Oroville Dam Main Spillway Waterfall Erosion Watch

[deirdre]

It suggests that the flow out of the reservoir might have decreased even before the flood control structure is used to ramp it down

I'm a bit confused by this conversation. According to the Weather Forecast thread the flow is lower. This photo is Feb 19th at 55,000 cfs.

either way DWR twitter says outflows to be reduced to zero this afternoon. so we'll be able to see more...
Source: https://twitter.com/CA_DWR/status/836263709018214400

 

[Hama Neggs]

Views of Oroville Spillway today 2/26/17


Source: https://www.youtube.com/watch?v=m9ar73kBlWg

That one really shows how sheer momentum is causing high water levels at the power plant outlet area, plus, I suppose, that plug of rock debris.

 

[Anna Reynolds]

I'm a bit confused by this conversation

These two photos shared by DWR on the 24th and 25th show water flowing at 50,000cfs but the flow coming down the main chute appears to be much less by the 25th (50,000cfs is mentioned in the description for both):
http://pixel-ca-dwr.photoshelter.co...I0000yYFnwfD7v7Q/KG-oroville-damage-14319-jpg

Image of 50,000 cfs flow on the 24th:
http://pixel-ca-dwr.photoshelter.co...I0000HEJuP5yfNLc/KG-oroville-damage-14111-jpg

But you are correct, things are about to get a lot more clear in the coming days .

 

[deirdre]

These two photos shared by DWR on the 24th and 25th

thank you. Marian's post said "from today" (26th), i should have double checked the reference links.

 

[Blaise]

50,000 cfs stands for 50,000 cubic feet per second which is a measure of velocity.

No, cfs is a measure of flow not velocity. In terms of this spillway, 50,000 cfs can be delivered as 10 ft of water travelling at 20 mph or 5 ft of water travelling 40 mph. Same amount of water but different splash pattern.

 

[whoosh]

KCRA filming the flow reduction live on 2/27:

https://www.facebook.com/KCRA3/videos/10155073025346514/

Slower flows kicking up a lot more sediment at the first plunge pool.

Infuriating that they waste so much time on the gate area.

At around 38m; another at 58m; good close-up footage of right side erosion taking place at the break as they cut back on flows. It shows the type of material being deposited in the div. pool.

Here's the DWR low altitude drone video of the zero flow spillway 2/27:
Much different perspective without the zoom from the chopper.

Source: https://www.youtube.com/watch?v=UyvDPt-HU3g

 

[SR1419]

Currently at zero (0) outflow...here are some screen shots from the KCRA coverage with outflow at one gate and some with all gates closed and residual water still pouring over.

 

[BrokenLug]

Boy they sure do have one heck of a fix-it up project ahead of them.
Is priority #1 still to dredge/ clear the Thermalito Diversion Pool and get the power plant back into action ?
That's an awful lot of material in "the pool" from the main spillway debris flow.

 

[deirdre]

Is priority #1 still to dredge/ clear the Thermalito Diversion Pool and get the power plant back into action ?

i cant find a link to the press conference this morning (10 am) but here is a press release from yesterday

Cutting off flows from the flood control spillway for several days will allow workers to aggressively attack the debris pile at its base and reduce the water level surface elevation of the channel that leads from the Hyatt Power Plant.

http://www.water.ca.gov/news/newsreleases/2017/022617ict.pdf

Press Conference 10 am this morning https://www.facebook.com/CADWR/videos/10154473698627449/
2:40 "we're going to focus on removing debris"
6:00ish "as we move through the next 5-7 days our goal is to aggressively attack this diversion pile"

 

[BrokenLug]

10-4, Thanks for the response deirdre.

They're going to have to get a whole heck of a lot of barges and earth removing equipment in there to remove that quantity of material.

I realize the perspective of my above image is skewed, but if you compare the amount of material that was placed/ poured at the foot of the emergency spillway to the amount of debris flow material that's currently in the diversion pool below the main spillway, oooff, that's a lot of material to dredge out of the water.

I wonder what the depth of the water in that location was before this event?

 

[Chris Owens]

Great pics! Thanks for capturing those, BrokenLug and SR1419!

I guess they're interested in clearing out the pool to reduce erosion around the edges of the pool? The more stuff that fills in, the more the water has to find more routes downstream.

Pity they can't be confident that there won't be any more big rains. That volume of material would probably be handy for whatever repairs they plan to make to the main spillway.

