Very tragic video and an equally sad situation in Yosemite.
Even though I live in the PNW now (water, water everywhere), the only time I was in a flash flood situation was at Slickrock Creek near Fontana Dam in Tennessee and North Carolina in the middle of summer. It was eye-opening to me given the location and time of year. The previous week of summer storms had lessened the temperature a bit (good) but added a lot of water.
We were sitting and eating on a small island in the middle of the creek, just downstream from a waterfall. Someone asked, "Where did that big rock go next to the waterfall?" By the time we realized what was happening and grabbed our stuff, we were wading in waist deep water towards the hillside and trail.