In the event of a major failure of the Conestogo Dam, a giant wave of water would rush downstream, threatening about 240 homes and about 800 lives.
The wave would reach Glen Allan in two hours and St. Jacobs in five hours, closing almost every bridge, leaving many roads under water and causing property damage that could reach tens of millions of dollars.
And its not just the Conestogo River that will be affected, there would be extensive flooding in the Grand River as well, downstream from where the two rivers meet, near Conestogo.
But Grand River Conservation Authority (GRCA) spokesman Dave Schultz said that catastrophic, worst case scenario is “extremely unlikely.”
He also stressed that plans to spend over $22.7-million to upgrade the 51-year-old dam are in no way indicative of any pressing safety concern.
“It’s just Conestogo’s turn,” Schultz said at an open house on June 25 at the Floradale fire hall. He explained the GRCA regularly does work on all of its seven major and 22 smaller dams in accordance with standards set by the Ministry of Natural Resources (MNR).
Regardless of how unlikely a dam failure may be – caused either by extreme flooding combined with an equipment failure or the dam developing a leak – the GRCA has to ensure the dam can pass the flow that would accompany the “probable maximum precipitation,” such as that received by a direct hit by a hurricane-sized storm.
Stilling basin upgrade
Starting this month, the GRCA will be spending $2.7-million to upgrade the dam’s stilling basin, which calms the water coming out of the gates to prevent dam erosion.
The cost of the project will be divided evenly between the GRCA and the MNR, through its Water and Erosion Control Infrastructure program (WECI).
Facca Inc., of Essex County, is the contractor and the project was designed by Hatch Energy, of Niagara Falls, the successor to H.G. Acres and Company Ltd., which also designed the dam and supervised its construction in 1958.
The project may be completed at the end of this summer, but most likely by next fall. Schultz said much of the work has to be done under water, which means construction can only occur in warmer months.
The upgrade involves raising the basin walls and end sill and building four dissipation blocks in order to double the maximum basin flow from 700 cubic metres per second to 1,400 to match the maximum flow of the dam gates.
Schultz said the average daily flow rate ranges from 200 to 300 cubic metres per second and the highest ever recorded flow was 590 in 1975.
However, in order to accommodate a flow of 2,100 cubic metres per second, which the GRCA estimates would accompany a hurricane-sized storm, more work is required.
Spillway construction
In order to prevent water from flowing over the top of the dam and causing it to collapse, the GRCA has proposed a $20-million emergency spillway at the south end of the dam.
The 1km-long channel, which will be two to three metres deep and 60m wide, will run from the Conestogo reservoir around the dam and into the river, several hundred metres downstream.
It will be concrete lined in a few spots but for the most part will appear natural and be lined with grass. It will include at least one bridge for traffic along County Road 11.
Joe Farwell, the GRCA’s assistant Chief Administrative Officer for resource management, said the mouth of the spillway will be closed over with concrete blocks and other materials, which would have to be removed with a backhoe in the event it is ever needed.
Farwell explained someone is always working at the Conestogo Dam, which negates the need for an opening device, although the process for accessing the spillway matters very little.
“It’s unlikely we’ll ever use it,” he said. The GRCA expects the spillway to be completed by 2015, but before it can even be started there remains the major issue of how the project will be funded.
“The province has established deadlines, so we’re hoping they’ll also provide a cheque,” Schultz said.
The GRCA annually spends about $1.2-million to maintain all of its dams and dikes, half of which comes from municipal levies and half from the MNR’s WECI program. And that program has a maximum annual total of $5-million for all 36 conservation authorities in Ontario.
“If we got our funding, no other conservation authority in the province would get any for the next four years,” Schultz said, pointing out that’s a very unlikely scenario.
So the GRCA is hoping for some other form of government funding and Schultz hinted municipal levies could also be increased to help cover the cost.
Also at the meeting last week, the GRCA unveiled a new emergency preparedness and response plan to help protect residents in the event of a dam emergency. Check future issues of the Wellington Advertiser or visit www.grandriver.ca for more information on the plan.
About the dam
The Conestogo Dam is one of seven dams built by the GRCA between 1942 and 1976 to reduce flood damages and augment river flows during the summer and fall.
Built in in 1958, it consists of a clay core overtopped with earth and rock (rip rap). County Road 11 crosses the top of the dam, which is 30 metres (96 feet) high and 545 metres long.
When the Conestogo reservoir is full it is 13 kilometres long, covers 725 hectares (1,800 acres) and holds 59.4 million cubic metres (13 billion gallons) of water. The reservoir is ringed by about 400 cottages and the Conestogo Lake Conservation Area.
