Percent Impervious/Runoff Coefficient for prior to sucess of revegetation?

[debgallery]

What percent impervious/runoff coefficient would you use for the Rational Method for an area that has just been re-graded and probably has hydromulching or something similar but no actual vegetation yet? Also has steep slopes. I'm using 5% for undisturbed areas and was thinking 30% was appropriate for interim conditions but a colleague thinks it should be more like 80% (he has yet to produce any backup for this).
Thanks for your opinions.

 

[Yakman256]

I got this link to the PA Erosion and Sediment Pollution Control Manual

http://www.elibrary.dep.state.pa.us/dsweb/Get/Document-87860/363-2134-008.pdf

Page 111 gives runoff coefficients for bare soil between 0.2 & 0.6 depending on the Hydrologic soil group and slope.

Hope this helps

 

[debgallery]

Yes, very helpful. I was able to back into an impervioussness of about 60%
Thanks!

 

[cvg]

unfortunately, hydromulch does little to prevent either runoff or erosion. benching or discing parallel to the contours, or silt fence or waddles will do more although still not much compared to good vegetation.

Another example from Maricopa County for un-disturbed land
The variation is for 2-year vs 100-year storm

rangeland (slopes less than 5%) .40 - .50
hillslopes (slopes greater than 5%) .55 - .69
mountain terrain slopes greater than 10%) .80 - .95

Table 6.4

http://www.fcd.maricopa.gov/Pub/Manuals/downloads/DPSM_Draft_011113.pdf