Flooding and Landslide Events Southern British Columbia 1808-2006 _______________________________________________________________________ D. Septer Province of British Columbia Ministry of Environment 1. Introduction Landslides, snow avalanches and flooding events may have devastating impacts on property and can result in the loss of human life. Many such events have been recorded while others only remain in the memories of those affected. A clear understanding of the magnitude and frequency of landslides, flooding and storm events may help in project planning, emergency planning and risk assessment. Thus the importance of storms, which create such events, is of interest to a wide audience ranging from emergency measure planners and foresters to engineers and historians. In order to understand current weather and possibly make predictions of future weather patterns, it is important to have a database of historical weather information. Septer and Schwab (1995) have made an attempt to assemble and catalogue such information for northwestern British Columbia. Their report contains detailed historical information pertaining precipitation, floods, landslides and snow avalanches and damage caused by these. 2. Scope The intent of this report is to produce a chronological list of flood events for the southern part of British Columbia. Together these two catalogues of historically important storm and flood events will provide a better understanding about the occurrence and magnitude of such devastating events in British Columbia. The geographic area covered in this study is the southern half of British Columbia roughly from Bella Coola in the west to Quesnel in the Interior and on to the Alberta border in the east and includes Vancouver Island. Some extreme wind and snow events have also been inckuded even though these did not cause flooding or landslide damage. The June 23, 1946 earthquake has been decribed as well. Unless stated otherwise, damage amounts are in dollar values of the time of the event. 3. Information reliability Weather is one of the most common topics of conversation. Extreme events, such as heavy rainstorms, hurricanes, and subsequent damage get lot of coverage; first in the newspapers, and later in reports and articles in journals and books. However, historically based information on weather events and their impacts is scattered throughout many different sources, including newspapers, diaries, ship logs, streamflow records. Terminology used to describe mass wasting events may vary widely in newspaper accounts. In order to be consistent, no effort has been made to confirm the correctness of the terms used in the original reports. Also as little as possible was changed in the original reporting. Eyewitness reports and many reported cases of “worst flooding on record” are all in quotation marks by the author. Such information must be taken at face value. Consequently, the information contained in this report is only as good as originally reported. Thus all the information and quotations must stand on their own merit, without benefit of the author’s analysis or comment, which could be influenced by personal opinions. 4. Community flooding and landslide archive In a report with a time span and geographic area like this, many flood and storm events will have been missed. Personal memories, diaries, newspaper articles all contain valuable information on historical storm and flood events. This local knowledge may also make this report more accurate and help planners in future flood prevention measures. Treating it as an open file, the author welcomes any new information to be included in future updates. Additional information about any missed events can be send to the author’s address at P.O. Box 1000, Whaletown, BC, V0P 1Z0. 5. Some climatic features The primary climatic features of the coastal region include relatively high annual precipitation with the wettest months occurring in fall and winter, and a relatively small annual range of temperature. Heaviest precipitation is caused by low-pressure systems imbedded in an intensified westerly flow move onto the coast, often in close succession. Moderate to strong southwesterly winds cause the heaviest rains. Due to the complex interaction between atmospheric circulation patterns and major topographic features distributed along the coast which serve as barriers to the movement of air masses, local variations exist in precipitation and temperature. Henderson Lake on the west coast of Vancouver Island, for example, shows an unusual high precipitation. From November 1922 to May 1936, the Dominion Fish Hatchery operated a weather station at the north end of the 32-km long lake that lies in a northwest to southeast direction. The rain gauge was situated close to a high mountain situated to the north and east of the station. Though the average annual precipitation on the west coast of Vancouver Island is over 100 in. (250 cm), for the first eight years of operation Henderson Lake recorded an average annual precipitation of about 250 in. (625 