Last week, almost 200,000 residents downstream of the Oroville Dam in California were evacuated when the dam risked failing. The problem started Feb. 7, when a massive hole appeared in the spillway during a water release. Because there was so much water flowing into the reservoir, operators had to keep the spillway open, despite the damage it was causing.
On Feb. 11, water overflowed into the emergency spillway prompting officials to order an evacuation out of fear it could fail. The evacuees weren’t allowed to return home for two days, when the situation was stablized.
Meanwhile, in Revelstoke, people wondered, “Could that happen here?”
On Friday, Feb. 17, I spoke to Stephen Rigbey, BC Hydro’s director of dam safety, about what safeguards are in place to prevent a similar situation from developing here.
What does BC Hydro have in place to safeguard against an incident like this?
Photo: Stephen Rigbey.
First of all, most of our largest spillways have been all used within the past year so we’re confident they’ll all work. We inspect them after they’re used then we repair them as required. It really doesn’t take much to keep these things in shape as long as you catch the damage early. We spent about $2 million in the last two years and have another half million planned for this year. That’s just ongoing general maintenance.
Some of our major spillways haven’t been used in the last 4.5 years now. After those spills, we closely inspected them and have effected repairs, but we’ve also done a lot more. If we see something we think could turn serious in future spills, we get on it and we do major rehabilitation work. At (W.A.C. Bennett Dam), we spent $64 million to replace areas of damaged concrete in the spillway chute, and to provide safe access for that work, and to make sure we can get in quickly to do any future repairs. We look really closely at our spillways after a major spill and spend money when and as required.
Are these visual inspections, or are they monitored by instruments?
These are visual because what you’re looking for is any damage of the concrete itself. That’s done by a strict visual inspection. That’s why I’m confident we’re not going to get one of these big holes appealing in one of our concrete spillways.
The free overflow spillway situation is very different. In Oroville they let it spill onto unprotected ground. We don’t have very many situations like that. Where we do have that, we have very strong bedrock. Just think of the Mica schist, that rock is hard. Wherever we have discharging water directly onto bedrock, it’s solid and we know it’s solid and it’s been safe for decades flowing across that bedrock, so we don’t have any issues like that whatsoever.
One of the big risks is landslides crashing into reservoirs, causing waves to spill over the dams. How do you monitor the slides?
There’s a couple of different situations. Upstream at Mica there’s two large landslides — Dutchman’s and Little Chief. Dutchman’s slide, we actively drain. We put a series of tunnels into that landslide and are actively draining it so we monitor it very closely. All of these slides are moving a couple of millimetres a year or so. That’s the standard landslide phenomenon in B.C — there’s many large landslides slowly creeping down hillsides, so we have to keep an eye on them closely.
Little Chief, we’re monitoring and at this point there’s no need for drainage because it’s very slow, very gentle and very steady creep.
We’re doing a heck of a lot with new technologies such as GPS, land-based LIDAR, etcetera, looking at how we’re monitoring these slides and always try to get a better handle on it for remote sensing. We have a heckuva lot of instrumentation on the slides itself in boreholes looking at displacements across the slide plain.
What would happen if there was a catastrophic failure beyond the norm and it triggered a wave?
These landslides have been looked at by specialists from all over the world. Everybody is in agreement that the likelihood of a catastrophic failure is very, very small and it would have to precipitated by some sort of event like a massive earthquake to make it happen. It’s an incredibly low probability event.
The Mica Dam was specifically built to withstand an over-topping wave. There was a lot of model testing to make sure it would be able to withstand an over-topping wave. At Revelstoke Dam, any over-topping wave is designed so it would go over the concrete portion of the dam, which would remain stable, and it would not go over the earthen dam.
In terms of landslides, we drain, for example, the Downie Slide, which is upstream of Revelstoke, and we have anchored the smaller Checkerboard Creek, because that one was of a size we could actually anchor it.
What would you do if they started sliding faster?
We would immediately put in more drainage. You slow them down and they slowly slump into the reservoir over a number of years.
How do you monitor the dam structures themselves?
We have hundreds of instruments in these dams. We watch deformations, we watch water pressures, we watch seepage flows. You name it, we watch it. All these dams are behaving quite nicely and have been since construction.
If the things you’re monitoring start to exceed their levels, what do you do?
By regulation, we have an outside firm come in once every five years and review the safety of these dams. We also have a five year external independent review of our management processes in terms of monitoring the safety of these dams. We get audited from two different directions in terms of insuring that these dams remain safe.
The first line of defence is lowering the reservoir until you figure out what’s going on. That’s why spillways are so important. Going back to Oroville, they don’t have a problem until they can’t keep that reservoir down.
Then, it depends. There’s so many possible variations. You have to figure out what is the problem before you could possibly figure out what to do about it. I can’t speculate what the action would be because I don’t know what the problem would be.
Watch a video by BC Hydro of a spill at the Mica Dam in July 2012.
On the coast, a lot of money is being spent to protect dams against earthquakes. What are the biggest concerns in the Interior?
I don’t know if I have any big concerns in the Interior. We are constantly doing reviews. For example, at Mica, we’re going to be undertaking another design review to look at a very specific seismic withstand of little parts of the structure. We’re initiating another of the retaining wall down the one side of the spillway just to make sure that it is stable in an extreme earthquake. We wouldn’t want to have that damaged because that would lower the possibility of using our spillway. It’s linking back to spillway use and that’s really what we’re focusing on now, is making sure we can use our spillways in a post-seismic situation. We’re doing a lot of work about that now.
Is there much of an earthquake risk around here?
No. It’s pretty low ground motions compared to (the coast).
In the unlikely event of a dam failure, what would happen? How much warning would the people of Revelstoke get?
You’d have lots of warning. Things in dam safety don’t happen fast, quite bluntly. You would have lots and lots of warning time. I can’t think of a catastrophic scenario, but in a slowly developing problem, we would have weeks to months to figure it out. We do have emergency plans we share with downstream responding agencies. We have internal emergency notification schemes OVERSET FOLLOWS:that are practiced.