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New report from Returning Rapids Project

jayfromtexas

Well-Known Member
Hello All,
Just in case any of you might find this interesting, the Returning Rapids Project people just posted their latest trip report. Their latest expedition was in late September 2021, maybe trying to get down there early just in case the 'guvmint shut down again? Also I think they were trying to recreate a historical photograph. Anyway here is the link so those who may wish to read it. Be forewarned that they are now affiliated with the GCI so some content may be "offensive".


Jay
 

JFRCalifornia

Escalante-Class Member
Very interesting stuff... thanks for sharing. Great photo comparisons, but the slide I keep coming back to is this graph on the 5th slide... that sediment profile is something to study. I had to do a little mental translating from meters to feet, but that amount of sediment up there is staggering... and you can really see how it's marched downstream with the falling surface elevation of Lake Powell....

Sediment Profile.jpg
 

Dougie

Well-Known Member
The report says over time approx 1 meter per year of sediment is cut down by the River. But take a look at those very impressive before/after pictures of the tributary at Dark Canyon after the monsoon flood of this past August! (pages 27/28 of the report) I'm thinking that if we get a 100,000 CFS flow as in a 1983 level spring, we we could see some major channel shifting and some impressive 100 foot high waterfalls as the river scours out new flow patterns running through those huge silt deposits.
 

Tiff Mapel

Well-Known Member
Very cool! Even if GCI is involved.... We did Cataract back in July of 2011, when the river was rocking at 60,000cfs! What a wild ride that was! The silt walls were impressive back then. I always wondered if there was a good way to get it out and redeposit it downstream, for instance, in the Paria. That way, the Grand Canyon could get their sand replenished. But, the downside is that the sediment will work its way downstream to Mead. And wasn't that a minor reason that Glen Canyon Dam was built--to trap sediments that would settle out in Mead and fill Mead up faster?

Tiff
 

drewsxmi

Well-Known Member
I was pondering the rate of silt deposition, and by the raw numbers it's about 700 years to fill the entire 24,322,000 af. That's far longer than I plan to be around to be concerned about it. On the other hand, as we see in North Wash, Farley, and White Canyons, the silt might not fill the side canyons. Although the silt spread out pretty evenly at Hite Marina, I don't think that it would deposit deep into Bullfrog, Halls, Warm Creek, Wahweap, and especially Last Chance Bays. If you subtract the big side bays out of the calculations, it might be only 350 years. If you factor in lower water levels, that might bring the sedimentation time to as short as one third of that, say 100 years. At that theoretical 100 year time, there would still be plenty of capacity (70% of 24,322,000 af) to store more water, but the lake at it's current level would all be shallow water or mudflats.

And just to make things even more complicated, some sediment will get deposited above the still water level because the current begins to slow as the river gradient decreases. This effect has already occurred along the San Juan River.

The Returning Rapids folks found an amazing amount of sediment. My folks ventured to Dark Canyon by houseboat in 1986 with the lake about 3,690' elevation, and found that to be the upper limit of practical navigation. The original river level there was around 3,490' elevation.
 

JFRCalifornia

Escalante-Class Member
I always wondered if there was a good way to get it out and redeposit it downstream, for instance, in the Paria. That way, the Grand Canyon could get their sand replenished. But, the downside is that the sediment will work its way downstream to Mead. And wasn't that a minor reason that Glen Canyon Dam was built--to trap sediments that would settle out in Mead and fill Mead up faster?

Tiff
Glen Canyon Dam's role as a sediment trap was not even a minor reason it was built (at least based on the text of the Colorado River Storage Project Act of 1956), but was used retrospectively in subsequent USBR publications as a further justification of its presence, especially once questions of sedimentation and dam life started to arise. That said, downstream flood control was always an important reason for building the dam, and one of the reasons cited in the original 1925 USGS report that evaluated many potential dam sites along the Colorado River and identified the one at Glen Canyon as the best site for the first dam, even over what eventually became the site of Hoover Dam.