cm). The heaviest monthly rainfall at Henderson Lake was recorded in December 1923 with 79.45 in. (2.02 m). Relatively warm southwesterly flows of fall storms may well generate free convection once initial lift has occurred. Radar measurements over the lower Fraser Valley (Bonser 1982) have documented cells of much higher than normal precipitation within the frontal storms. The heaviest rainfall from these convective cells is quite local (Jackson et al. 1985). Most storms in mountainous western Canada are cyclonic, but are modified by orographic effects. Within these storms, cells of very intense rainfall as small as 5 km across exist. However, the course grid of weather stations might not record these. Bruce (1961) clearly shows the large increases in rainfall amounts as one goes from the Western Fraser Delta-Boundary Bay area to the mountainous terrain north and east of Vancouver. Areas along west facing mountain slopes tend to have more clouds and subsequently receive more precipitation than the eastern faces of the mountains. The southeastern lowlands of Vancouver Island, the islands of the Straits of Georgia and the Fraser River estuary lie in the rain shadow of the Vancouver Island coast range and the Olympic Mountains in Washington state. Consequently, this is the driest zone of the coastal climatic region. It is also the warmest with more hours of bright sunshine during the summer. The climate of the coastal region is controlled on a seasonal basis by macro-scale atmospheric processes. Storm tracks in the late fall, nicknamed “Pineapple Express” or “Hawaiian Express” start in the tropics but sweep away to the north and east and eventually may hit the British Columbia coast. But the jet stream has another branch that can send all that wind and rain over California, causing flooding and landslides there while not affecting British Columbia. During the winter, vigorous circulation is produced by a strong temperature gradient between tropical and polar latitudes. During this period, low pressures over the Gulf of Alaska and high pressures in the interior combine to produce strong pressure gradients over British Columbia, Oregon and Washington and southerly surface winds prevail. Winter storms are not the only destructive storms in British Columbia. In fact, the more devastating disturbances occur during the spring and fall months. High winds and pelting rain often combine to smash houses, trees, and down communication circuits, often causing millions of dollars of damage. One of the most damaging storms ever to hit the Pacific Northwest was typhoon “Freda” on October 11-12, 1962. 6. Flood events In the study area, extreme floods do not result from the same flood producing mechanisms on all drainage basins. Some watersheds have a snowmelt-induced flood regime in spring or summer, some a rainfall- induced flood regime either as rainfall runoff only or as rain-on-snow runoff in fall or winter, while others have both of the two flood regimes. On most drainage basins in the coastal region, rainfall-induced floods occur in the fall and early winter. In spring and summer extreme floods primarily occur only on the main stem of major rivers that flow through the region. Areas in the coastal region that have both a fall/winter rainfall-induced and a spring/summer snowmelt-induced flood regime, extreme rainfall-induced floods are greater than those estimated on the same basin for snowmelt floods. Unit discharges of extreme floods on basins with only rainfall-induced flood regimes are greater than those on other basins in the coastal region with only snowmelt- induced flood regimes (Melone 1985). Extreme weather events in the southern coast mountains, with its close proximity to the lower mainland, have always received much coverage in the media and reports. Recent flooding and debris torrents in the steep catchments on the east shore of Howe Sound have resulted in costly damage to Highway 99, the tracks of the former BC Rail, and a number of settlements between West Vancouver-Squamish. The climate of the Howe Sound area is strongly influenced by topography and by the confined nature of the inlet. In 1956, the Pacific Great Eastern Railway (later renamed BC Rail) was completed along Howe Sound. Two years later, Highway 99 between North Vancouver-Squamish was opened. Soon after, settlement and associated development occurred in the area. Prior the 1950s, there are only a few reports of property damage or loss of life. Charles Creeks (also known as Strachan Creek or Strachan-2 Creek) is the most active of all the Howe Sound creek with six debris torrents in the 26 years between 1969-1985. The debris slides are controlled by a combination of topographic, geologic, and hydrometeorological factors. Deforestation and particularly clear-cut logging and road building are often blamed for flooding and landslide events. Clear-cutting large areas on steep slopes plus poorly constructed logging roads, and the increase of logging at higher altitudes where the soils are fragile, increases the runoff in many areas.