Environment, Coasts, Oceans, & Infrastructure

What a situation! And what lessons that will need to be learned and applied!

For the moment, I want to discuss what I have been able to gather (from general media) about the supposed worst case scenario.  The worst case scenario appears to be a failure of the emergency spillway structure, which is threatened by toe erosion during the prior release. (Given the site features, there is no way that the dam itself could be over-topped.) From what I have seen in media stories, DWR officials appear to be describing this scenario as the release of about the top 30 feet of storage in Lake Oroville.  As best as I can determine, this seems to roughly equate to the height of the emergency spillway structure.  

If that is the basis for the estimates of potential release, I would like to suggest that this significantly underestimates the amount of water that is likely to be lost in that scenario.  If the spillway does fail due to toe erosion, the spillway would initially fail only at that zone; much of the rest of the emergency spillway would remain as water poured through a notch created by the failure zone. As that water surges through the opening, the water would then likely scour the underlying bedrock at the notch where failure occurs.  It seems to me that the situation would be analogous to the Lake Missoula floods in eastern Washington, which formed the channeled scablands.  In the scablands we can see flow surges of this type created scour channels  more than a hundred feet deep.  

Given the differences in flow magnitude we might not see scouring that deep at Oroville.  But I can easily envision where water escaping through a notch in the emergency spillway might deepen the notch an additional 20 feet, perhaps more.  If that is the case, the reservoir would lose considerably more than 30 feet of storage volume.

------------------------------
Stephen Nelson P.E., M.ASCE
Consultant
Coal Creek Envtl Assoc
Bellevue WA
(425) 746-7509
------------------------------

I agree it could be a significant event, except that it looks like the spillways are on solid rock off to the side.  I don't know that for sure, but the erosion and head cut could be limited because of that.  I think a worse case was the Teton Dam failure: 
https://en.wikipedia.org/wiki/Teton_Dam

That blew out right through the middle.  If the cut works over to the built section, I'm not sure how the levee overtopping wouldn't get to Sacramento.

Looks like just a basic strong winter storm coming up this weekend though, so might be just the 20 or 30 ft or so cut released.  Once it starts piping underneath the structures...

Will be interesting.  I wonder how many responses are in the queue here...

------------------------------
Stephen Hemphill P.E., M.ASCE
Semi-Retired
Rio Rancho NM
------------------------------

The crest of the Oroville dam is higher than the  top of the ridge where the spillways are located.  That is a good design feature - it is almost impossible for the dam itself to be overtopped. 

Thus, the worst case scenario is not a complete emptying or the reservoir; rather the worst-case scenario is the loss of reservoir contents above the elevation of the ridge where the spillways are located.  An impetus for my original posting on this topic is that I believe that the volume of water that would be release in case of a catastrophic failure of spillway control structures has been underestimated because bedrock erosion during such an event has not been considered.

------------------------------
Stephen Nelson P.E., M.ASCE
Consultant
Coal Creek Envtl Assoc
Bellevue WA
(425) 746-7509
------------------------------

Original Message:
Sent: 02-15-2017 01:35
From: Stephen Hemphill
Subject: Oroville Dam - worst case scenarios

I agree it could be a significant event, except that it looks like the spillways are on solid rock off to the side.  I don't know that for sure, but the erosion and head cut could be limited because of that.  I think a worse case was the Teton Dam failure: 
https://en.wikipedia.org/wiki/Teton_Dam

That blew out right through the middle.  If the cut works over to the built section, I'm not sure how the levee overtopping wouldn't get to Sacramento.

Looks like just a basic strong winter storm coming up this weekend though, so might be just the 20 or 30 ft or so cut released.  Once it starts piping underneath the structures...

Will be interesting.  I wonder how many responses are in the queue here...

------------------------------
Stephen Hemphill P.E., M.ASCE
Semi-Retired
Rio Rancho NM
------------------------------

In my opinion, the channeled scablands in eastern Washington suggest that we can't simply assume that bedrock will resist erosion during cataclysmic flow events.  The Columbia Basin coulees show the tremendous incisive power into bedrock of a massive flow once channelization in bedrock starts to develop and focus the energy of the flow into a more restricted location.  .

The video of the area below the emergency spillway after the emergency spillway suggests to me that incision was probably already occurring.. 

