I'm going with the 6.40" in the gauge here, I saw somewhere that someone measured a rate of 12" per hour at one point.
Anyway, when we created this rock area out back, in 2014, we put a 6" pipe in to handle non-heavy rainfalls, and figured that the "3 year flood" would just overflow and run across the grass. We contoured the grass so the water would be spread out over a 40 to 50' wide area. It's worked pretty well overall, except we seem to get 2 or 3 "3 year floods" a year now. Up until today we had never had a single rock wash out onto the grass, at the most some debris from the woods (like Tuesday). This rain today was so far beyond anything we anticipated, I don't think we could have designed for it. First pic, my son was having some fun in it, lol. 2nd pic is during the rain. 3rd pic is the aftermath. Those are river rocks out on the grass, some of them are 8 or 9" long. Guess I won't be mowing there for a few days.