Posted 7/19/16 (Tue)
By Kevin Killough
What would happen if the Tioga Dam broke?
City officials Thursday, working with engineers and fire and rescue volunteers, explored a number of such scenarios and what the most effective response would be to keep residents safe.
The scenarios considered everything from small structural mishaps to catastrophic failures resulting from severe weather.
Thomas Johnson, senior water resources engineer with Ackerman/Estvold, presented a series of map projections showing how water would flow under certain situations, and how deep the water would be.
In some areas under some conditions, the water is as deep as 20 feet.
Engineer Antonio Conti said the likelihood of any of the emergency scenarios happening are “pretty minimal.”
The North Dakota Century Code requires every city in the state with a dam to develop an emergency action plan, including responses to catastrophic failures, and to update the plan every five years.
For the past 20 years, the state has been sending letters to the city asking for this plan to be developed, but it was never complete.
Recently, the city received a grant from the state, which will pay 80 percent of the $50,000 price tag for the plan.
Conti said the city inspects the dam on a regular basis, and the state inspects it every two years, further mitigating the chance a structural breach will occur.
Under the “sunny day breach” scenario in which the dam experiences a structural failure, a leak “starts slow but ends fast,” Johnson said.
The most likely situation under this scenario would be a breach of the east wall, which is the deepest part of the dam and therefore has the most pressure.
The projection of this case had varying shades of blue across Tioga’s neighborhoods indicating the water’s depth, as it ran south, east of Main Street, and then flowed east down the tracks, putting many houses along that corridor under as much as two feet of water.
Mayor Drake McClelland pointed out this would flood his house.
The worst case scenario involves a torrential downpour flooding the reservoir with more water than the dam can handle, resulting in breaches on both sides of the dam.
In that case, the path of the water flows west of the high school and down the railroad tracks corridor to the west, where it meets a flood river from the east-side breach going down the N.D. 40 corridor.
This scenario basically creates an island out of the neighborhood from near the elementary school to the high school and between the reservoir and the railroad tracks.
Johnson said, while the worst case scenario results in significant flooding, it’s unlikely to occur without warning, unlike the “sunny day breach.”
With early warning systems in place for approaching severe weather, the possibility of a problem would be known before it hit the dam.
So city officials could be monitoring the dam for any problems before they occur and begin appropriate level responses as the situation looks more likely to produce a structural failure.
With a breach resulting from structural mishaps, it would only be spotted if someone happened to notice indications of a problem.
“It’s the sunny day one that’s a bit scary,” Johnson said.
Chain of command
The team at Thursday’s meeting began the preliminary discussion on how to respond to the scenarios according to their severity.
In each case, the chain of alerts begins with the city’s water department. Jeff Moberg, head of the city’s water department, will assess the situation at the dam and if he identifies a potential emergency, he alerts the city’s fire department and mayor.
At that point, the mayor and emergency volunteers may decide an evacuation is necessary and will begin organizing it.
The chain of contacts continues from there, bringing in engineers and Williams County emergency response teams, and alerting Montana-Dakota Electric.
The team discussed where to evacuate people or where they could meet for safety. The primary areas of high ground were the high school and city shop west of town.
The high school, while safe, is complicated by the fact it would be surrounded by rivers of water in the worst case scenario, leaving no means to get out of the area.
The team discussed other responses, such as shutting off the city’s water mains.
This idea was dismissed since it could result in back flow and could take weeks before the city’s water service could be restored.
“Leave your mains full,” Moberg warned.
The group also looked at where material resources, such as sand for barriers, buses for evacuating people, and heavy moving equipment could be acquired to aid in the response.
They also discussed how people in the flood path could be notified.
Since county records of land ownership may not reflect who is living at the residence or what phone number they use, the group decided to compile a list of city utility billing addresses for the best contact information.
They will also utilize the town’s weather siren system and automated phone calling system to let the general population of Tioga know of the emergency.
With the information gathered at the meeting, Johnson will develop a draft for the response plan within a month, which will be discussed and further detailed in a future meeting.