Riprap channel design using Manning's equation is pretty straight forward:
n=d^(1/6) / [21.6 log(d/d50) + 14], where d = flow depth (NOT channel depth) in feet and d50 = average rock size, as follows:
NSA size d50, ft
R1 0.0625
R2 0.125
R3 0.25
R4 0.5
R5 0.75
R6 1
R7 1.25
R8 1.5
You will have to run a few iterations to get n, but 0.07-0.10 are good starting points.
Size for velocity (under 10% long. slope, s) or shear (over 10%):
NSA size v, fps shear, lb/ft = Shear stress = d x s x density of liquid
R1 2.5 0.25
R2 4.5 0.5
R3 6.5 1
R4 9 2
R5 11.5 3
R6 13 4
R7 14.5 5
R8 14.5 8
It should be noted that R1 and R2 are generally not accepted in central PA, because they are not really available (small graded stone is usually classed by AASHTO or a DOT system). Also, for all but the largest of projects, riprap is not actually graded, so spec on the high side (higher NSA #). Finally, when costing out riprap versus synthetic liners, add a significant amount for machine placement, as the per ton or cf price is just what it will cost your client to have a big pile of rocks dumped in her driveway, not what is will cost labourers and machines working all week to place the stuff. In reality, a reno mat is easier to place than R7 or R8. After adding in the aesthetics of the final product (grass versus rocks), I usually go with a synthetic liner.