------------------------------
Stephen Nelson P.E., M.ASCE
Consultant
Coal Creek Envtl Assoc
Bellevue WA
(425) 746-7509
------------------------------

Original Message:
Sent: 02-15-2017 01:35
From: Stephen Hemphill
Subject: Oroville Dam - worst case scenarios

I agree it could be a significant event, except that it looks like the spillways are on solid rock off to the side.  I don't know that for sure, but the erosion and head cut could be limited because of that.  I think a worse case was the Teton Dam failure: 
https://en.wikipedia.org/wiki/Teton_Dam

That blew out right through the middle.  If the cut works over to the built section, I'm not sure how the levee overtopping wouldn't get to Sacramento.

Looks like just a basic strong winter storm coming up this weekend though, so might be just the 20 or 30 ft or so cut released.  Once it starts piping underneath the structures...

Will be interesting.  I wonder how many responses are in the queue here...

------------------------------
Stephen Hemphill P.E., M.ASCE
Semi-Retired
Rio Rancho NM
------------------------------

Interesting thoughts pertaining to eroding  a deep wedge and the relationship to rock to resist the erosion.  Don't I recall that, while the dam is earth fill, was there not a concrete core wall constructed which is keyed into rock at the base and both sides of the dam?  Then construction continued with the rail cars transporting the earth fill to be dumped and compacted on both sides of the core, or am I wrong?

------------------------------
Charles Thompson P.E., M.ASCE
Principal
Fair Oaks CA
(916) 965-5607
------------------------------

Of perhaps greater concern to me is what was the proximate cause of the failure of the primary spillway? I've stood at the bottom of the chute in the past, and it resembles a giant ski jump. Did the spillway slab settle and crack just enough to create a lip that served as a nick point that allowed cavitation and negative pressure to pluck the concrete? Or was some other causative action in play here? Certainly, DWR, who designed the structure, was negligent in their periodic inspections of the spillway to not have detected an incipient crack that probably was developing for some time. There are some important lessons on how to conduct future inspections of long chute spillways that we all need to learn.

------------------------------
Lawrence Magura P.E., D.WRE(Ret.), M.ASCE
Consulting Water Resources Engineer
Lake Oswego OR

------------------------------

There are a couple potential failure mode scenarios for a concrete spillway like Oroville.  They are cavitation as you discussed and stagnation pressure (sometimes referred to as hydraulic jacking).  Both require an offset to occur in the  spillway with the offset being higher on the downstream side of the joint for stagnation pressure.   Both of these potential failure modes can be fairly easily calculated.  I recommend using USBR Hydraulic Laboratory Report DSO-07-07 for the stagnation pressure.   USBR Engineering Monograph No. 42 is a good resource for cavitation issues.  Aeration would prevent the cavitation issue.   As for the rock under the main and limited use spillway, much information is needed to determine the amount of erosion that may potentially occur or not occur. 


------------------------------
Jeffrey McClenathan P.E., M.ASCE
Civil Engineer Hydraulics
Omaha NE
(402) 894-2958
------------------------------

Original Message:
Sent: 02-15-2017 10:24
From: Lawrence Magura
Subject: Oroville Dam - worst case scenarios

Of perhaps greater concern to me is what was the proximate cause of the failure of the primary spillway? I've stood at the bottom of the chute in the past, and it resembles a giant ski jump. Did the spillway slab settle and crack just enough to create a lip that served as a nick point that allowed cavitation and negative pressure to pluck the concrete? Or was some other causative action in play here? Certainly, DWR, who designed the structure, was negligent in their periodic inspections of the spillway to not have detected an incipient crack that probably was developing for some time. There are some important lessons on how to conduct future inspections of long chute spillways that we all need to learn.

------------------------------
Lawrence Magura P.E., D.WRE(Ret.), M.ASCE
Consulting Water Resources Engineer
Lake Oswego OR

------------------------------

Jeffrey brings up a good point - there is a tree very close to the wall at the approximate location of the initial failure.  Could root pressure have lifted the concrete and provided an infiltration point for the lifting pressure to initiate?  Once a slab starts lifting even minutely leading to a piece of the slab popping out erosion will have its way.

Also, look at Google Earth Historical Imagery for 5/2/2013.  There seems to be a flow discontinuity in that area.  Possibly (probably) a chaotic combination of factors.  Definitely a case to be made for observing the flow regime from zero all the way to maximum.  Although not the case here, standing waves at intermediate flows can wreak havoc when horizontal and vertical curves are mixed. 

