
How long are dams like Hoover Dam engineered to last? (2006) - monort
http://www.straightdope.com/columns/read/2666/how-long-are-dams-like-hoover-dam-engineered-to-last
======
yourapostasy
For those who wanted the answer to the title in these comments, paraphrased it
was: with proper maintenance, indefinitely. The "with proper maintenance"
phrase should be pointed out to management next time they think something we
build should last forever; even these remarkable structures that appear
formidably unchanging undergo continuous, hands-on, expert maintenance.

There is a tangentially-related Straight Dope thread [1] asking "Is the Hoover
Dam concrete still curing?", because if you take the tour (highly recommended
for hackers with an affinity for industrial- and megastructure-scale
engineering), you will be told "the concrete is still curing!" by the guides.

This is apparently not technically precise; someone from the concrete industry
please correct me if I'm wrong here. I gathered from a civil engineering page
that Hoover Dam's concrete is considered cured, but it is still undergoing the
hydration process as long as water is present in the concrete. [2] I think the
Hoover Dam guides can be cut some slack for colloquially using the terminology
"curing" in light of this technical distinction that most laypeople won't
understand.

[1]
[http://boards.straightdope.com/sdmb/showthread.php?t=258217](http://boards.straightdope.com/sdmb/showthread.php?t=258217)

[2]
[http://www.engr.psu.edu/ce/courses/ce584/concrete/library/co...](http://www.engr.psu.edu/ce/courses/ce584/concrete/library/construction/curing/curing.html)

~~~
cmsmith
As you probably gathered from your references, concrete curing is a continuous
process - every time a stray water molecule binds with some cement the
concrete will get a little bit stronger. We use 28 days to call the concrete
'cured' because it's a reasonable time on construction schedules (why it's in
an even number of weeks) and hits a nice fat part of the exponential strength
curve. It's a bit like asking when a pile of plutonium is done decaying. What
will happen is that at some point, the strength gains are so marginal that
various degradation processes will cause the concrete to start losing overall
strength.

But - the idea that the concrete is curing and will be finished in 50 years is
fairly goofy. No one is doing high-temperature long-duration concrete curing
research, but the idea that you can extrapolate the laws of heat transfer to
figure out how long some bubble of superheated wet concrete in the middle of
the dam will last isn't very reasonable.

~~~
ethagknight
Fun fact on the topic of curing, the Hoover Dam was built with chilled water
coolant lines running throughout the bulk of the dam in order to dissipate
heat through the curing process. Without cooling, the sheer mass of the dam
and concrete's exothermic curing process would have basically caused the dam
to self destruct from expansion/contraction.

[http://www.usbr.gov/lc/hooverdam/history/essays/concrete.htm...](http://www.usbr.gov/lc/hooverdam/history/essays/concrete.html)

------
Rexxar

        With a small dam the water can sometimes be drained,
        allowing repairs to be done safely. But imagine trying to 
        drain the Hoover Dam to repair cracks at the bottom--it 
        can't be done
    

I don't understand why they think it's a problem to drain the dam. It's done
regularly to check and repair them. (Every 30/40 years, at least in France).

Recent video of this in France :
[https://www.youtube.com/watch?v=JflFg5un5zg](https://www.youtube.com/watch?v=JflFg5un5zg)

It is this dam, this is not a small one :
[https://fr.wikipedia.org/wiki/Barrage_de_Sarrans](https://fr.wikipedia.org/wiki/Barrage_de_Sarrans)

~~~
psuter
According to their respective Wikipedia articles, the lake behind the Hoover
Dam has a volume 108x larger than the lake at Sarrans. [1] It's unclear
whether the same techniques apply.

[1]
[http://www.wolframalpha.com/input/?i=32.22+km^3+%2F+296*10^6...](http://www.wolframalpha.com/input/?i=32.22+km^3+%2F+296*10^6+m^3)

~~~
Rexxar
IMHO, the main parameter seems to be how many time is needed to fill/empty the
dam and to apply a reasonable ratio to ensure the dam is working 95% of the
time.

From Wikipedia on Hoover Dam: "Filling of Lake Mead began February 1, 1935,
... ... In the latter half of 1936, water levels in Lake Mead were high enough
to permit power generation ..."

To empty Hoover Dam once every 60/80 years doesn't sound absurd to me.

~~~
hyperbovine
Give us a few years, we'll drain it for you.

\--Love, the Western US.

------
pp19dd
By last count (few years old), the number of dams was estimated at ~84,000
(not 76,000). Close, but that numerical discrepancy should point out one
thing: it's an estimate. USACE maintains an authoritative database of dams in
USA (and also 32 in Puerto Rico and 1 in Guam) and the numbers are always a
moving target. Believe at this point they're working with pattern matching and
satellite imagery to fine-tune the database. USACE is currently more concerned
with inspections near urban populations than cataloging.

The write-up is a bit vague and misses one major point: not all of these dams
in the U.S. are concrete. Not even close. Here's a construction type
breakdown:

    
    
      70,278 Earth
       9,031 (Unknown)
       1,446 RCC
       1,215 Gravity
         724 Concrete
         455 Other
         420 Rockfill
         201 Masonry
         136 Buttress
         103 Arch
          58 Stone
          51 Timber Crib
          16 Multi-Arch
    

Only 847 of those dams are tailings dams, used for industrial processing. More
than 0, but less than 84,000. Of those, none are listed as concrete (730
Earth, 61 Unknown, 43 Other, 13 Rockfill).

