22-23 September Flooding: Post Event Analysis

Conservation authority put flood contingency plan into action

Highlights from the October, 2021 issue of fyi, the Upper Thames River Conservation Authority (UTRCA) newsletter.

The UTRCA watershed’s most significant flood event since before the COVID-19 pandemic occurred as a result of 50 mm to 135+ mm of rain falling over a 24-hour period on 22-23 September 2021.

The watershed was already partially saturated from 45 mm of rainfall from earlier in the month; much rain was not absorbed but rather ran off into local streams and rivers. Waterways rose to bank full levels and beyond, in harder hit areas.

The UTRCA Flood Contingency Plan was put into action. A series of flood messages went out to flood coordinators and the public, beginning on Sept. 21. Fortunately, UTRCA flood control reservoirs at Fanshawe, Pittock, and Wildwood conservation areas were at seasonal levels, storing up some runoff to reduce downstream flooding and later released after the storm. Operations prevented serious flood damages from occurring.

This event was equivalent to a one-in-five-year or one-in-ten-year event, meaning a 10%-20% chance of flooding near this magnitude could occur in any given year. Note: It is very rare for a flood of this magnitude to occur in the fall rather than the spring, when snow meltoff occurs.

The UTRCA Flood Contingency Plan guides coordination of a number of departments and their personnel. In particular, Water Management staff perform flood potential assessment at least once a day, year-round, to identify risks in a timely manner. During flood conditions, assessments are increased to continuous monitoring of watershed conditions and weather. Water Management staff use assessment data 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 municipal flood coordinators who have vulnerable infrastructure or flooding impacts.

During a flood event, Communications staff assist Water Management employees by fine tuning and issuing flood bulletins. During more significant events, Communications personnel will also follow up with municipal flood coordinators to confirm that bulletins have been received.

The magnitude and timing of the flood event may necessitate contributions from other departments such as Regulations, whose staff may contact specific permit holders who are undertaking activities near a waterway, to ensure awareness of the potential for flooding. In major flood event, employees from across the organization may be called to assist with documentation or recording of high-water marks.

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