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Thames River Flood - December 2008
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Thames River Flood - December 2008The Thames River watershed experienced a
significant flood at the end of December 2008. The month was very
wet and snowy overall, with precipitation 50% above normal (recorded
by Environment Canada at the London Airport). A major melt in
mid-December helped to saturate the ground and fill depressions, and
a significant snow pack redeveloped across the watershed by late
December. These two factors - above normal December precipitation
and saturated watershed conditions - set the stage for a major flood
event. Most of the snow pack melted quickly due to the prolonged warm temperatures, running off the saturated ground and into watercourses. Preliminary estimates suggest the flood levels were as high as a 1:50 year occurrence (i.e., a flood that would occur once every 50 years) in some upstream areas, including the Avon River through Stratford and the North Thames River through St. Marys and into Fanshawe Reservoir. Flood levels on the South and Middle Thames were generally between a 1:5 year and 1:10 year occurrence. The combined operations of Wildwood, Fanshawe
and Pittock Dams reduced flows in the Thames River through London to
below a 1:10 year flood. Without the UTRCA’s flood control system,
London would have experienced close to a 1:100 year flood along the
North Thames River and below the Forks of the Thames. Friday, December 26thThe UTRCA issued a Flood Monitoring Safety Bulletin near mid-day on December 26 to advise municipal Flood Coordinators of the changing weather and possibility of flooding, and to warn residents to stay away from waterways. Saturday, December 27thVery warm temperatures moved into the area early on December 27, causing a sharp increase in flows across the watershed beginning that afternoon. The UTRCA issued a Flood Advisory Bulletin for the entire watershed that evening, with details of estimated peak flow magnitudes and timing over the next two days at key flood threatened areas. Late on Saturday evening, the UTRCA advised London City Police that the Thames River would exceed levels established for activating a Boating Ban in the City of London just after midnight. The ban remained in effect until December 31. Sunday, December 28Water levels continued to rise Sunday morning. Air temperatures dropped to near 0ºC by noon, and remained within a few degrees of freezing for the rest of the day. Water levels began to reach peak levels in some of the northern subwatersheds late on Sunday morning, including on the North Thames River at Mitchell. The UTRCA issued a Flood Advisory Bulletin update before noon, with updated estimates of peak flow magnitudes and timing at key areas. Stream flows in the upper portions of the watershed, including the North Thames in St. Marys and into Fanshawe Reservoir, Avon River in Stratford, Middle Thames in Thamesford, and South Thames in Woodstock and Ingersoll, peaked early Sunday afternoon. The peak flows in this event were very similar to those observed during the spring melt in April 2008. The peak flow in St. Marys was the highest since 1948, and just slightly higher than flows recorded in April 2008. Operations at Wildwood Dam reduced the peak flows in Trout Creek by approximately 80%. The St. Marys Flood Wall, which was constructed in 1990, kept floodwaters from entering the downtown core of St. Marys. Monday, December 29thWater levels in the South Thames in London and the Thames River in Byron peaked early Monday. Fanshawe Reservoir also peaked Monday morning at approximately 7.7 metres. (A quick check of the last 25 years of data indicates this was the highest reservoir elevation in 25 years). Adjustments to Fanshawe Dam controlled peak releases from the reservoir to approximately 470 cms (cubic metres/second) from mid-day Sunday until the reservoir peaked on Monday morning. The release from the reservoir was gradually reduced as the water level began dropping. The
UTRCA issued a Flood Advisory Bulletin update, with a summary of
peak flow magnitudes and timing from the flood and updated
information on plans for reservoir management over the next several
days. Without the UTRCA’s flood control system, London would have experienced close to a 1:100 year flood along the North Thames River and below the Forks of the Thames. These flows would threaten to over-top lower sections of the West London Dyke, possibly prompting a large evacuation. The West London Dyke helps to protect approximately 1,000 homes and numerous businesses and other facilities from flooding. The combined operations of Wildwood, Fanshawe and Pittock Dams reduced flows in the Thames River through London to below a 1:10 year flood level. The following chart outlines peak flows in seven areas of the watershed, and compares them to the spring snowmelt in April 2008. Note: Flow volumes are preliminary and subject to review. Click HERE
for information on flows in the Thames, including a map showing stream gauge
locations. Click HERE for more information on flooding in the Thames River.
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This page was last
revised on
April 20, 2012.
Copyright © 2012 Upper Thames River Conservation Authority 1424 Clarke Road, London, Ontario, Canada N5V 5B9 tel: 519-451-2800 fax: 519-451-1188 All rights reserved. |