[off topic text removed]

 

[Pat Dilling]

I notice that the residual water was still eroding under the left side of the spillway. That was a concern expressed by the Director at the presser. At 50k release the water is over shooting that area. At lower releases it may fall closer and cause more of the upper spillway to fall in. He stated the helicopter would be filming at all stages of slowed release to determine when erosion happens so if they need to turn it back on they will have an idea what the safest flow is.

 

[Jody Bourgeois]

I notice that the residual water was still eroding under the left side of the spillway. That was a concern expressed by the Director at the presser. At 50k release the water is over shooting that area. At lower releases it may fall closer and cause more of the upper spillway to fall in. He stated the helicopter would be filming at all stages of slowed release to determine when erosion happens so if they need to turn it back on they will have an idea what the safest flow is.

Interesting--I just posted that thought (low flows might be locally more under-cutting) on my facebook page along with the new no-flow photos.

 

[DeejayB]

Boy they sure do have one heck of a fix-it up project ahead of them.
Is priority #1 still to dredge/ clear the Thermalito Diversion Pool and get the power plant back into action ?
That's an awful lot of material in "the pool" from the main spillway debris flow.

View attachment 25572

Interesting--I just posted that thought (low flows might be locally more under-cutting) on my facebook page along with the new no-flow photos.

At last we see some excellent views from videos made by drone flights for DRW:
Source: https://www.youtube.com/watch?v=UyvDPt-HU3g;

both parts show that upstream from the hole, the rock directly beneath the spillway is weathered hydrothermally altered bedrock/intrusive dyke material, and as with the material that was the source of the original hole that was scoured out, is also likely to scour out with further drain failure. How far back this material extends beneath the spillway cannot be gauged accurately from the video, but in my opinion as a geologist I would expect that it extends back to near the area adjacent to the power pylons, the red/brown material is the weathered altered bedrock, the grey the the metabasalt, which resists erosion strongly. Sampling from the areas on both sides of the spillway would give a reasonable guide, but considering the age of the spillway, and the fact that some of the drains have collapsed, I think it would be prudent to demolish the whole of the remaining spillway and start afresh.

[off topic content removed]

But the priority now facing the DRW is the removal of the damming rubble that was washed into the river by the stream coursing down the south side of the spillway; this rubble is preventing the operation of the power station, which is preventing further controlled lowering of water in the reservoir.

 

[NoParty]

At last we see some excellent views from videos made by drone flights for DRW:
Source: https://www.youtube.com/watch?v=UyvDPt-HU3g;

Those are awesome, priceless images!

 

[SFX]

Source: https://www.youtube.com/watch?v=GlSGDGTaCh0#t=127.095827


Part 2

 

[DeejayB]

Source: https://www.youtube.com/watch?v=GlSGDGTaCh0#t=127.095827


Part 2

And as you can see at the one minute mark, the brown/red weathered zone has eroded considerably further back than the unweathered metabasalt on the south side of the spillway.

The patchy nature of the country rock is a problem to be faced, it will determine how rebuilding will be done.

But in the meantime a large effort has to be made to clear the detritus that has been washed down into the river as it is damming the pond below the power station, preventing it operating and contributing to the control of the water level in the reservoir.

 

[Stack Dump]

Velocity is correct. "per second" denotes a measure of time.

Velocity is incorrect. Velocity is measured as distance/time.

Cubic feet per second is a unit of flow rate. It is independent of velocity, as area needs to be taken into account also. It is volume per second, which is what your worked example demonstrated. The volume flow can be derived from area multiplied by velocity (distance x distance x distance / time), but velocity on its own is incorrect. Cubic feet per second is not "a measure of velocity".

Cubic feet per second (cfs) should rightly be denoted cf/s. Calling it cfs is convenient, but sloppy engineering.

 

[JCL]

The scale of this "debris pile" is shocking.

 

[SFX]

(Off topic) Yes, but all they have to do is use it for fill to repair both spillways.

 

[Stack Dump]

The scale of this "debris pile" is shocking.

The scale of the Oroville dam, its associated structures, the lake, and everything else to do with it continues to amaze me. I'm still trying to come to terms with it. When I see a man or a machine next to something, I think, "oh, that's how big that thing really is -- about five times bigger than I thought".

I could add, that is one reason why things like design safety factors, operation, and the need for easy routine and long-term maintenance need to be very carefully thought through in advance. There are a few shortcomings readily apparent with the Oroville dam (other than the obvious), yet now the thing is built and operating, very hard to go back and fix.

