On July 31, a large landslide near the town of Hanceville BC has dammed the Chilcotin River. A lake is forming upstream of the dam; downstream of it, the river is dry.
This is obviously temporary: the lake will rise and flow over the dam, or leak through it and erode it, or (most likely) some combination. The danger is rapid erosion of the dam, which could release a major flood with potential to cause serious damage extending far downstream.
Previous landslides have happened in this area, though apparently not in historic times.
The Chilcotin is a tributary of the Fraser River, which flows past Hope, Abbotsford and Vancouver on its way to the ocean.
I’m not sure: the video article said that they are stopping service of the Lytton ferry which looks to be over 100 miles downstream, but they didn’t say if that was due to low water levels due to the lack of water from the tributary or due to flooding risk.
If the landslide dam magically evaporated it would take 5 hours for water moving at 20mph to travel 100 miles downstream to Lytton. If the river course is flatter, or the dam failed more gradually, it would take even longer. Which would be plenty of time to notify the ferry to seek a dock and offload before the floodwaters got there.
So I strongly suspect the actual reason is lack of water in the river course to float the boat.
Or they’d prefer folks to get out of there gradually (as opposed to trying to evacuate everyone at once in the five hours warning before a flood), and they’re hoping that making things inconvenient by stopping ferry service will encourage that.
The Lytton ferry is a small cable ferry that crosses the Fraser River. The Fraser River is not running out of water. The river is not slow here. Lytton burned down two or three years ago. BC is very dry right now - a tinderbox like last year. I would hazard a guess that the concern is more earth making it’s way into any fast surging water - even more destructive than the water itself.
No major developments. The lake caused by the landslide dam is now 11 km long.
Expectation is that the water will eventually flow over the top of the dam, eroding it, rather than seeping through the dam, which could cause it to collapse more quickly.
Just spitballing, here… Is it possible to install a big-ass siphon over the top of the dam, to keep the water from backing up more, without uncontrolled erosion?
I’m not an engineer, obviously, but three thoughts come to mind:
To be effective, any such siphon would have to be able to handle the upstream flow of the river. I think that would be a huge requirement, not something that could be whipped up in a few days.
The area is pretty isolated, I think, which probably means it doesn’t have the infrastructure (roads, residences, power supply) to allow for the siphon to be built on short notice.
The dam is likely pretty unstable. Could it bear the weight of such a large piece of equipment? More importantly, would it be safe for engineers and work crews to be on the dam, installing it? The speculation that the dam could collapse on short notice suggests it would be extremely hazardous.
I saw the flow rate reported as 200 cubic meters per second.
A pipe to handle that would indeed be formidable. My back-of-the-envelope calculation says that in a pipe 2 meters in diameter the water would have to be flowing at a speed over 200 kph.
The landslide all being cleared away all of a sudden is exactly what we’re trying to avoid. That gets you a wall of water full of abrasive grit all suddenly coming downstream at high speed. What you want is some way to remove the blockage gradually, so there’s at worst a temporary slight rise in the water line downstream. The difficulty is that, as soon as you remove the blockage down to below the level of water behind it, water starts flowing over the top, which will itself remove more of the blockage, and very quickly grows out of control to the “all of a sudden” scenario.
If a big-ass siphon isn’t available, then I suppose they could instead try a big-ass tarp over the crest, to slow erosion. That might be easier to procure and deliver.
Another article with details on the models of best and worst case scenarios.
Best case: river tops the dam and then it takes about 24 hours to cut a channel deep enough to release the water from the temporary lake. Steady flow, similar to spring run-off, which is normally around 300 cubic metres per second. High point rise in the river: about 2 m.
Worst case: water tops the dam and dam breaks, releasing all water in a few hours. Estimated rise in the river: 10 m; estimated water flow: 6,500 cubic metres per second.
“the dam is estimated to be 1,000 metres in length, 100 metres high and 30 metres in depth.”
“It also said the sudden lake from pooling water behind the dam is continuing to grow at a rate of 22 cm per hour.”