A massive ice jam that caused major flooding in a portion of east downtown Bolton, Ont. in 2019 was the most recent in a string of flood and ice risks the community has wrestled with since it was founded next to the Humber River in the early 1820s.
While the ice jams may still occur, a now underway two-phase Toronto and Region Conservation Authority (TRCA) project is intended to provide protection in that area against a 500-year flood.
“This is quite a delicate operation,” says TRCA project manager Rudra Bissoon on the complexities of the $1.5-million, two-kilometre-long Bolton Berm Major Maintenance Project.
It consists of installing two berms, one 250 metres long and a second 500-metre-long one on top of existing ones along the south side of the Humber River. Technically it’s one berm, but the project has been divided into two phases for logistical reasons, he says.
Consisting of rip rap stone along the river banks and engineered landfill on the flat areas above, the berms are being built by TRCA crews with the assistance of heavy equipment operator TMI Contracting Ltd.
Dimensions of the berms vary, with widths ranging up to 12 metres at the base to three metres at the crest. The heights go up 3.5 metres on the creek side and up 1.8 metres on the dry side.
Although almost all the construction is occurring on TRCA lands, access agreements with a number of adjacent property owners along the route, including an apartment building, will allow for the movement of several pieces of equipment and had to be in place before construction could start, says Bissoon.
Completing the access agreement on one particular property was a little more detailed because the owners owned the land right to the river’s edge.
“Then the house was sold and we had to sign an agreement the new owner.”
Hemmed in by those properties and the river, TMI Contracting has to maneouvre its machines within very tight spaces. The machines currently being used are a Cat 308 Excavator and a Cat 299 Skidsteer, says Bissoon.
What has now been erected are major enhancements of existing berms installed in the mid-1980s. However, a study conducted a few years ago concluded those earlier structures weren’t providing the level of protection they were originally intended to provide, he says.
In 2017 the authority and its consultants conducted various studies and technical assessment of the existing flood control structure including geotechnical and botanical ones. An original concept design was developed by Valdor Engineering, and then a preferred design was developed by the KGS Group in 2019.
Besides negotiating the access agreements, the project presented a number of design and planning challenges, “due to the sensitive nature of the work” and the location of that work at the rear of a number of residential and commercial properties.
“There was a high level of interest on the project as the (original) berm structure itself had been used as an informal trail system by the community for a number of years and contained a large number of trees which provided privacy to the adjacent residential properties.”
A major source of concern was that approximately 300 trees were pinpointed for removal in the design. There were a number of reasons why the trees, which were actually invasive species, had to go, he says.
“Trees can be uprooted by natural processes, such as during heavy rain and wind. When this occurs, the tree roots can rip up sections of the berm as they fall. This leaves large voids in the berm that can lead to the failure of the berm during flood events.”
Tree roots could have created seepage pathways within the soil of the berm, allowing water to flow through, weakening it and possibly leading to structure failure. Ice can also form around trees and then, when warmer weather arrives, uproot trees and carry them into the berms. Trees can also trap ice and that increases the risk of ice flow jamming, he explains.
The residents’ concerns were alleviated by TRCA’s impact mitigation and restoration plan which includes replacing the trees with native shrubs and the installation of non-intrusive bat boxes on the berm, says Bissoon.
Erection of the first-phase berm at the project’s most easterly section in the area behind the apartment building got underway last September. Considerable site work had to be conducted before construction could commence, including the tree removal, the installation of erosion and sediment control safeguards, plus the excavation of existing topsoil and replacing it with compacted engineered fill, he says.
Phase one is substantially complete, although there are some minor works and outstanding restoration measures which will be completed this spring and summer. Construction of phase two, which will take the berm westerly to Queen Street, the main north-south artery serving Bolton, got underway in January. Scheduled to be completed by September, the berm will help protect a Peel Region pump house as well as the adjacent properties.