------------------------------
Stephen Hemphill P.E., M.ASCE
Semi-Retired
Rio Rancho NM
------------------------------

Original Message:
Sent: 02-16-2017 12:25
From: Jeffrey McClenathan
Subject: Oroville Dam - worst case scenarios

There are a couple potential failure mode scenarios for a concrete spillway like Oroville.  They are cavitation as you discussed and stagnation pressure (sometimes referred to as hydraulic jacking).  Both require an offset to occur in the  spillway with the offset being higher on the downstream side of the joint for stagnation pressure.   Both of these potential failure modes can be fairly easily calculated.  I recommend using USBR Hydraulic Laboratory Report DSO-07-07 for the stagnation pressure.   USBR Engineering Monograph No. 42 is a good resource for cavitation issues.  Aeration would prevent the cavitation issue.   As for the rock under the main and limited use spillway, much information is needed to determine the amount of erosion that may potentially occur or not occur. 


------------------------------
Jeffrey McClenathan P.E., M.ASCE
Civil Engineer Hydraulics
Omaha NE
(402) 894-2958
------------------------------

I agree with the original poster that a head-cut significant enough to fail the parapet wall would likely cause a greater loss of reservoir volume than the height of the wall. How much more is open to debate and speculation... I hope we do not find out for certain!

Insofar as the dam itself, it is on the other side of a large mass of bedrock so it is not in any significant danger.

Spillways have the unfortunate characteristic of being somewhat impervious to direct observation of impending failure of the underlying material. The main method of determining condition is to look at the exposed concrete for dislocation at joints or cracks and inspect seep water at drains for turbidity. Unless an indication of an issue is presented the expense of further ground penetrating type testing is not (typically) warranted. Let alone destructive investigation like drilling or excavation.

It will be interesting to see if California dam safety makes more rigorous testing mandatory after this. 


------------------------------
Michael Anderson P.E., M.ASCE
Owner
AEU
Carson City NV
(775)772-2056
------------------------------

There are several discussion points earlier that are looking for cracks and other markers that should have been detected and corrected. 

During the 1960s through the 1980s I was involved for over 3 years of residence and subsequently in the planning, design and construction of Reza Shah Kabir Dam in Iran on the Karun River (now named otherwise). It is sited at Km 490 on the Karun River. It is a bit more than 200 meter high double curvature concrete arch dam. It has a ski jump main spillway on the left abutment. Four generators and turbines for 250,000 KW capacity are located in a powerhouse at the toe of the dam. The concept was that in the future a 500,000 kw powerhouse could be located in the limestone under the spillway. 

This ski jump spillway was of unprecedented height. Sometime after operations commenced a large flood  was passed. About mid point the water was flowing without turbulence against the concrete; plucking of the concrete occurred through its reinforced depth. We of Harza Engineering Company analyzed the situation and constructed a remedy that created cross aeration of the repaired spillway with the result that cavitation no longer occurred has not after more than two decades to my knowledge. As I recall it was proposed to use steel fiber reinforced concrete but was not there for the actual repair.

Working from the premise that Oroville Dam will not fail in this weather cycle, I suggest that the height of the Oroville spillway be analyzed and reconstructed accordingly. Should conditions be similar and there be a request I will undertake to identify former Harza employees who were directly involved, though a number of them have crossed the bar.

priestjohn@...

------------------------------
John Priest P.E., F.ASCE
RETIRED
Littleton CO
(303) 730-2929
------------------------------

Indeed. Ogee type spillways including those with ski-jump style energy dissipators are resistant to the cavitation in a fairly narrow flow range. The concrete itself must resist outside of that range or, as you stated, other measures must be taken to address the concern. Usually step-type.

Oroville's service spillway flowed 150,000 cfs in 1997 with no apparent damage so the 100,00 cfs it was discharging was not a concern for cavitation by design and demonstrated ability. Indeed they have been lowering the reservoir by continuing to utilize the failed service spillway and monitoring the extent and progression of damage (no more so far).

The auxiliary spillway is another matter entirely.

------------------------------
Michael Anderson P.E., M.ASCE
Owner
AEU
Carson City NV
(775)772-2056
------------------------------

Original Message:
Sent: 02-15-2017 15:35
From: John Priest
Subject: Oroville Dam - worst case scenarios

There are several discussion points earlier that are looking for cracks and other markers that should have been detected and corrected. 

