the floodway (right).

Canada’s most costly flood ever was the June 2013 flood in southern Alberta. The flooding was initiated by snowmelt and worsened by heavy rains in the Rockies due to an anomalous flow of moist air from the Pacific and the Caribbean. At Canmore, rainfall amounts exceeded 200 millimetres in 36 hours, and at High River, 325 millimetres of rain fell in 48 hours.

Figure 13.5.4 Map of the communities most affected by the 2013 Alberta floods (in orange).

In late June and early July, the discharges of several rivers in the area, including the Bow River in Banff, Canmore, and Exshaw, the Bow and Elbow Rivers in Calgary, the Sheep River in Okotoks, and the Highwood River in High River, reached levels that were 5 to 10 times higher than normal for the time of year (see Exercise 13.5). Large areas of Calgary, Okotoks, and High River were flooded and five people died (see Figures 13.5.2 and 13.5.3). The cost of the 2013 flood is estimated to be approximately $5 billion. Learn more about Alberta’s flood of the century.

Figure 13.5.5 Flooding in Calgary (June 21, left) and Okotoks (June 20, right) during the 2013 southern Alberta flood.

Figure 13.5.6 shows the highest discharges per year between 1915 and 2014 on the Bow River at Calgary. Using this data set, we can calculate the recurrence interval (Ri) for any particular flood magnitude using the equation: Ri = (n+1)/r (where n is the number of floods in the record being considered, and r is the rank of the particular flood). There are a few years missing in this record, and the actual number of data points is 95.

The largest flood recorded on the Bow River over that period was the one in 2013, 1,840 cubic metres per second (m3/s) on June 21. Ri for that flood is (95+1)/1 = 96 years. The probability of such a flood in any future year is 1/Ri, which is 1%. The fifth largest flood was just a few years earlier in 2005, at 791 m3/s. Ri for that flood is (95+1)/5 = 19.2 years. The recurrence probability is 5%.

  1. Calculate the recurrence interval for the second largest flood (1932, 1,520 m3/s).
  2. What is the probability that a flood of 1,520 m3/s will happen next year?
  3. Examine the 100-year trend for floods on the Bow River. If you ignore the major floods (the labelled ones), what is the general trend of peak discharges over that time?

See Appendix 3 for Exercise 13.5 answers.

One of the things that the 2013 flood on the Bow River teaches us is that we can’t predict when a flood will occur or how big it will be, so in order to minimize damage and casualties we need to be prepared. Some of the ways of doing that are as follows:

  • Mapping flood plains and not building within them
  • Building dykes or dams where necessary
  • Monitoring the winter snowpack, the weather, and stream discharges
  • Creating emergency plans
  • Educating the public

Image Descriptions

Figure 13.5.1 image description: The highest and lowest daily average discharge from the Stikine River by month in 2013 in cubic metres per second (m3/s).
Month Lowest daily average discharge (m3/s) Highest daily average discharge (m3/s)
January 100 100
February 100 100
March 100 100
April 100 100
May 150 2,450
June 1,250 1,800
July 700 1,450
August 300 700
September 250 300
October 250 750
November 150 600
December 100 100

[Return to Figure 13.5.1]

Figure 13.5.2 image description: The highest and lowest daily average discharge from the Qualicum River by month in 2013 in cubic metres per second (m3/s).
Month Lowest daily average discharge (m3/s) Highest daily average discharge (m3/s)
January 5 24
February 6 14
March 8 50
April 7 21
May 7 20
June 5 12
July 3 7
August 2 3
September 3 8
October 3 37
November 3 13
December 3 4

[Return to Figure 13.5.2]

Figure 13.5.6 image description: Peak annual discharge from the Bow River from 1915 to 2014 in cubic metres per second (m3/s)
Year Peak Annual Discharge (m3/s)
1915 1,125 (major)
1916 800
1919 475
1920 525
1922 375
1923 850
1924 400
1925 400
1926 275
1927 575
1928 580
1929 1,325 (major)
1930 425
1931 325
1932 1,520 (major)
1934 525
1935 400
1936 425
1937 300
1938 475
1939 325
1940 325
1941 200
1942 375
1943 375
1944 250
1945 375
1946 400
1947 425
1948 600
1949 225
1950 500
1951 500
1952 575
1953 425
1954 400
1955 375
1956 325
1957 275
1958 325
1959 300
1960 400
1961 250
1962 450
1963 375
1964 475
1965 350
1966 425
1967 275
1968 475
1969 325
1970 400
1971 425
1972 350
1973 350
1974 250
1975 250
1976 225
1977 300
1978 150
1979 375
1980 425
1981 300
1982 225
1983 225
1984 200
1985 425
1986 200
1987 250
1988 250
1989 550
1990 400
1991 300
1994 300
1995 500
1996 300
1997 300
1998 350
1999 400
2000 225
2001 175
2002 350
2003 250
2004 250
2005 791 (major)
2006 250
2007 375
2008 325
2009 175
2010 200
2011 350
2012 475
2013 1,840 (major)
2014 350

[Return to Figure 13.5.6]

Media Attributions

  1. Mannerström, M, 2008, Comprehensive Review of Fraser River at Hope Flood Hydrology and Flows Scoping Study, Report prepared for the B.C. Ministry of the Environment [PDF]. Available at: http://www.env.gov.bc.ca/wsd/public_safety/flood/pdfs_word/review_fraser_flood_flows_hope.pdf ↵

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