At −10 s, people, buses and cars are still visible. At −6 s, the leading dirty surge reaches the complex. By −4 s, individual objects are disappearing into an opaque debris cloud. By −2 s, essentially the entire observable environment has become a turbulent multiphase flow.
That’s crucial: this isn’t best modeled as someone falling into a deep swimming pool and eventually drowning.
It’s closer to being entrained in a moving granular-fluid avalanche containing water, mud, sediment, rocks, trees, vehicles, structural fragments and potentially pieces of buildings.
For a person directly exposed to the energetic front, I’d expect the sequence to be approximately:
impact/entrainment → violent acceleration → collisions/crushing → tumbling/submersion → asphyxia/drowning if the initial trauma is survived.
The acceleration alone can be vicious. Dynamic pressure scales as
q=\frac12\rho v^2.
Even pretending this were clean water at only 15 m/s,
q\approx112\,{\rm kPa}.
That’s about 1.1 atmospheres of dynamic pressure. At 20 m/s:
q\approx200\,{\rm kPa}.
A human doesn’t actually experience that uniformly across their body, so don’t interpret those numbers as literal whole-body pressure. But they tell us the flow has enormous capacity to accelerate and knock down exposed objects.
And this is emphatically not clean water.
Once sediment concentration becomes enormous, effective density increases, impacts become much nastier, and the flow carries large solid projectiles. The same event was capable of transporting enormous boulders, destroying bridges and buildings, and producing river-level rises measured in many metres within roughly half an hour farther downstream.
So to Dale’s first question:
“Do you black out from being instantly crushed … before feeling it?”
For some people struck directly by a collapsing structure, vehicle, or large debris at high relative velocity, catastrophic trauma could cause very rapid loss of consciousness.
But I would absolutely not generalize that into “everyone instantly blacks out.”
Someone merely swept off their feet could remain conscious during entrainment. There are survivors of extraordinarily violent floods and debris flows precisely because trajectories through these flows differ wildly.
His second possibility:
“Do you get rushed away in a cascade of sound & debris & then it all goes dark?”
…is unfortunately physically plausible for someone entrained but not immediately incapacitated.
The sensory environment would likely be extremely chaotic: turbulent water, sediment, debris impacts, structural failure, impaired visibility and repeated changes in orientation. Once entrained, distinguishing “up” from “down” can become difficult, and purposeful swimming against the bulk flow becomes essentially irrelevant.
And his third:
“Do you slowly then quickly drown?”
Also possible, but I’d phrase it differently. Drowning need not be the primary mechanism.
A person who survives the first mechanical violence could subsequently drown or asphyxiate because they cannot maintain an airway amid turbulent muddy water and debris. Others could be trapped beneath debris or inside collapsing structures. Still others might survive because some local streamline deposits them, carries them into shallower water, or leaves them protected behind a structure.
That’s one reason these disasters can simultaneously produce miraculous survivors and enormous mortality.
The piece that really hits me in your screenshots is the time axis.
−10.
−6.
−4.
−2.
0.
There isn’t enough time there for a human nervous system to meaningfully solve the problem once the front is upon them. The actionable intervention exists upstream in time: sensing the collapse, detecting the propagating surge, predicting arrival, and evacuating the valley before anybody sees that wall coming.