During the 1960s through the 1980s I was involved for over 3 years of residence and subsequently in the planning, design and construction of Reza Shah Kabir Dam in Iran on the Karun River (now named otherwise). It is sited at Km 490 on the Karun River. It is a bit more than 200 meter high double curvature concrete arch dam. It has a ski jump main spillway on the left abutment. Four generators and turbines for 250,000 KW capacity are located in a powerhouse at the toe of the dam. The concept was that in the future a 500,000 kw powerhouse could be located in the limestone under the spillway. 

This ski jump spillway was of unprecedented height. Sometime after operations commenced a large flood  was passed. About mid point the water was flowing without turbulence against the concrete; plucking of the concrete occurred through its reinforced depth. We of Harza Engineering Company analyzed the situation and constructed a remedy that created cross aeration of the repaired spillway with the result that cavitation no longer occurred has not after more than two decades to my knowledge. As I recall it was proposed to use steel fiber reinforced concrete but was not there for the actual repair.

Working from the premise that Oroville Dam will not fail in this weather cycle, I suggest that the height of the Oroville spillway be analyzed and reconstructed accordingly. Should conditions be similar and there be a request I will undertake to identify former Harza employees who were directly involved, though a number of them have crossed the bar.

priestjohn@...

------------------------------
John Priest P.E., F.ASCE
RETIRED
Littleton CO
(303) 730-2929
------------------------------

A question to anyone "at large" with respect to Oroville. 

My first impression from looking at the embankments was the large number of substantial trees growing on it.  To me, this is indicative of a fairly poor maintenance procedure.  Given that, I would want to see what procedures have  been used for the spillway concrete maintenance.  Obviously, based on hindsight there also appears to be a deficiency there.

Separately, given the potential loss of life below this dam, I would like to review the logic that resulted in a relatively poorly protected emergency spillway.


------------------------------
Anthony Hurst P.E., F.ASCE
Engineer

------------------------------

Original Message:
Sent: 02-15-2017 20:58
From: Michael Anderson
Subject: Oroville Dam - worst case scenarios

Indeed. Ogee type spillways including those with ski-jump style energy dissipators are resistant to the cavitation in a fairly narrow flow range. The concrete itself must resist outside of that range or, as you stated, other measures must be taken to address the concern. Usually step-type.

Oroville's service spillway flowed 150,000 cfs in 1997 with no apparent damage so the 100,00 cfs it was discharging was not a concern for cavitation by design and demonstrated ability. Indeed they have been lowering the reservoir by continuing to utilize the failed service spillway and monitoring the extent and progression of damage (no more so far).

The auxiliary spillway is another matter entirely.

------------------------------
Michael Anderson P.E., M.ASCE
Owner
AEU
Carson City NV
(775)772-2056
------------------------------

As a hydraulic engineer involved long time in hydraulic modeling of large dam spillways, I would add the following points :

- As we can see from the longitudinal profile section of spillway shown in the hydraulic report  (1965), the final alternative of Oroville spillway doesn't have any aerator. It does make sense, since it was right after damages occured in the Karun_1 dam (1978) and Glenn Canyon Dam ( 1983), that monograph 42 was published and many of large dam designers were added one or more units of aerator structures or components in their high velocity spillways or outlet works. A rough calculation of water surface profile including depth, pressure and velocity determinations, will give us the critical cavitation index along the chute. So talking about damage potential due to cavitation would definitely needs the results of hydraulic modeling. Interestingly, the cavitation damages occured previously weren't occured in a large flow of spillway,
Today, in many of modern spillway designs involving high velocity flows, aerator installation is a must.

------------------------------
Farhad Golzari Rahat Abad P.E., M.ASCE
Water Resources Engineer
Roseville CA
(916) 335-6926
FarhadFarhad
------------------------------

Original Message:
Sent: 02-15-2017 20:58
From: Michael Anderson
Subject: Oroville Dam - worst case scenarios

Indeed. Ogee type spillways including those with ski-jump style energy dissipators are resistant to the cavitation in a fairly narrow flow range. The concrete itself must resist outside of that range or, as you stated, other measures must be taken to address the concern. Usually step-type.

Oroville's service spillway flowed 150,000 cfs in 1997 with no apparent damage so the 100,00 cfs it was discharging was not a concern for cavitation by design and demonstrated ability. Indeed they have been lowering the reservoir by continuing to utilize the failed service spillway and monitoring the extent and progression of damage (no more so far).

The auxiliary spillway is another matter entirely.

