September 2021 flood event (UTRCA Weekly, November 26, 2021)
The Upper Thames River Conservation Authority (UTRCA) watershed’s most significant flood event since before the pandemic, occurred as a result of 50-135+ mm of rain falling over a 24 hour period on September 22-23. With the watershed already saturated from rain earlier in the month, much of this rain was not absorbed into the ground, but rather ran off into local streams and rivers.
It’s quite rare for a flood of this magnitude to occur in the fall rather than the spring. This event was equivalent to a 1:5 to 1:10 year event, meaning there is a 10-20% chance of a flood near this magnitude occurring in any given year.
UTRCA water management staff were monitoring watershed conditions and the weather, and provided advance warning of the event. The UTRCA Flood Contingency Plan was put into action with a series of flood messages going out to flood coordinators and the public.
The UTRCA flood control reservoirs at Fanshawe, Pittock, and Wildwood Conservation Areas were at seasonal levels, allowing some of the runoff to be stored to reduce downstream flooding, and then gradually released once the event had passed. These operations prevented serious flood damages from occurring.
Water management staff undertake a planning cycle at least once a day, year round, to identify any flood potential in a timely manner. During flood conditions, the planning cycle becomes a continuous monitoring of watershed conditions and weather. Staff use the information to develop flood bulletins for municipal flood coordinators. Depending on the magnitude, timing, and location of flooding, multiple bulletins may be required, and staff may be in frequent contact with specific flood coordinators who have vulnerable infrastructure or flooding impacts. Water management field staff can also be very busy with dam operations at various locations as well as trouble shooting at various structures and stream gauges, and rely on conservation area staff to be able to operate the flood control dams.
Conservation Areas Unit staff at Fanshawe, Pittock, and Wildwood Conservation Areas performed dam operations throughout the week. This work involves operating dam gates, as directed by water management staff, at all hours of the day and night. The CA Unit staff have an excellent understanding of reservoir benchmarks for triggering action within their parks, such as moving trailers or closing trails as the reservoirs rise.
Once a flood event is over, staff evaluate the modeling, review data, and look at the accuracy of the forecasts for precipitation, runoff, and watershed conditions. Understanding how an event unfolded provides valuable information for the future. UTRCA staff are hosting a Flood Coordinators virtual meeting on December 1st and will be reviewing this flood event and the communication procedures.