~~~
uxp
> The write-up is a bit vague and misses one major point: not all of these
> dams in the U.S. are concrete

The write-up is in response to a question asking how long dams like the Hoover
Dam, which is a large concrete dam, are designed to last. It wasn't suggesting
that all dams are concrete.

The single sentence: "Indefinitely with regular maintenance" is a rather short
article length, so Straight Dope decided to educate their readers that the
large dams, which make great action-packed movies when they fail
catastrophically, are the very least of our worries since earthen or other
natural material based man-made dams, which generally have significantly less
water capacity, are in fact at more risk of failure and potentially even more
damage (like the cascading failures in SC this past year, or the Banqiao Dam
incident).

~~~
pp19dd
> The write-up is in response to a question asking how long dams like the
> Hoover Dam, which is a large concrete dam, are designed to last. It wasn't
> suggesting that all dams are concrete.

But the author mixes concrete dams with tailings dams, of which none
(according to USACE) are concrete. That's what I meant by vague: starts with a
large large number talking about US dams with, talks about concrete, mentions
danger of tailings and then talks about foreign dams. The data is chosen for a
narrative and ends up implying a lot without clear references.

------
TheCapn
Anecdotal story: This summer I was out at a Dam spillway installing some of my
systems and got talking with the operator out there. He was commenting about
the engineering of the facility and how impressive it was for the age it was
built (50+ years ago) in comparison to the modern dam my government was
currently building and all the lawsuits and issues its has even before
opening. To him, at least, and I'd agree from his comments, business got too
involved with engineering. They started running too many cost estimates and
too many lowest-bidder contracts instead of building things for a century plus
lifetime.

Shame to see that. I hope that our engineers continue to fight for proper
surveys and designs rather than a race to the bottom.

~~~
alanctgardner3
For reference, ~100 people died in terrible conditions building the Hoover
Dam. It was a tremendously unsafe worksite. The environmental impact of the
project was barely considered.

I think all things considered we've made good progress.

~~~
jacquesm
On a relative scale the number of accidents building the Hoover dam was high
but not spectacularly high when compared to the number of people dying in
high-rise construction and road building.

------
jacquesm
The Hoover Dam hasn't even properly set yet!

Curing is an exponentially decreasing process, there will always be a little
bit more concrete that still needs to cure, much like radioactivity half-life.

The Hoover Dam is quite an interesting piece of civil engineering, it broke
ground in many ways other than the physical one.

Tons of interesting stuff here:

[http://www.usbr.gov/lc/hooverdam/history/essays/concrete.htm...](http://www.usbr.gov/lc/hooverdam/history/essays/concrete.html)

Concrete, while on the surface very boring is actually a super interesting
engineering material.

Another interesting tidbit: for the longest time the dam was actively cooled
to whisk away the heat from the curing concrete (concrete curing is an
exothermic process).

------
cc438
The threat posed by small dams and their short designed life expectancy was
demonstrated in dramatic fashion earlier this year. The historic flooding that
impacted South Carolina's midlands earlier this year was driven by a cascade
of dam failures.

Everyone saw the pictures of Columbia, SC but that flooding was mostly due to
"normal" causes. The city's drainage system was overwhelmed by rainfall that
exceeded historic levels by several orders of magnitude (higher than even a
1,000 year storm) and the Broad river consequently overflowing its banks.
However, that wasn't the only place impacted by the storm. South Carolina has
an enormous number of small earthen dams, built on private land with the land
owner responsible for maintenance.

Obviously, these dams were not built to handle such a storm but even if they
were, the lack of oversight led to a lack of maintenance, worsening their
chances of surviving the initial floods. As upstream dams broke, downstream
dams were subjected to massive surges, causing a cascading series of failures,
greatly increasing the amount of damage caused by the storm.

There are 10,000 to 20,000 unregulated small dams in the state according to
the state. They pose an enormous risk as the flooding clearly demonstrated but
SC isn't alone in sharing this burden.

~~~
cjslep
Accounting for failure frequency due to natural disasters occurs at the design
(initial or retrofit) stage, not the maintenance stage. Should every mom-and-
pop dam adhere to the once-in-ten-thousand-year-rule (eg: nuclear powerplants)
or once-every-hundred-years (eg: steel framed commercial construction)? Cost
effectiveness is also a huge concern.

