Comment by _52jq | Hacker News Reader

Comment by _52jq | Hacker News Reader

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).

> 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).

> 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.

That number is nowhere near accurate as we learned during the South Carolina floods back in October. The state estimates there are between 10,000 and 20,000 unregulated dams in South Carolina alone: http://www.thestate.com/news/local/article41315256.html

Those are the dams that pose the greatest threat. You can't plan for something you don't know about and the threat posed by such an extensive system of "invisible" dams is immense. A small earthen dam on a farm may not pose much of a threat by itself but its failure can cause a cascade of failures further downstream if it breaks when those dams are already at their limit.

So, in South Carolina, The State reports that the state estimates that there are between 10,000 and 20,000 dams in the state which are not regulated by the state.

You'd think that a publication called 'The State' would be a lot more careful about throwing that term around, but within that article, it talks about "the state" meaning South Carolina as a geographical area, "the state" meaning the SC government as a regulatory authority, "the state" meaning government actors in general, and "the state" as a newspaper (though they do refer to themselves as "The State newspaper". Presumably they aren't claiming to be the official state newspaper, just a newspaper called 'The State').

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