 

[Stack Dump]

The water was flowing clear at high volume, but now, at a very low flow rate, water collecting in the pools is muddy brown. So yes, they need to find a range of usable flow rates that don't hasten its demise. They also need to figure out how to shut it off completely, so the last trickle doesn't flush it down the toilet.

The mere fact of the north-west side advancing further up the slope shows where the weakness is, and those slabs seem to have the smallest overhang. Less overhang means more washout at low flow rates.

The overhanging slabs will be a safety hazard when it comes to repairs, at whatever point they stop being undercut and dropping off.

 

[Soilmaker]

The scale of the Oroville dam, its associated structures, the lake, and everything else to do with it continues to amaze me. I'm still trying to come to terms with it. When I see a man or a machine next to something, I think, "oh, that's how big that thing really is -- about five or ten times bigger than I thought".

I could add, that is one reason why things like design safety factors, operation, and the need for easy routine and long-term maintenance need to be very carefully thought through in advance. There are a few shortcomings readily apparent with the Oroville dam (other than the obvious), yet now the thing is built and operating, very hard to go back and fix.

I agree. The logistics of removing the pile are daunting; excavating, hauling off spoil, dealing with materials, etc. In looking at the

 

[whoosh]

The scale of this "debris pile" is shocking.

also showing yellow iron on the pile.

Source: www.youtube.com/watch?v=E3gtNUljcN0

We'll see if they store it on site for reconstruction fill.

 

[SeanT]

The scale of this "debris pile" is shocking.

DWR hopes to have it cleared as early as Thursday...

The water in the channel below the spillway dropped 23 feet in less than a day, enough to get at least one of the turbines up and running, perhaps as early as Thursday. Electrical crews are working to get transmission lines running from the plant reattached to the powergrid, Vogel said Tuesday.

source http://www.sacbee.com/news/local/article135422349.html

 

[DeejayB]

View attachment 25588

also showing yellow iron on the pile.

Source: www.youtube.com/watch?v=E3gtNUljcN0

We'll see if they store it on site for reconstruction fill.

An excellent choice of photographs, Whoosh, a photo that clearly shows the variable depth of weathering in the south side of the channel formed by the stream that broke free of the confining spillway, and the resistant unweathered rock beneath it; and notice that the only material in the rubble that forms the unwanted dam is that of the unweathered rock, showing just how much tougher it is and how much better it withstands transport by a very vigorous stream. As you suggest, it could well be very useful as reconstruction fill.

 

[Soilmaker]

I also am amazed by the volume of debris accumulated at the base of the spillway. It will not be trivial moving it. But their immediate focus seems to be removing debris from in front of the dam so they can restart the power plant. I've been looking at available images and monitoring the water elevations in the reservoir published on the CDEC website and it looks like without outflow to the spillway the water elevation in the reservoir is increasing about a foot every 8 to 9 hours.

The weather forecast calls for rain starting this weekend. At the current rate the water elevation will be 12' higher by then or more, still low well below capacity but at some point they will need to restart the flow to the spillway. It's impossible to estimate from photos the volume of debris to remove from in front of the spillway, but it is doubtful this work will be completed any time soon. Obviously starting and stopping flow to the spillway will be part of the equation. It will be interesting to watch how they juggle turning on and off the flow to the spillway and managing the clearing of debris.

It seems from their news briefs that they want to clear sediment from the bottom of the dam in order to lower the water level and restart the power plant, thus allowing them to release water from that source.
As a civil engineer looking at the problem I think it would be a good idea to excavate and deepen an outside channel around the spillway debris that would allow the water to drain from in front of the dam. I would pile up the debris in such a way that it blocks water flow from the spillway upstream towards the bottom of the dam. In doing so they make use of the debris, since they have no roads to move it by anyway. And when they have to restart the flow over the spillway it will be prevented from pushing sediment back upstream and depositing more sediment in that direction.

This is going to be an on going and significant challenge for several months as snow melt and rainfall proceeds.

 

[harkin]

The scale of this "debris pile" is shocking.

What an incredible photo - thx.

Wonder if they're finding any gold in there. In Jimtown they're finding gold all over which happens in very wet years.

 

[Soilmaker]

DWR hopes to have it cleared as early as Thursday...

The water in the channel below the spillway dropped 23 feet in less than a day, enough to get at least one of the turbines up and running, perhaps as early as Thursday. Electrical crews are working to get transmission lines running from the plant reattached to the powergrid, Vogel said Tuesday.

source http://www.sacbee.com/news/local/article135422349.html

Based on photos it's hard to believe that water levels dropped 23ft. How deep is that channel?

