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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.
* 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.
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