If you are looking at that pic tom posted, the length and width of it is about 35’X20’ based on people and object height. For math-ez, let’s just say 50X20, or 1000 square feet. Now picture that surface area moving towards you as a solid wall of water at 1 foot per second. That’s 1000 cubic feet per second of water.
Pulling what I think is a reasonable number off the top of the dome of 20 feet per second (bit over 13 mph), that area would produce a flow of 20,000 cubic feet per second on a not seriously inclined slope.
Hopefully that gives a bit of perspective when you hear numbers like the 20,000-50,000 cfs going over the thrashed spillway, or the potential to have as much as 200,000 cfs coming out of the damaged overflow spillway come next rain (along with a betting pool on how long those 4,000 pound plastic bags of rocks will last).