10-foot steel coupling ring? What 10-foot steel coupling ring?

An interesting theory, but it assumes things that even more improbable, though not impossible. If workers did not put the coupling on, what did they do with it? Surely someone would notice a spare ring lying around. And why did the pipe not fail much earlier? Uncoupled pipes leak quickly in virtually any situation, including your own basement, let alone under the particularly high-pressure situation here. Or are you suggesting that none of the couplings anywhere on the pipe have rings--i.e., none were even ordered? It seems unlikely that would go unnoticed now, or that it would avoid similar problems. It does not seem particularly unreasonable that a large piece of metal--or more likely, now several pieces of metal, contrary to your assumption--could be lost under silt following a massive flood of millions of gallons of high-pressure water. This was not simply an intact piece of metal tossed into the river and then sought for; you seem to be vastly underestimating the power of large-scale water flow. You still might be right, but you are presenting unfairly limited possibilities. I would add that $137,000 spent on a search for the broken coupling is not an excessive amount to attempt to find a key piece of evidence in what likely will be forthcoming litigation involving sums far greater than that.