Where was the video filmed? I don’t know. SoCal mountains have rain water flow from mountain storms down washes called arroyos. SoCal is basically desert. The arroyos are rock strune, dirt and 99% of the time dry washes. A mountain rainstorm brings water, mud, rocks, boulders and any other debris rumbling down these arroyos that can and do move cars, buildings just about anything in their way. Bridge washouts are not uncommon. As another poster commented, the water undercuts banks and any/all of that debris adds to the mix.
Is it accurate to term this a “lahar flow”?
Because that’s what we were told to call these as kids.