The Idlewild Dam Breach Inundation Study sheds some light on the in-camera decision to decommission the Idlewild Dam.
The report was prepared by Urban Systems Ltd and completed as part of the Dam Safety Review of Idlewild required by the Dam Safety Officer from the Ministry of Forests, Lands and Natural Resource Operations.
The report was prepared by A. Glen Zachary, Senior Hydrology and Hydraulics Engineer at Urban Systems.
Zachary wrote that the results of the analysis confirmed that the Dam Consequence Classification should remain as “High” based on the current dam conditions.
High severity is defined as occurring “when the flood sweeps the area clean and nothing remains. High flood severity should be used only for locations flooded by the near instantaneous failure of a concrete dam, or an earthfill dam that turns into ‘jello’ and washes out in seconds rather than minutes or hours.
“In addition, the flooding caused by the dam failure should sweep the area clean and little or no evidence of the prior human habitation remains after the floodwater recedes.”
Zachary wrote that the most severe flooding would be caused by a dam breach during overtopping of the dam by the Inflow Design Flood (IDF).
“The peak flood flow would be approximately 287 m3/s, but this flow rate would occur only at the dam within a time span of approximately 10 minutes,” Zachary noted in the report. “This peak flow rate also includes the IDF flow rate of approximately 31 m3/s.”
The report noted that 21 per cent of the city would be impacted by the flooding, with an average flow depth of less than half a metre.
“The flood wave and associated flooding due only to the dam breach would dissipate within an hour due to the relatively small volume of water stored in Idlewild Lake,” Zachary wrote. “Most of the inundation identified during analysis, especially the duration of flooding, is due to the relatively large volume of water associated with the Inflow Design Flood.”
The estimated loss of life could be as high as eight people. That is primarily because there would be no warning time and takes into account potential for flooded basements within residential areas — and residents who live in basements.
The economic and social losses would likely be high, but would only be temporary. The study also estimates the environmental and cultural losses to be high, but restoration would be possible.
The report noted that the most significant potential environmental loss would be that of the Joseph Creek riparian area between Idlewild Dam and 1st Street South.
“Restoration of grasses and shrubs could occur relatively quickly (a couple of years). However, it would take two or three of decades for lost trees to be re-established,” the study read. “The other potential environmental impact would be the flooding of the waste water treatment plant lagoons. This has the potential to carry and deposit bio-wastes downstream. However, based on the velocities and flow durations from the inundation modeling, the impacts would probably be low.”
The report noted the most significant cultural site within range of a breach is the Canadian Museum of Rail Travel, now the Cranbrook History Centre. However the inundation analysis showed that it would not be affected at all.
The most critical mode of failure of the dam was identified as overtopping, which raises the water level over the dam crest significantly increasing the volume of water stored behind the dam if a breach occured. Other modes of failure identified include the low hydraulic capacity of the spillway, its potential for debris blockage and increased storms combining also identified as the most likely to occur — and capacity in joseph Creek being reduced or used up by overtopping flows, which would increase the extent of flooding by the breach.
“The probability of a dam breach by overtopping could be reduced if the city increased the spillway capacity to accommodate the Inflow Design Flood or armoured the downstream embankment of the dam to prevent erosion,” Zachary noted.
The city elected to decommission the dam and begin a process of consultation on the future of Idlewild Park, which has been extended until June 12, 2015.
The turtles and
the fence
At Monday’s meeting, Coun. Norma Blissett noted she had been getting inquiries about what is happening with the fencing at Idlewild Park, and whether it needs to stay up.
“I think there’s been some concerns with turtles getting caught in the fence,” Blissett said.
CAO Wayne Staudt said they have made some modifications to the fence.
“We have allowed for the turtles to pass under the fence in certain spots where we believe the turtles are moving around,” Staudt said. “We need to do a bit of correcting on the fence. We looked at that issue today and I believe we’ve spoken to the individuals including the Ministry of Environment about that. So I think we will correct that.”
Staudt said as a safety concern, the fence will probably have to stay up for a while.
“As the lake draws down, the mud is very, very dangerous — you get a young child in there and he may not get out of there so it is very dangerous. People should not cross the fence and get on the other side of it to see what the turtles are doing or anything like that.”
Staudt noted that if they create an opening and a child falls through that onto the other side of the fence, it would be of grave concern.
“The fence is up for safety reasons,” he said.
Coun. Ron Popoff noted he’d been on a tour of Idlewild as the dam is decommissioned and drained.
Popoff said the water drainage was already well on its way at that time, noting there was an odour coming off the mud as it dried.
“Bare with that, it will dry out,” Popoff said.
Popoff also said he noticed the fence.
“My perspective is it is there as more of a flag to warrant people to stay out as opposed to a barrier,” Popoff said. “It’s pretty tough to physically keep people and dogs and kids out of there.”