Does flow area reduce by 2/3 when sheet flow recollects to shallow concentrated flow?

[meatymatt]

I have been told that it is a rule of thumb that the sheet flow area is reduced to approximately 1/3 area when it becomes shallow concentrated flow, therefore to sheet flow velocity should be limited to 33% downstream erosive velocity.

So if max allowable velocity of a thick grass is 6fps, I should limit my sheet flow velocity to 2fps since when the sheet flow recollects it will only cover approximately 1/3 the area as shallow concentrated flow.

Agree? Do you use 33%? 40%? 50%? Why? Any links to scholarly articles or texts showing that flow will channelize to 1/3 land downstream?

Thanks

 

[eea]

I've not heard of such an approximation before, but I limit my design velocity to what the ground cover is rated for. 2fps sounds correct for grasses, you should be able to source different treatments and their allowable flow rating thru your local DOT design handbook. I seem to recall the SCS TR-55 worksheets defining the maximum length of sheet flow as 300' before it becomes shallow concentrated, but I'm reading online that has been revised downward to 150'