Note that maintenance doesn't increase the expected failure frequency, it
maintains it at the design spec. And I'm willing to bet that even with proper
maintenance those small dams in SC would not have survived; they were way
outside their designed specs.

------
r0m4n0
I drive past Folsom dam (1952) every day and have watched the incremental
maintenance as well as a few large scale projects progress first hand. One of
the most amazing things to witness was the construction of the spillway they
have been building around the outer perimeter. They essentially are creating a
second dam around the main dam to release water from a lower point due to a
scare many years ago. Months of sorting dirt and rock at some kind of large
facility nearby and a good year of constant trucks moving the dirt down the
road. Even the asphalt on the road began to sink from the constant pressure of
the heavy loads.

A few of the old spillway gate failures over the years are outlined here:

[https://en.m.wikipedia.org/wiki/Folsom_Dam](https://en.m.wikipedia.org/wiki/Folsom_Dam)

------
nroets
Currently, the Kariba dam is the one most likely to fail:

[http://citizen.co.za/820414/kariba-dam-a-ticking-time-
bomb/](http://citizen.co.za/820414/kariba-dam-a-ticking-time-bomb/)

------
sizzzzlerz
How long a dam lasts and how long the reservoir behind it is usable are two
different things. A well constructed dam may last 100+ years but if the water
flowing into the lake carries heavy sediment loads, the lake will fill up with
mud before then if not periodically drained and the muck removed. This is
definitely an issue with Three Gorges as well as Lake Powell/Glen Canyon Dam
on the Colorado River. In fact, one of the reasons the latter dam was
constructed was to act as a catch basin so the downstream Lake Mead behind
Hoover dam wouldn't fill so quickly.

~~~
WildUtah
Sediment loads aren't really a problem. The sediment drops out as soon as the
water goes slack so it all accumulates at the far upstream section of the
reservoir and raises the river level rather than filling in much capacity.

The Grand Canyon is filling in with Hoover Dam sediment while the reservoir
remains unfilled. In fact, the Grand Canyon is now a mud lagoon starting above
Separation Canyon and the water below Pearce Ferry has very little sediment.
The upstream sediment slackwater continues to migrate upcanyon, not
downcanyon.

On Glen Canyon, the sediment accumulated 40 meters deep over Hite but did not
slip downcanyon. Instead, Cataract Canyon continues to fill in upstream. Moab
will be covered in Glen Canyon's reservoir before any serious capacity is lost
because sedimentation moves upriver, just as in any ocean delta.

The same effect has stopped the main flow in the San Juan and covered all the
rapids up to Slickhorn and even above.

\---

Above I said, "sediment loads aren't really a problem." I mean they aren't a
problem for dam operators. They're bad for the environment, of course.

But that should be obvious. For all the selling of hydroelectric as green and
sustainable, we should know that it's the most destructive and damaging of all
electric generation technologies. Even a failed nuclear plant in full meltdown
isn't as destructive and harmful as a properly operating hydroelectric dam.
Hoover dam makes Tchernobyl look good.

------
snowwrestler
Worth a read:

[https://en.wikipedia.org/wiki/Risks_to_the_Glen_Canyon_Dam](https://en.wikipedia.org/wiki/Risks_to_the_Glen_Canyon_Dam)

This gives a hint as to why huge dams cannot be drained for maintenance: there
is so much water behind them that it would take too long to drain them via the
spillways. In fact, the spillways would probably be irreparably damaged by
experiencing full flow for that long.

------
melling
"Engineers, about 30 percent of the more than 76,000 dams in the United States
are older than 50 years--and by 2020, that number will increase to more than
80 percent. That's a lot of old dams, some of which hold back not just water
but toxic sediments from early industrial operations."

~~~
ZeroGravitas
It wasn't clear to me why having toxic sediment sitting behind the dam was
that much better than it moving downstream?

~~~
Someone
Do you want it 100 km away or in your garden and living room?

~~~
Cerium
100 km away is another family's garden and living room.

~~~
bronson
They're living at the bottom of Lake Mead?

(cue the Yorkshiremen sketch
[https://www.youtube.com/watch?v=Xe1a1wHxTyo](https://www.youtube.com/watch?v=Xe1a1wHxTyo)
)

------
peter303
Some Roman concrete like Pantheon Dome is still good after 1900 years. The
Dome was poured in 126 C.E.

------
jonawesomegreen
I remember hearing somewhere, perhaps on a tour of the Hoover Dam that the
concrete is very good for the time because although the ideal chemical
properties of concrete were not well understood at the time, the local
aggregates used were relatively pH neutral which has contributed to the dams
longevity. I'm trying to find a source for this though and struggling. Has
anyone else heard this about the dam?

~~~
jjp
Don't know about pH but very interesting read on history primarily from a
civil engineering perspective -
[http://web.mst.edu/~rogersda/hoover_dam/Civil_Eng-
Nov-2010-H...](http://web.mst.edu/~rogersda/hoover_dam/Civil_Eng-
Nov-2010-Hoover_Dam.pdf)

