HEC-RAS and Manning's Roughness for large rock 1

[jleiben]

Does anyone have any experience with using manning's values in HEC-RAS for a streambank stabilization as part of a flood study? We will need to use rock with an avg. Diameter of approximately 3' or more for very large flows. I can't seem to really nail down any definitive values. And most of the information i've found is for channel protection more so. Also, the depth varies from about 1.5' at OHW to 27' at the 100-yr and i'm trying to determine the best way to model something such as this. Any assistance would be great. Not that it matters, but i'm in VA.
Thanks in advance.

 

[cvg]

try chapter 5

U.S. Army Corps of Engineers
Engineer Manual EM 1110-2-1601
Engineering and Design HYDRAULIC DESIGN OF FLOOD CONTROL CHANNELS