 

[Marian]

Based on photos it's hard to believe that water levels dropped 23ft. How deep is that channel?

If you watch the end of the hour and a half video posted by KCRA3 yesterday, you will see that immediately after the water stopped flowing down the spillway, the level dropped quite dramatically, as it is no longer being forced in the upstream direction by the rushing force of the waterfall.

 

[Stack Dump]

Of the rock that remains, the parts that received a severe pounding by the water over several days are that blue-green colour we are now familiar with. Furthermore, the outline of the blue-green rock through the newly carved ravine shows the path of the heaviest, turbulent water flow. It's that exact.

Following the discussion about weathering rock and loose material washing out, the video shots of the ravine and massive concrete slabs hanging in the air put that to rest. It's the same problem we saw with the emergency spillway: even if the concrete was originally laid on competent rock, the combination of weathering and/or chemical transformation, plus water erosion, has weakened the rock, and large chunks are just gone. The washout is uneven, with deep fissures opened up. It's as much a geology failure as it is an engineering failure.

 

[StickyMockwell]

DWR hopes to have it cleared as early as Thursday...

I'll state the obvious - I don't think they will be able to clear all that material by Thursday. More feasibly, they need to enlarge the channel enough so that upstream and downstream water levels equalise (more or less), even when they get the powerplant running. If the channel is big enough, the water flowing through it from the powerplant to downstream should look calm and tranquil. If its still causing an obstruction, the flow will look more like whitewater rapids.

 

[Stack Dump]

I'll state the obvious - I don't think they will be able to clear all that material by Thursday.

Agreed. I think the source of that idea is from post #225 above:

The water in the channel below the spillway dropped 23 feet in less than a day, enough to get at least one of the turbines up and running, perhaps as early as Thursday.

But getting a turbine up and running doesn't equate to moving that hill of rubble by Thursday.

 

[David L. Hagen]

The critical requirement is to cut a channel sufficient for the power plant flow.

 

[JCL]

DWR hopes to have it cleared as early as Thursday...

The water in the channel below the spillway dropped 23 feet in less than a day, enough to get at least one of the turbines up and running, perhaps as early as Thursday. Electrical crews are working to get transmission lines running from the plant reattached to the powergrid, Vogel said Tuesday.

source http://www.sacbee.com/news/local/article135422349.html

What an incredible photo - thx.

Wonder if they're finding any gold in there. In Jimtown they're finding gold all over which happens in very wet years.

The critical requirement is to cut a channel sufficient for the power plant flow.

What about when the main spillway comes back on, will the water in the channel rise 23' again and the power plant is in the same spot with potential flooding? Don't you think just cutting a channel for the power plant is going to be redundant as soon as they turn on the main again? That pile is massive and will only grow as soon as they turn on the main again...

 

[Pat Dilling]

It will truly be interesting to watch how they attack the debris pile

As a civil engineer looking at the problem I think it would be a good idea to excavate and deepen an outside channel around the spillway debris that would allow the water to drain from in front of the dam. I would pile up the debris in such a way that it blocks water flow from the spillway upstream towards the bottom of the dam. In doing so they make use of the debris, since they have no roads to move it by anyway. And when they have to restart the flow over the spillway it will be prevented from pushing sediment back upstream and depositing more sediment in that direction.

This is going to be an on going and significant challenge for several months as snow melt and rainfall proceeds.

It will be interesting to watch how they attack the debris pile and if they try to move some of it to block upstream flows and direct more flow downstream as Soilmaker suggests. I am astounded not only by the overall size of the debris pile but the size of some of the debris pieces. Seeing them in relation to that big Cat (assuming D10) crawling across shows how powerful the water stream was to move huge rocks that far. There will certainly be attrition on the equipment keeping the mechanics busy. I also wonder how much of the pile is still underwater downstream and how far below what we can see they will have to start dredging in order to reach the pile.

 

[Soilmaker]

https://ww2.kqed.org/news/2017/02/0...lle-dam/?_ga=1.148472255.942819923.1483464978

Update, 2:20 p.m. Monday, Feb. 27:

"There are a lot of numbers being thrown around, anywhere from 150,000 cubic yards all the way up to a million," DWR Acting Director Bill Croyle said in an interview Monday.

He said with flows down to zero, laser mapping technology will be used to assess just how much debris now obstructs the channel.