It's also notable that in a 1956 summary report of the CRSPA, the USBR cited an important reason for Glen Canyon Dam was to regulate the wildly fluctuating annual river flows (store water during bad times), which it said ranged from 4 to 22 maf annually. While that was undoubtedly a reason, they made the low flows sound worse than they really were. Deliberately? Although we just had a year with a 4 maf flow, in 1956 there had never been an inflow less than 5.5 maf (only happened once, in 1934), and only a couple of times in the range of 7-8 maf. Otherwise flows were always much higher. So it was a little disingenuous (at best) of USBR to make it seem the sky was falling in order to justify the dam, although in retrospect those low flows eventually did happen...
 
Last edited by a moderator:

bob london

Active Member
Hello All,
Just in case any of you might find this interesting, the Returning Rapids Project people just posted their latest trip report. Their latest expedition was in late September 2021, maybe trying to get down there early just in case the 'guvmint shut down again? Also I think they were trying to recreate a historical photograph. Anyway here is the link so those who may wish to read it. Be forewarned that they are now affiliated with the GCI so some content may be "offensive".


Jay

Very interesting, jay. Thanks for posting. What's "offensive" about a bunch of people getting their feet wet, recording the situation on the ground and then posting their findings in a blog?

From where I sit, the RRP seems to conduct its research in a thorough and unbiased manner: they report what they find and then put it in the public domain. What's "offensive" about that?

One glaring error I spotted in this report is that 'dead pool' occurs at 3490 when we all know that figure should read 3370, right?

To be honest, I have little interest in rapids reappearing but I am fascinated by the inexorable march downstream of the deltas towards the *whispers* dam.
 

jayfromtexas

Well-Known Member
Bob,
Yup. Deadpool is indeed 3370'. To be honest I don't know why they don't have lower outlets on dams than that but they don't. Perhaps someone proficient in engineering can help explain why they don't build them with river level outlets and such. Also what's "offensive" is absolutely nothing haha. It's just a way of poking fun at certain people who take things too seriously sometimes.
 

jayfromtexas

Well-Known Member
Glen Canyon Dam's role as a sediment trap was not even a minor reason it was built (at least based on the text of the Colorado River Storage Project Act of 1956), but was used retrospectively in subsequent USBR publications as a further justification of its presence, especially once questions of sedimentation and dam life started to arise. That said, downstream flood control was always an important reason for building the dam, and one of the reasons cited in the original 1925 USGS report that evaluated many potential dam sites along the Colorado River and identified the one at Glen Canyon as the best site for the first dam, even over what eventually became the site of Hoover Dam.

It's also notable that in a 1956 summary report of the CRSPA, the USBR cited an important reason for Glen Canyon Dam was to regulate the wildly fluctuating annual river flows (store water during bad times), which it said ranged from 4 to 22 maf annually. While that was undoubtedly a reason, they made the low flows sound worse than they really were. Deliberately? Although we just had a year with a 4 maf flow, in 1956 there had never been an inflow less than 5.5 maf (only happened once, in 1934), and only a couple of times in the range of 7-8 maf. Otherwise flows were always much higher. So it was a little disingenuous (at best) of USBR to make it seem the sky was falling in order to justify the dam, although in retrospect those low flows eventually did happen...
JFR,
Very interesting stuff, all this about sediment. But yes I think I did read somewhere too that a secondary purpose of Lake Powell was to extend the life of Lake Mead by reducing sediments flowing down the Grand. Also I do find it ironic that yes indeed the west is beginning to experience some fairly low inflows.
 

drewsxmi

Well-Known Member
Bob,
Yup. Deadpool is indeed 3370'. To be honest I don't know why they don't have lower outlets on dams than that but they don't. Perhaps someone proficient in engineering can help explain why they don't build them with river level outlets and such. Also what's "offensive" is absolutely nothing haha. It's just a way of poking fun at certain people who take things too seriously sometimes.
I can think of a couple of reasons for not having outlet works at the extremely low levels of the dam. The main reason is that they would likely get blocked by sediment somewhat early in the life of the reservoir, so they would lose their usefulness. Each special feature of the dam costs more money, so they probably wanted to avoid having multiple sets of outlet works at different levels.

