One of the main concerns in the lower section of the Chilliwack/Vedder River is the undercutting of dykes by water pushing directly into it, which can potentially lead to damages and collapses. The pits are not directly used to prevent flooding, but rather to divert or ease the flow where water is hitting straight into the dyke.
Some clarification to what someone may have told you on this issue that I was able to obtain today.
"The issue is that the dykes were built too close together in the floodplain (and maybe too low) and the sediment (gravel) recruitment is great enough in the stream that when it gets out of the Chilliwack River, and through to the low-gradient parts of the Vedder River, it deposits.
If enough gravel settles down in critical parts between the diking system, which should have been spread out further and provided more floodway capacity, and maybe built higher, it pushes up the water profile to the point that it will overtop the dikes for big floods.
What really makes it complicated is the railroad bridge is at a really awkward angle so the river has to make the big dog-leg above the bridge and causes the stream to backwater and many of the coarse sediments drop out at this really vulnerable part of the river.
The amounts and locations are largely determined by measured cross sections (the change in topography, or river bed height, since the last removal), and hydraulic models (how high the stream will be for a given flow), and flood-return models (how often a big flood will happen of a certain magnitude). These are all used to calculate how high a particular design-flood will be based on a probability of how often it returns. In the old days they used a 1:200 year return period and the estimated water surface couldn’t exceed 0.6m below the top-of-dike.
Historically there was quite a bit of my smelly socks surrounding this process, but I think they cleaned some of it up when a few others started to really question what was going on. In fact, the word was by the old Water Management Branch engineers that there was quite a bit of collusion in the early days when it came to bidding for the gravel. I think that Tara (Chilliwack’s engineer) keeps a pretty tight ship on the whole process.
The gravel removal appears to be effective (?) in the part of the river between Yarrow and Peach Road. Down in the Canal, the reason it is so high is because of Fraser River backwatering in the spring time, and largely has little to do with the fall/winter floods that threaten Yarrow and the Vedder Crossing area. So there is little reason to take out gravel from the Canal as it is basically a stillwater issue, particularly from below the KWR bridge, and related to the Fraser River backwatering.
One of the things about the Canal is that it has recruited some very nice gravel for pink spawning over the years since I started fishing there. Also, some really nice pool riffles, which are good for angling as well as probably some good rearing habitat. Additions of some constructed log jams would make it fabulous habitat, but the diking authorities freak out over that stuff. The Province’s steelhead biologist, Ron Ptolemy, once told me that some of the highest densities of juvenile steelhead parr were sampled rearing in the Canal.
If there is an instance of the river heading for the dike, and gravel is taken out to reduce erosion, I would be surprised. It may happen, on occasion, but it would be the first time I saw it. Again, I have been away from the issue for over 10 years, so maybe there are shenanigans going on that I don’t know about, but on the whole, the above description is how I remember it."
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Topic: Gravel Mining The Vedder is on Again (Read 28680 times)