Rational Method "C"

[bigriver]

I feel as though this is a silly question, but one that has alluded me until a recent review.

I submitted plans/calcs for a residential subdivision that is being phased in construction. Along a primary roadway of which residential lots are planned, but not platted, I have calculated ditch capacities using a VA standard "C" value for the expected build-out within the drainage shed. In my calculations I did not designate a separate impervious area, only "C" based on land use.

Reviewer is asking why no impervious areas are designated for the roadway that is being constructed.

Question - Does a land use "C" include impervious areas for access roadways? Or only for platted lots leaving right-of-ways assigned for a different "C"?

 

[bpattengale]

The land use "C" usually include the associated access roads. If you want to check that the "C" value you used is valid calc out the composite "C" value for an average lot. Assume a maximum impervious for the lot based on the zoning code and then include half the roadway width along the lot frontage.

 

[bigriver]

Thanks for the reply.

I have went through the zoning / build area / driveways routine prior to this and it seemed reasonable to me that the "C" were composite from CL of roadway to lot limits. It seems when I check existing lots from aerial photo's I get less than the composite I am using.

Guess I will make my case and see what happens.