Thames River Spring Flood – April 2008

A significant snow pack developed across the Upper Thames watershed during the winter of 2008, with the equivalent of 50-130 mm of water remaining in the northern portion of the watershed at the end of March. The southern portion of the watershed experienced more melting throughout the winter and had much less snow remaining at the end of March and only 10-25 mm of water content. This snow pack, combined with 10-25 mm of rain, temperatures above 10^o C for two days with no cooling temperatures at night, and saturated soils, set the stage for a significant flood event in the Thames River watershed.

The spring melt could be characterized as a moderate to high spring storm event with the potential for about 75 mm depth of runoff to occur over a few days. Normal spring runoff may take upwards of a week.

Preliminary estimates suggest the flood levels were as high as a 1:100 year occurrence (i.e., a flood of that magnitude would occur once every 100 years) in some upstream areas. Operations at Wildwood Dam reduced flows in St. Marys from 1:100 years to 1:25 years. The combined operations of Wildwood, Fanshawe and Pittock Dams reduced flows in London to a 1:10 year flood.

A big 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 so effectively and professionally in getting the safety and flood messages out to the public.

Monday, March 31st

The UTRCA issued a Flood Monitoring/Safety Bulletin to advise residents of the changing weather and the possibility of flooding, and to warn residents to stay away from waterways. A Flood Advisory Bulletin was issued for the northern parts of the watershed, where it was evident that a larger than average spring melt was imminent.

There are three levels of information bulletins that the UTRCA may issue during a flood event to municipal Flood Coordinators and emergency response personnel:

• The Flood Safety/Monitoring bulletin reports general conditions and river safety issues,
• The Flood Advisory bulletin is issued when the potential for flooding exists in the Upper Thames watershed, and
• The Flood Warning bulletin is issued for specific areas where serious flooding involving damage to property and evacuations appears inevitable.

Tuesday, April 1st

The UTRCA issued a Flood Advisory Bulletin for the entire watershed on April 1. Water levels rose quickly in the St. Marys area and the UTRCA was in regular communication with the Flood Coordinator for the Town of St. Marys. The peak flow in St. Marys was 702* cubic metres/second (cms), which was the highest flow since 1977. To put this volume into perspective, one in-ground swimming pool holds approximately 100 cubic metres of water.

Operations at Wildwood Dam reduced the flow of Trout Creek by 87%, from 84 cms flowing into Wildwood Reservoir, to 11 cms flowing out of the Reservoir. The flood plains at the Flats in St. Marys and the St. Marys Golf Course on Trout Creek did their job of handling the increased flow without damaging buildings. The St. Marys Flood Wall, which was constructed in 1990, was also put to the test and kept floodwaters from entering the downtown core of St. Marys. Without Wildwood Dam, St. Marys would have experienced a 1:100 year flood.

Late Tuesday evening the UTRCA updated the Flood Advisory for London, based on high flows into Fanshawe Reservoir and in consideration of the MNR’s forecast for 40 mm of rain later in the week. During the night, the City of London Flood Coordinators and the University of Western Ontario Flood Coordinators began preparing for flooding in London. City of London staff were mobilized from their control centre to close floodplain parks, Windermere Road near Adelaide Street North, and to check on low lying areas for flooding.

Wednesday, April 2nd

Early Wednesday morning, flows of the North Thames into Fanshawe Reservoir rose to 820 cms. With the operation of Fanshawe and Wildwood Dams, UTRCA staff were able to reduce peak outflow on the North Thames into the City of London by 40% to 500 cms. To help reduce flooding in London, the South Thames was controlled using Pittock Dam in Woodstock to reduce the peak inflow of 121 cms to 47 cms, a reduction of 61%.

The City of London and the UTRCA held a news conference at Fanshawe Dam at 11:30 am to provide residents of the watershed with the most up to date information. At 1:00 pm, UTRCA staff took members of the media in two planes to fly the area and see the extent of the flooding and flood control operations.

Without the UTRCA’s flood control system, London would have experienced close to a 1:100 year flood along the North Thames River, and a 1:50 year flood below the Forks of the Thames. The West London Dykes would have had about 970 cms flowing past at the peak. These flows would have been at the top of the West London Dyke, with flood waters likely spilling over at some lower sections, possibly prompting evacuations. The dykes overtop at about 815 cms if the south branch of the river is also experiencing a significant flood. During this April flood, the south branch only experienced about a 1:2 year level and, with these lower water levels at the forks, the West London Dyke would have been near maximum capacity but significant breaches would have been unlikely. Certainly the newly constructed section of dyke at Labatt Park would have had over 1 metre of freeboard before threatening overtopping.

The flood control system also mitigated flooding issues in Chatham by reducing flow out of London considerably.

• See chart comparing peak flows in April and December 2008 flood events

* Note: Flow volumes are preliminary and subject to review.
Flows are measured in cubic metres/second (cms). One cubic metre (1,000 litres) = 2 bathtubs; 100 cubic metres = 1 in-ground swimming pool. The peak flow passing through Fanshawe Dam is 470 cms, or nearly 5 in-ground swimming pools, every second.