------------------------------
Michael Anderson P.E., M.ASCE
Owner
AEU
Carson City NV
(775)772-2056
------------------------------

Your post makes a lot os sense

Anthony Hurst

------------------------------
Anthony Hurst P.E., F.ASCE
Engineer
Hurst Engr Serv
Denver CO
(303) 388-8613
------------------------------

Original Message:
Sent: 02-15-2017 15:35
From: John Priest
Subject: Oroville Dam - worst case scenarios

There are several discussion points earlier that are looking for cracks and other markers that should have been detected and corrected. 

During the 1960s through the 1980s I was involved for over 3 years of residence and subsequently in the planning, design and construction of Reza Shah Kabir Dam in Iran on the Karun River (now named otherwise). It is sited at Km 490 on the Karun River. It is a bit more than 200 meter high double curvature concrete arch dam. It has a ski jump main spillway on the left abutment. Four generators and turbines for 250,000 KW capacity are located in a powerhouse at the toe of the dam. The concept was that in the future a 500,000 kw powerhouse could be located in the limestone under the spillway. 

This ski jump spillway was of unprecedented height. Sometime after operations commenced a large flood  was passed. About mid point the water was flowing without turbulence against the concrete; plucking of the concrete occurred through its reinforced depth. We of Harza Engineering Company analyzed the situation and constructed a remedy that created cross aeration of the repaired spillway with the result that cavitation no longer occurred has not after more than two decades to my knowledge. As I recall it was proposed to use steel fiber reinforced concrete but was not there for the actual repair.

Working from the premise that Oroville Dam will not fail in this weather cycle, I suggest that the height of the Oroville spillway be analyzed and reconstructed accordingly. Should conditions be similar and there be a request I will undertake to identify former Harza employees who were directly involved, though a number of them have crossed the bar.

priestjohn@...

------------------------------
John Priest P.E., F.ASCE
RETIRED
Littleton CO
(303) 730-2929
------------------------------

Effects of high velocities in spillway chutes are often underestimated. The Oroville spillway will experience very high velocities which result in cavitation risks. This could eventually be avoided if the chute surface is entirely smooth without any offsets or local slope changes. However over years every perfect chute surface will show minor or larger deficiences and this might be the cause for cavitation erosion. The Oroville chute erosion damages I have so far seen on the internet do not allow to clearly identify cavitation as only cause for the damage. It could also be due to imperfect joints and stagnation pressures acting on the concrete slabs as uplift forces. Or it could be a mixture of the two.

I know from cases where, due to stupid mortar patches not removed after construction, cavitation erosion occured with not too high velocities. Therefore I would recommend aeration of chutes for all cases where velocities higher than about 25m/s will occur. If planned from the beginning it is a very cheap and entirely safe measure against cavitation. The Oroville chute has not a constant slope over its entire length but upstream of the damage an increase in slope is visible. Tendentially this would result in pressure reductions and would inccrease the cavitation risk.

When looking at the videos I also had the impression that shock waves and free surface flow aeration of the chute where not considered for the design of the side wall heights including a suitable freeboard. However this is somewhat speculative.

When I startet my professional career as an academic hydraulic engineer performing physical model studies we had experts in the board of experts of larger dam projects specialized in dam hydraulics. They were experienced in phenomenas like high velocity flows, shock waves, cavitation, free surface aeration, vortex formatition etc, This is not the case any more for nowadays projects even though the hydraulic effects have potentially higher risk for failures then dam static issues. I am a little bit disappointed that this damage happens in the US. I was myself one of the first ones working on free surface aeration in chutes and on spillway chute aeration using aeration devices. I know the expertise of the US Bureau of Reclamation (e.g. Hank Falvey and Warren Frizzell). I do not see that this expert know-how has in any way influenced the Oroville design.

------------------------------
Peter Rutschmann Aff.M.ASCE
Academic
Professor and Director, Chair and Lab of Hydraulic
and Water Resources Engineering
TU Munich
490049 8964911082
------------------------------

Another problem I can see is deeply weathered reddish saprolite zones EXPOSED beneath the spillway chute, where the initial problem occurred.  There are some significant problems with the Paleozoic age metavolcanics, referred to in the old days as “Amphibolite Schist.”   This material is what the dam and spillways are founded on, where it was most recently exposed by the downcutting of the Feather River.  But, up on the hilltops you have weathering surfaces that have been exposed for as much as 9 million years (a long time in a dynamic terrain like California).  Because it’s been sitting there for such a long, long time the material is deeply weathered, with bands of saprolite.  Saprolites are the almost disintegrated remains of a parent igneous rock, which looks good on first glance, but when you “bit” into it, you find you can excavate it with your hands…  It would appear that the lion’s portion of the first big holes in the floor of the service spillway were founded on saprolite, which was rapidly removed by uplift and erosion.

