More Flood History

Over the last month or so, when listening to the flood stories and comparison of the Great Flood of 2010 to past floods, I realized that there was almost no talk of floods in the Bella Coola Valley from the huge flood of 1936 until another big flood in 1965.  I thought there must be something in there.  I checked my prime source, the publication Rainstorm and Flood Damage: Northwest British Columbia, 1891-1991 and another publication, Bella Coola River Flood and Erosion Control, W. Tempest, September 1974 commissioned to do planning after the big 1968 flood.  The only mention of any significant floods was one in 1950, but other than that the period is referred to as having moderate floods between 1936 and 1965 -- a 29 year period with only one significant flood!

Nusatsum Forest Service Road 1.0 km Sep 2010

In Tempest's 1974 report he has a nice summary of the flood history in the valley from 1874 to 1974:

“Moderate floods occurred in the winters of 1905, 1917, 1924, 1932.  In October 1934 a large flood occurred which washed out many bridges and destroyed much of the road.  Engineer Swan (Department of Public Works) reported at the time that this was the largest flood in 60 years. Then in November 1936 a flood occurred higher that [sic] in 1934 and devastated the country once more.  Moderate floods occurred at frequent intervals up to the high floods of 1965 and 1968. The 1968 flood was the largest flood recorded during the period of record of the gauging station on the Bella Coola River.  Since the Hagensborg area had been settled from 1894 and the original settlers would still be alive in 1934, Swan’s report of the 1934 flood being the largest in 60 years appears reasonable.  From river level surveys taken at the time and estimated discharge at Hagensborg the flood in 1934 was slightly less in magnitude than the 1968 flood.  Field surveys of historical flood marks indicate that the 1936 flood was higher than the 1968 flood by approximately one foot.”  Tempest, 1974

Most of the anecdotal information I've gotten is that the Great Flood of 2010 was higher than the water in 1968 and in most areas more than a foot or two higher.  My records of floods since the 1991 report have moderate floods in 1996, 2004, 2006 and 2009, none anywhere close to the Great Flood of 2010.  You can see where I'm going here to suggest that Great Flood of 2010 was the largest flood since 1874 - a period of 136 years.  

Later on in the report he goes on to calculate flood frequency curves using some math and statistics.  It’s interesting to note:

“Since the 1934 flood had apparently not been exceed in 60 years  the length of the historical record is from 1874 to 1972, a period of 99 years.  Including the 1934, 1936 and 1968 floods, the number of events equaling or exceeding the lowest historical flood is 3 and the total number of events in the array is 27”

Tempest covered the period of 1874-1974, the Rainstorm and Flood Damage: Northwest British Columbia, 1891-1991 covers the period of 1975 to 1991  My personal notes cover the period of 1992 to 2010. When you add the number of events from 1975 onwards to Tempest's total number of events of 27 then the likely total flood events of moderate size and greater since 1874 in the Bella Coola Valley is 36, or expect a moderate or greater flood every 3.8 years.  Whopper floods seem to have occurred 6 times in 136 years; 1874, 1934, 1936, 1968, 1980 and 2010 or every 22.6 years.

If you are looking at your situation, and trying to crystal ball the future then plan accordingly.  Me, I'm hoping it's 2010 plus 22.6 years for the 2032 fall/winter before we get a whopper flood again, but I know enough about statistics that means it could also be next year... Grizzly