On June 23rd the North Maitland River watershed (which extends from Wingham up to Wellington North) was hit with an extremely intense storm. In just seven hours over 175 mm (6.9 inches) of rain was recorded upstream of Harriston. The resulting runoff caused extensive property damage in the community of Harriston. The flood also had a significant impact on the Gorrie Conservation Area. At 4:00 p.m. on June 24th the berm to the south of the Gorrie dam was breached. The force of the water eroded the emergency spillway, gradually creating a large opening. This allowed flood water to move through the berm.
Gorrie Dam Operations
At 6:00 a.m. on June 23rd Maitland Conservation contacted the Dam Committee in Wroxeter and recommended that the boards be lifted at that dam. No boards at the Gorrie dam could be lifted until the boards in Wroxeter were removed. Additional flows from Gorrie would have made it more difficult for the boards to be lifted in Wroxeter.
The Dam Committee in Wroxeter responded to the request immediately. The elevated flows in the river meant that heavy equipment had to be brought in to lift the boards.
The boards at the Wroxeter dam were lifted by 9:15 a.m. and work began to lift the boards in Gorrie. By 10:00 a.m. one top section of boards were lifted. At that point the Township of North Huron notified Maitland Conservation that the Howson dam in Wingham was at full capacity and requested that the lifting of boards be halted so conditions in Wingham could be monitored. Maitland Conservation complied with this request as recommended by Ministry of Natural Resources and Forestry (MNRF) information on the operation of the three dams along the North Maitland River.
At 11:50 a.m. the Township of North Huron gave the go ahead to lift additional boards in Gorrie, however, by that point the flows in Gorrie were too high to remove any additional boards. Even if the remaining boards had been removed, it would only have delayed the breach of the berm by approximately two hours.
Regulation of Dams on the North Maitland River
The MNRF is responsible for overseeing the operation of dams in Ontario. Each dam must have an operational plan. Operational decisions at Gorrie must not impede the ability to safely operate the dams in Wroxeter and Wingham.
Maitland Conservation is responsible for operating the dam in Gorrie, while the Township of Howick is responsible for operating the dam in Wroxeter and the Township of North Huron is responsible for the operation of the dam in Wingham. None of these dams are flood control structures and they are not designed to be operated during a major flood event.
How Big Was This Storm?
Many area residents remember when the Gorrie dam was damaged by flooding in May 1974. In 1974 the peak flow rates reached 200 m3/s. In comparison, the peak flow rates on June 24, 2017 were 526 m3/s.
Assessing damage to the berm and determining follow up action will be a long process. As mentioned previously, the MNRF is responsible for regulating dams in Ontario. Maitland Conservation has contacted the MNRF to begin the process of determining what our next steps should be. The standards and criteria governing dams in Ontario have changed greatly since the 1970’s and any action taken must meet today’s standards and criteria. Once we have met with MNRF staff, Maitland Conservation will be in a better position to determine what the options are for the dam. Maitland Conservation will provide the community with updates when we have more information. Again, we would like to stress that this will take time.
As everyone knows, in addition to the breach of the berm, Gorrie Conservation Area sustained considerable damage from the flood. For public safety reasons Maitland Conservation has decided to close the park. We are working on determining the best course of action for dealing with all of the damage to the site. We are unsure how long this will take but we will keep the community informed of our progress.
Could it Happen Again?
Unfortunately yes! Our climate is changing rapidly. In the past, flooding was normally associated with spring conditions. Ice on the river, combined with large amounts of melting snow, would result in flood conditions.
In recent years we have seen a trend towards more localized, intense thunderstorms resulting in large amounts of rainfall dropping in parts of the watershed, while completely missing other areas. These storms are very difficult to forecast. These changes mean that we are likely to experience more floods during the summer months in the years ahead.
If you have questions or concerns please contact: