The Grand River Conservation Authority says the flooding associated with a record rainfall event on June 23 was the direct result of more than 100 millimeters of rain falling in the span of two to three hours across a wide section of the northern portion of the watershed.
In a flood debriefing posted on its website, the GRCA states rain gauges at Luther Marsh indicated the flooding, which heavily impacted and caused damage in Drayton and Glen Allan, was due to the highest recorded one-day total rainfall for that area since record-keeping began in 1950.
“This is an unprecedented volume of rainfall at any time of year, let alone in such a condensed time period, over a relatively large area during the summer,” GRCA officials state.
The authority notes the heavy rainfall was not identified in any weather forecasts in advance of the event. Environment Canada did identify a chance of “localized, severe thunderstorms” in a message put out shortly before midnight on June 22, but there was no indication that any of those storms would contain a rainfall event of this magnitude over such a widespread area. The original forecast on the afternoon of June 22 was for 5 to 10 mm of rainfall over areas of the watershed on June 23.
The GRCA operates a sophisticated series of rainfall and river flow gauges throughout the watershed. They are designed to alert staff on duty 24-hours a day in case river flows increase quickly, or heavy rainfall occurs.
As a result of the heavy rainfall on June 23 GRCA staff began reacting to alarms around 3:15am, with dam operators dispatched to attend major water control structures around 4:17am.
At 5:40am the GRCA opened a flood operations centre and began notifying municipal flood coordinators in the northern part of the watershed of significant rainfall and flooding. Municipal flood coordinators began notifying residents and closing roads at risk of flooding, the authority states.
By 9:15am, GRCA staff completed an assessment of flooding forecast and issued a flood message, warning all townships and municipalities north of Cambridge that were at risk of flooding. A series of seven additional flood warning messages were subsequently issued between June 23 and 27, providing actions and updates to municipal flood coordinators, emergency services, municipal staff media and GRCA social media feeds and email subscribers as the event unfolded.
At this time of year, GRCA reservoirs are approximately 90 to 96 per cent full to support river flow augmentation. This provides up to 10% of reservoir capacity for flood storage. The authority estimates this additional storage space helped delay peak flows downstream by approximately two hours.
Where significant rainfall events are forecast days in advance, water levels in reservoirs may be drawn down when possible, to help provide additional flood storage capacity. However, as there was no advance warning of the level of rainfall that occurred, there was no opportunity to proactively reduce water levels in reservoirs.
GRCA reservoirs were operated following standard operational procedures in order to protect the integrity of the dams themselves.
Once flood storage capacity was reached, discharge from the dams was gradually adjusted to match flows in the river upstream of reservoirs. As soon as flows into the reservoirs began to decrease, so too did discharge from those reservoirs.
Maximum discharge from GRCA dams does not exceed maximum flow into the reservoirs – this is to ensure that downstream flooding is not made worse due to the presence of these major dams, the authority explains.
GRCA officials note each flooding event is a learning opportunity for GRCA staff, emergency services and municipal flood coordinators throughout the watershed.
“As with any significant flood, staff are in the process of meeting with municipalities, emergency services, flood coordinators, businesses and residents. From these meetings, improvements to warning systems will be established, allowing all agencies to be even more prepared for future flooding events,” the authority states.