"I suspect it's going to be between a half-million and a million cubic yards," Croyle said. "But again we won't know until that mapping tomorrow."

Croyle said contractors have been tasked with clearing a channel 30 feet deep, 150 wide and 1,500 feet long to help facilitate flows below the dam."

I calculated the amount of debris from such a channel:
30*150*1,500 = 6,750,000 ft3 or 250,000 cubic yards. The weight of a cubic yard of rock depends on the size of the pieces so I used sand as a minimum boundary. A cubic yard of sand weighs 2.2 tons. So there will be 550,000 tons of debris to remove from the channel Mr. Croyle is saying contractors are going to dig. A quad-axle dump truck can haul 20 tons of gravel. At 20 tons per load there will be 27,500 truckloads. Not a fast process!

 

[EricL]

https://ww2.kqed.org/news/2017/02/0...lle-dam/?_ga=1.148472255.942819923.1483464978

Update, 2:20 p.m. Monday, Feb. 27:

"There are a lot of numbers being thrown around, anywhere from 150,000 cubic yards all the way up to a million," DWR Acting Director Bill Croyle said in an interview Monday.

He said with flows down to zero, laser mapping technology will be used to assess just how much debris now obstructs the channel.

"I suspect it's going to be between a half-million and a million cubic yards," Croyle said. "But again we won't know until that mapping tomorrow."

Croyle said contractors have been tasked with clearing a channel 30 feet deep, 150 wide and 1,500 feet long to help facilitate flows below the dam."

I calculated the amount of debris from such a channel:
30*150*1,500 = 6,750,000 ft3 or 250,000 cubic yards. The weight of a cubic yard of rock depends on the size of the pieces so I used sand as a minimum boundary. A cubic yard of sand weighs 2.2 tons. So there will be 550,000 tons of debris to remove from the channel Mr. Croyle is saying contractors are going to dig. A quad-axle dump truck can haul 20 tons of gravel. At 20 tons per load there will be 27,500 truckloads. Not a fast process!

I believe that the reason your calculation of the volume of material moved is only half of the minimum figure estimated by Mr. Croyle is because you assumed a square-cut channel, cut "to size", while he probably assumed that the sides would be sloped back to some stable configuration. Still, though I'm not disputing your conclusion that a huge amount of material will be moved, I will nit-pick your methodology a little bit.

Your assumption that loose-dumped sand in a truck box weighs 2.2 tons per cubic yard, which works out to 163 pounds per cubic foot, is awfully generous. I started out planning an explanation why sand's density in a loose-dumped condition would be around 100 to 120 pounds per cubic foot (1.3 to 1.6 tons per cubic yard), depending on the water content, but on seeing that, people would just roll their eyes. So check this out:

http://scruggscompany.com/resources/conversions-calculators/

Standard Weight for Sand = 2,700 LBS per Cubic Yard

This was in pretty close agreement with a handful of other references I found, and to put it in the same units that you used, this equals 1.35 tons per cubic yard.

The material at the Oroville site is mostly rock, so here's a more applicable generalization:

http://petelien.com/21-crushed-faqs/50-how-many-tons-of-riprap-are-in-one-cubic-yard

The conversion for riprap varies by size but generally speaking, a conversion of 2 tons per cubic yard is sufficient. Limestone has an in situ unit weight of approximately 160 pounds per cubic foot but voids on average reduce the unit weight of riprap to closer to 150 pounds per cubic foot.

Not all rock has the same density, of course, but it will generally have a specific gravity ranging from 2.65 to 2.7, so this site's figures are probably "close enough". The in-situ density that they cite for limestone is for undisturbed bedrock, and in my experience, I would say that their generalization regarding the proportion of voids in loose rock and the resulting density of 150 pounds per cubic foot is high, and lower densities would not be unusual. However, if we go with 150 pounds per cubic foot, that's 2.0 tons per cubic yard, so even though your sand-density estimate was far too high for that material, it wasn't far off from this figure for loose rock. Using your overall volume and 20 tons per load, that works out to being 25,000 truckloads, which is not an overwhelming difference from the 27,500 loads you calculated, and like I said, I don't dispute that this is basically a huge undertaking!

For those really interested in the picturing this in terms of the number of truckloads, if they move the material somewhere nearby, they'll most likely use off-road trucks, which are loaded "heaping-full" without regard to the weight carried. These trucks carry more than a quad-axle dump truck, but for estimating the actual number of loads I don't know a reasonable conversion. It's "a lot of material" either way!

