Ol' G.K. Gilbert once said that there are three events that all geologists have to experience in their lives: a glacier, a volcanic eruption, and an earthquake. Its a good list, don't get me wrong, but I think I'd have to add "Debris Flow" into the mix (maybe between volcano and earthquake, in terms of difficulty).
Brian, in a comment on the previous post, makes the rad suggestion of a Fan Cam, set up in some debris flow prone region; heck, a few checks every month, for a decade or so, and I reckon we'd catch a few good ones. Until then, however, I have videos of debris flows and hyperconcentrated flows that I have lovingly harvested from the bounty that is YouTube.
The videos below are all from a single USGS film on Debris Flow Dynamics, and are narrated by John Costa, who was one of the big wheels in the USGS debris flow hazard group. The footage is a little iffy in some spots, and there are some clearly turbulently supported hyperconcentrated flows (which are pretty different from cohesive debris flows) thrown into the mix, but all in all, its pretty slick. There is also a more recent debris flow video put out by the USGS that has some awesome footage from China; if I can find it, I'll post it up later.
Now, onto the films!
PART THE FIRST
PART THE SECOND
and, PART THE THIRD