What was the point of clearing the trees in the path of the water topping the auxiliary spillway at the Oroville Dam? If the spillway fails, isn’t a bunch of timber in the water the least of the area’s worries?
No. If the water comes down, the trees would act as batterying rams, or little dams themselves, and demolish things in their path.
Is one thing for a building bridge or other man made structure or object to be inundanted by flood water, its another for it to be pressed upon by a a whole tree with the flood water behind it. The idea is that the flood may damage things, but the trees in it would contribute to destruction.
Oh the spillway is designed to bypass the town, but if the trees taken down by the water were to form a dam the water might damage more.
The tress might form a dam where the floodway goes under a bridge or similar structure.
Over 100,000 people evacuated. The gas stations south of Oroville have run out of gas and have had to shut down, leaving drivers stranded.
The spillway is not “designed to bypass the town.” The main spillway, and the auxiliary spillway are designed to return the water to the Feather River below the dam, but above a secondary dam that is used to impound water for the Thermalito power dam outside of Oroville.
The cutting of the trees had nothing to do with the emergency spillway failing. There were being cut since that spillway was being used.
They didn’t want a bunch of dangerous stuff being sent downstream.
(The media coverage of this is atrocious. Many online pics don’t even correctly label which spillway is which.)
It doesn’t help when the NWS issues an Flash Flood Warning stating the dam itself was failing … later versions made the correction … but still, nice frantic panic there, guys, getting a little bored with your job?
Don’t the trees help prevent erosion?
Yes, in a general sense. But not when a 100,000 cfs of water is rushing down on them. Then they are just huge, damaging missiles. There is no tree(s) that is going to hold soil intact in the face of that kind of onslaught.
Just cutting the trees down doesn’t effect the root system holding the soil in place …
You trying to get a job in the new EPA?
So…a reservoir in Northern California is so full, it’s in danger of overtopping?
Well, that part’s good news, I guess…
Here’s a link to another thread on the topic that helps explain things a bit better.
Well, confusing linquistic difference I guess, but that is what was happening. The dam was failing, in the way that dams fail. The edge the water was going over was failing, undermining the downhill side. That is a failing dam.
It’s not a /failed/ dam, but if you waited until after the dam had failed, it would be too late. And it wouldn’t be “failing” any more, in the technical sense. Although I guess you could then say that it was “failing to hold any water”.
The dam wasn’t failing, the spillway was failing. No water is or was going over the edge of the dam itself. Water was going over the emergency spillway, as was designed to occur. The problem is the area below the spillway is eroding, leading to concerns that the spillway might collapse. That would release a catastrophic flood, but nowhere near what would happen if the dam itself collapsed (which isn’t likely to happen).
The local news in Sacramento has been sometimes confusing, so I can only imagine how this event is unfolding around the country.
The Oroville Dam is not involved at all with any of what is going on. The height of the dam is about 20 feet above the highest level of the lake. There is no chance of the lake topping the dam, unless a meteor strikes the lake.
The problems began last week with the main spillway - the one that is concrete-lined. A hole developed in the main spillway about halfway down, that has continued to be eroded as they try to lower the lake to make room for high inflow from the drainage basin from recent rain storms. They could not release as much water as they wanted, and had to stop releases for a time while they inspected the pot hole.
In the mean-time, inflows topped-out about 3x as much as they could release thru the main spillway and the power plant that is built into the dam. The lake level rose. There is an emergency “auxiliary” spillway next to the main spillway - this is essentially a weir that will let water top-over via gravity when the lake reaches maximum capacity. It is uncontrolled, and the gully below this auxiliary spillway is (was) just a natural area with forest, trees, and boulders. They cleared the gully as best they could so as to minimize the amount of debris flowing into the main river, which may have caused more flooding in downstream communities.
Bottom line: Oroville Dam is fine. The main spillway is damaged, but they can still release water thru it without flooding the river below. The Emergency (Auxiliary) spillway has never been used before, and experienced more erosion than expected, and there was a fear it would fail - which is what prompted the evacuations. Now that the lake is below the level of the weir, they can re-inforce it in prep for the next round of storms, expected Thursday.
Congressman Garamendi said today that the possibility of the emergency spillway failing was known at the time of the dam’s construction, and it was ignored.
I remember looking at the dam just before it was finished thinking it was going to fail … I think everybody will say they did … uh huh …
Clearing away trees would not only reduce the amount of flotsam and jetsam in the flood waters, it will also enable crews to better see and repair damage when this is all over.
That hole in the main spillway is being filled with heavy bags full of rocks, to reduce further erosion.
The north Highlands of Scotland used to get incredible floods ( and extreme weather: however even when the Highlands had more people few lived in Sutherland, and far less now — and it is a massive place ). I was just reading in one of Alistair Alpin MacGregor’s books of how the Iron Bridge Thomas Telford threw up at Bonar-Bridge along the Kyles of Sutherland in 1811 was destroyed.
‘Through the narrows at Bonar pass the combined waters of the Oykell, the Einig, the Cassley, the Shin and the Carron. The first two drain an area of over 200 square miles of mountainous territory. The Cassley drains roughly a hundred. The Shin and its tributaries drain about 250 square miles. The Carron, together with such affluents as the Calvie and the Blackwater conducts to the Kyle of Sutherland, within a few hundred yards of Bonar-Bridge, the accumulated flow from a watershed of over 150 square miles. Add all these watersheds, and one realises what volume of water, especially in time of deluge, passes through the narrows at Bonar to reach the sea at the head of the Dornoch Firth.’
After a few months in 1812 it was tested when huge piles of ice piled up on the landward side of the piers and vast amounts of fir logs being sent down ( as in Canada ) broke loose and repeatedly smashed into the sides, embedding themselves in the massed ice. Telford’s Bridge had ‘no flaw’ – yet the greater floods of 1892 broke it.
‘Apparently this event occurred as predicted by the Brahan Seer.’ Wikipedia
No, I don’t think that is correct. They would not drop the bags of rocks into the hole in the main spillway (the damaged concrete one). The force of the water (100K cfs) is so great, that would only add to the debris being pushed into the river below. They will wait until there is no water in there so they can assess what can be done near and long term.
The 2-ton bags of rocks are for the emergency spillway, where it appears a large “gouge” eroded toward the base of the weir which threatened the integrity of that structure, during the overflow on the weekend. They want to get that ready for the next round of storms in case they cannot lower the level of the lake enuf by Thursday, and need to use the emergency spillway again.
This story dispenses with the sensationalism for the most part.