------------------------------
J. David Rogers Ph.D., P.E., PG, CEG, CHG, F.ASCE
Dept of Geological Engineering
Rolla MO
(573) 341-6198
------------------------------

Original Message:
Sent: 02-17-2017 02:53
From: Peter Rutschmann
Subject: Oroville Dam - worst case scenarios

Effects of high velocities in spillway chutes are often underestimated. The Oroville spillway will experience very high velocities which result in cavitation risks. This could eventually be avoided if the chute surface is entirely smooth without any offsets or local slope changes. However over years every perfect chute surface will show minor or larger deficiences and this might be the cause for cavitation erosion. The Oroville chute erosion damages I have so far seen on the internet do not allow to clearly identify cavitation as only cause for the damage. It could also be due to imperfect joints and stagnation pressures acting on the concrete slabs as uplift forces. Or it could be a mixture of the two.

I know from cases where, due to stupid mortar patches not removed after construction, cavitation erosion occured with not too high velocities. Therefore I would recommend aeration of chutes for all cases where velocities higher than about 25m/s will occur. If planned from the beginning it is a very cheap and entirely safe measure against cavitation. The Oroville chute has not a constant slope over its entire length but upstream of the damage an increase in slope is visible. Tendentially this would result in pressure reductions and would inccrease the cavitation risk.

When looking at the videos I also had the impression that shock waves and free surface flow aeration of the chute where not considered for the design of the side wall heights including a suitable freeboard. However this is somewhat speculative.

When I startet my professional career as an academic hydraulic engineer performing physical model studies we had experts in the board of experts of larger dam projects specialized in dam hydraulics. They were experienced in phenomenas like high velocity flows, shock waves, cavitation, free surface aeration, vortex formatition etc, This is not the case any more for nowadays projects even though the hydraulic effects have potentially higher risk for failures then dam static issues. I am a little bit disappointed that this damage happens in the US. I was myself one of the first ones working on free surface aeration in chutes and on spillway chute aeration using aeration devices. I know the expertise of the US Bureau of Reclamation (e.g. Hank Falvey and Warren Frizzell). I do not see that this expert know-how has in any way influenced the Oroville design.

------------------------------
Peter Rutschmann Aff.M.ASCE
Academic
Professor and Director, Chair and Lab of Hydraulic
and Water Resources Engineering
TU Munich
490049 8964911082
------------------------------

I have not looked over any plans for the dam, spillway or otherwise, but it appears from the videos that there are chute blocks right below the gates creating a large hydraulic jump and lots of aeration. Similarly, this is not a particularly steep spillway. I want to see what the post-event analysis says but it does not look like a concrete erosion issue (cavitation or otherwise) to me.

------------------------------
Michael Anderson P.E., M.ASCE
Owner
AEU
Carson City NV
(775)772-2056
------------------------------

Original Message:
Sent: 02-17-2017 12:43
From: J. David Rogers
Subject: Oroville Dam - worst case scenarios

Another problem I can see is deeply weathered reddish saprolite zones EXPOSED beneath the spillway chute, where the initial problem occurred.  There are some significant problems with the Paleozoic age metavolcanics, referred to in the old days as “Amphibolite Schist.”   This material is what the dam and spillways are founded on, where it was most recently exposed by the downcutting of the Feather River.  But, up on the hilltops you have weathering surfaces that have been exposed for as much as 9 million years (a long time in a dynamic terrain like California).  Because it’s been sitting there for such a long, long time the material is deeply weathered, with bands of saprolite.  Saprolites are the almost disintegrated remains of a parent igneous rock, which looks good on first glance, but when you “bit” into it, you find you can excavate it with your hands…  It would appear that the lion’s portion of the first big holes in the floor of the service spillway were founded on saprolite, which was rapidly removed by uplift and erosion.

------------------------------
J. David Rogers Ph.D., P.E., PG, CEG, CHG, F.ASCE
Dept of Geological Engineering
Rolla MO
(573) 341-6198
------------------------------