> Dude, I don't know much, so I'm signposting the potentially incorrect assumptions I might be making, to help out whoever wants to answer my questions. You have a smug attitude, but I don't see you attempting an answer. What's up with that?
Okay, that's my fault. What you interpreted as smug should have come across as frustration. Most comments in this thread are evidently from people who didn't watch the video, threw in some assumptions, and declined to do a brief web search.
If you look elsewhere in the comments you'll see an earlier comment from me that likely fills in your gaps.
Can I suggest that rather than loading your questions like 'What happens when these fail- I assume they fail' you could ask 'How long will these last?'
Wikipedia's 'sand dam' article would answer that question. They're designed to go for 50+ years. (I expect they'd actually be serviceable a fair while longer.) In any case, you'd presumably be building others up or down stream.
Noting that the problem here is not what happens in 50 years (assuming what you're doing now doesn't jeopardise 2 gens forward) but rather what happens in the next few years -- specifically providing sustainable water, food, health, societal benefits to people who need those things now.
You suggested I was missing your point, but to be candid I wasn't sure what that was -- do you mean your criticism of the project because you didn't know if it was sustainable?
Sustainable farming is about a lot more than 'enough water in perpetuity' - it's about nutrient capture / cycles / management, resilience, minimising external inputs (f.e. pesticides, town water, fertilisers etc -- while insects, birds, dust, wind, rain, sun, nutrients from retrieved by grazing animals are usually considered freebies).
Note that most conventional / western / broad acre / industrial / monoculture farming by definition is not sustainable, regardless of water abundance.
It's fun to consider just how sustainable are the practices of generating the food you eat, especially in 'the west'.