I was thinking about the higher levels of pressure for the valves if the inlets are deeper, but the pressure just depends on the elevation difference between the water surface and the valve outlet.

They used the diversion tunnels as temporary outlet works for a while while the lake was filling, until the water level reached high enough to get sufficient flow through the outlet works. The gates in the diversion tunnels were described somewhere as vertical steel gates, which might not operate very well with a huge pressure differential. After they closed them off they filled the diversion tunnels below them with concrete plugs.

The dam designers probably never envisioned that the reservoir would get so low as to need outlet works at the lowest levels, or the USBoR decided to not even allow for that possibility in the design of the dam.

The operators of the Teton Dam learned some valuable lessons about the need for adequate outlet works. I can also envision that river-level outlet works would be needed for full repairs on a dam.
 

jayfromtexas

Well-Known Member
Thanks Drew that makes sense. It certainly is possible that the Bureau never thought that there would be a need to be able to release water below a certain level. Back when the dam was envisioned I think the lowest flows were usually in the lower tens. I believe JFR found that prior to 1956 flows were around 5.5 only once (1934) and there were maybe two times in the 7-8 range. And yes the pressure at those depths with a full reservoir would probably be enormous on metal gates. Fortunately we are still a long way off from dead pool but then again I believe this is the second time in the last two decades that there were two low flow years. Predictions are only predictions but lets hope this next winter doesn't make for a third year of bad inflow.
 

KCYakker

Member
Very interesting, jay. Thanks for posting. What's "offensive" about a bunch of people getting their feet wet, recording the situation on the ground and then posting their findings in a blog?

From where I sit, the RRP seems to conduct its research in a thorough and unbiased manner: they report what they find and then put it in the public domain. What's "offensive" about that?

One glaring error I spotted in this report is that 'dead pool' occurs at 3490 when we all know that figure should read 3370, right?

To be honest, I have little interest in rapids reappearing but I am fascinated by the inexorable march downstream of the deltas towards the *whispers* dam.
3490' is (more or less) the bottom of Minimum Power Pool and the cutoff for generation - I think . . . . (feel free to correct as needed).
 

drewsxmi

Well-Known Member
That is a sobering statement.....but would the 60+ years of accumulated sediment effect those figures?
I'm still hoping for somebody to get a depth finder reading in the main channel in line with the entry station on Lakeshore Drive, but the bottom elevation in that area is around 3,180' according to the USGS bathymetric data. Sediment would not change the minimum power and "dead" (minimum reserve) pool levels until it reached the level of the intakes in the dam. At the minimum reserve level the lake would still reach to between Halls Crossing and Moki Canyon.

The major effects of sedimentation are above The Horn in the main channel, and above the Great Bend in the San Juan Arm. If the water level went down to minimum power pool the deposited sediment would probably erode more quickly and fill uplake areas faster than the normal deposition rates.
 

bob london

Active Member
I can think of a couple of reasons for not having outlet works at the extremely low levels of the dam. The main reason is that they would likely get blocked by sediment somewhat early in the life of the reservoir, so they would lose their usefulness. Each special feature of the dam costs more money, so they probably wanted to avoid having multiple sets of outlet works at different levels.

I was thinking about the higher levels of pressure for the valves if the inlets are deeper, but the pressure just depends on the elevation difference between the water surface and the valve outlet.

They used the diversion tunnels as temporary outlet works for a while while the lake was filling, until the water level reached high enough to get sufficient flow through the outlet works. The gates in the diversion tunnels were described somewhere as vertical steel gates, which might not operate very well with a huge pressure differential. After they closed them off they filled the diversion tunnels below them with concrete plugs.

The dam designers probably never envisioned that the reservoir would get so low as to need outlet works at the lowest levels, or the USBoR decided to not even allow for that possibility in the design of the dam.

The operators of the Teton Dam learned some valuable lessons about the need for adequate outlet works. I can also envision that river-level outlet works would be needed for full repairs on a dam.

'The main reason is that they would likely get blocked by sediment somewhat early in the life of the reservoir ...'

Floyd Dominy thought the sediment build-up could be a problem sometime around the year 2960.

Are you calling him out as a liar, drew?
 
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