Floodplain mapping shows Howson Dam actually causes flood concerns
BY DENNY SCOTT
Both North Huron and Morris-Turnberry received updated floodplain map documents from the Maitland Valley Conservation Area during two recent council meetings.
During North Huron’s March 1 meeting and Morris-Turnberry’s March 2 meeting, Steve Jackson of the Maitland Valley Conservation Authority (MVCA) presented the document. The last update to the mapping, he explained, was over 40 years ago in 1978.
Jackson said the document has been in the works for the past three years. It was delayed due to COVID-19, he said, because public information centres weren’t able to be held due to lockdowns.
Late last year, however, those meetings were held as one-on-one affairs, Jackson said.
“Public meetings were held Oct. 28 through Oct. 30 with 27 appointments,” he said. “The meetings lasted 20 to 60 minutes per appointment.”
He said the format was a welcome change from the more traditional open-house style meetings. He added he wants to continue to use it after the pandemic as it makes it easier to answer questions.
Jackson explained that the floodplain area is dictated by the province and is determined by the 1954 rainfall that accompanied Hurricane Hazel. The floodplain is not determined by the impact that storm had locally, however, but by the worst-hit area: Woodbridge.
Jackson said that, in Woodbridge, there was approximately 12 inches over the brief period during which the hurricane moved through the area. He said, to compare that, the rain and snow melt in December of 2008, which resulted in widespread flooding and service disruptions in the area, was only about five inches.
Jackson explained that, using updated mapping, the floodplain was set by taking that 12 inches, or 300 millimetres of rain, and pouring it over the Wingham area mapping.
The updated mapping comes thanks to a project undertaken by North Huron and Morris-Turnberry which used airborne Light Detection and Ranging (LiDAR) technology to measure the elevation variances in the area. While it’s rated to provide accurate elevations within four inches, Jackson said, in most cases, the MVCA found it was accurate to one to two inches.
The overall floodplain had some minor changes to it, with some areas being added and others being removed, meaning that some areas may not be able to be developed while new developable land opened elsewhere. For full details, visit mvca.on.ca.
During both the North Huron and Morris-Turnberry presentations, councillors questioned a slide that Jackson included that showed how the presence of the Howson Dam, the focus of a possible rehabilitation by North Huron, actually resulted in more lands being in the floodplain.
“The dam increases the floodplain area,” Jackson said, explaining it isn’t a flood control apparatus. “If the Howson Dam isn’t in place, it opens up more land for development.”
North Huron’s Deputy-Reeve Trevor Seip said that didn’t seem to be logical, but Jackson explained that, due to the structure, there was a great concern of flooding.
When answering the same question from Morris-Turnberry Deputy-Mayor Sharen Zinn, Jackson explained that flood control dams are different.
“[We] found if the Howson Dam is removed, there wouldn't be any change in flooding downstream,” he said. “The Howson Dam causes flooding upstream. It’s more of a hindrance in flooding than a help.”
Jackson explained that the mapping is still not officially complete due to one Morris-Turnberry ratepayer not submitting information “in a timely manner”, so the deadline for comment from that ratepayer has been extended to 120 days.
Morris-Turnberry Councillor Jim Nelemans asked if all the other issues were resolved, and Jackson said some were, however some people still had qualms about the modeling itself.
“There are outstanding items in the minds of those at the [public meeting appointments] who are concerned with flood forecasting,” Jackson said.
Those concerns, however, aren’t anything that the MVCA can remedy as it isn’t responsible for the formula behind flood forecasting. “We have to follow the province’s flood standard,” he said.