Never mess with water, water always wins

Short answer: water always wins. Long answer: flash floods can surge and move thousands of liters of water in minutes. All it makes is half a meter of water to take your feet out from under you, and you could be washed downstream or hit your head. It could be nothing and you'd be fine. Or that water could continue for days, ending up pulling all that loose sand along with it, including 10m wider on each side. It's hard to tell what's up stream.