Thames River Flood – December 2008
The 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.
A warming trend began near midnight on Friday, December 26 and lasted until mid-day on Sunday, December 28. Temperatures reached a high of +14ºC, and averaged 9ºC on December 27. After the unusual warm period, temperatures remained within a few degrees of freezing for several days.
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.
A sincere thank you goes out to all of the municipal Flood Coordinators for fast action and support during this flood event. Also, thank you to watershed media representatives for responding efficiently and professionally in getting the safety and flood messages out to the public.
Friday, December 26th
The 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 27th
Very 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 28
Water 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 29th
Water 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.
Both Wildwood and Pittock Reservoirs reached peak levels from this event late on Monday. Peak release from Wildwood Dam was approximately 16 cms to Trout Creek (a reduction of peak inflows of approximately 80%) and coincided with peak reservoir levels late on Monday. Peak release from Pittock Dam actually occurred on Sunday, December 28 at approximately 35 cms (a reduction of peak inflows of >50%) to the South Thames River. With the operation of Fanshawe and Wildwood Dams, UTRCA staff reduced peak flows in the North Thames River in the City of London by approximately 50%.
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.