 

[alec]

I believe that the reason your calculation of the volume of material moved is only half of the minimum figure estimated by Mr. Croyle is because you assumed a square-cut channel, cut "to size", while he probably assumed that the sides would be sloped back to some stable configuration. Still, though I'm not disputing your conclusion that a huge amount of material will be moved, I will nit-pick your methodology a little bit.

Your assumption that loose-dumped sand in a truck box weighs 2.2 tons per cubic yard, which works out to 163 pounds per cubic foot, is awfully generous. I started out planning an explanation why sand's density in a loose-dumped condition would be around 100 to 120 pounds per cubic foot (1.3 to 1.6 tons per cubic yard), depending on the water content, but on seeing that, people would just roll their eyes. So check this out:

http://scruggscompany.com/resources/conversions-calculators/

Standard Weight for Sand = 2,700 LBS per Cubic Yard

This was in pretty close agreement with a handful of other references I found, and to put it in the same units that you used, this equals 1.35 tons per cubic yard.

The material at the Oroville site is mostly rock, so here's a more applicable generalization:

http://petelien.com/21-crushed-faqs/50-how-many-tons-of-riprap-are-in-one-cubic-yard

The conversion for riprap varies by size but generally speaking, a conversion of 2 tons per cubic yard is sufficient. Limestone has an in situ unit weight of approximately 160 pounds per cubic foot but voids on average reduce the unit weight of riprap to closer to 150 pounds per cubic foot.

Not all rock has the same density, of course, but it will generally have a specific gravity ranging from 2.65 to 2.7, so this site's figures are probably "close enough". The in-situ density that they cite for limestone is for undisturbed bedrock, and in my experience, I would say that their generalization regarding the proportion of voids in loose rock and the resulting density of 150 pounds per cubic foot is high, and lower densities would not be unusual. However, if we go with 150 pounds per cubic foot, that's 2.0 tons per cubic yard, so even though your sand-density estimate was far too high for that material, it wasn't far off from this figure for loose rock. Using your overall volume and 20 tons per load, that works out to being 25,000 truckloads, which is not an overwhelming difference from the 27,500 loads you calculated, and like I said, I don't dispute that this is basically a huge undertaking!

For those really interested in the picturing this in terms of the number of truckloads, if they move the material somewhere nearby, they'll most likely use off-road trucks, which are loaded "heaping-full" without regard to the weight carried. These trucks carry more than a quad-axle dump truck, but for estimating the actual number of loads I don't know a reasonable conversion. It's "a lot of material" either way!

Well 24 hours x 60 minutes/hr = 1,440 minutes / day
25,000 truckloads / 1,440 truckloads / day = 17.36 days....if you can run 1 truck per minute ?(any thoughts on trucks/minute????)
Basalt density might be closer to 3.0.

 

[Soilmaker]

I believe that the reason your calculation of the volume of material moved is only half of the minimum figure estimated by Mr. Croyle is because you assumed a square-cut channel, cut "to size", while he probably assumed that the sides would be sloped back to some stable configuration.

Your assumption that loose-dumped sand in a truck box weighs 2.2 tons per cubic yard, which works out to 163 pounds per cubic foot, is awfully generous.

Standard Weight for Sand = 2,700 LBS per Cubic Yard

This was in pretty close agreement with a handful of other references I found, and to put it in the same units that you used, this equals 1.35 tons per cubic yard.

The material at the Oroville site is mostly rock, so here's a more applicable generalization:

http://petelien.com/21-crushed-faqs/50-how-many-tons-of-riprap-are-in-one-cubic-yard

The conversion for riprap varies by size but generally speaking, a conversion of 2 tons per cubic yard is sufficient. Limestone has an in situ unit weight of approximately 160 pounds per cubic foot but voids on average reduce the unit weight of riprap to closer to 150 pounds per cubic foot.

Thanks for catching the incorrect density for sand. My apologies for the mistake. And I agree, rip-rap would be a more appropriate material for comparison.

As for the volume of material moved, I assumed the channel they plan to excavate isn't going to be the entire debris pile, but large enough to do the job of opening up drainage from the bottom of the dam. I wanted to get a handle on the scope of the work, and I think we both agree that it's a lot of material. A large amount of it could certainly be used to repair damage in front of the emergency spillway. Difficult to say what the best approach will be to fixing the concrete spillway and the gaping hole alongside of it. Obviously it will be more cost effective to store as much on site as possible.