TR-55 - Runoff Curve Number 1

[PMAB32]

Hey guys,

Question for you. when I use rational method, I'm used to just looking at the drainage area and visually determining what percentage of the land is residential vs. forested, vs. grass and coming up with a composite C value. But when we do RCN number sometimes I've seen where people use the land use maps for that specific county and assign RCN values based on the percentage of that map that is residential or open space, etc. I believe I was taught that way too but then it was recently questioned by a coworker who was from a different region and she brought up a good point - once you have the drainage area why wouldn't you treat it the same way you do rational and just look at the drainage area and come up with and RCN value based on what you see at the mixed areas? How do you guys approach getting your RCN value? I guess another defense is that land use maps will show ultimate buildout so you probably need to be using that to be more conservative, right? Looking at what is now does not tell you what it could be.

 

[Twinkie]

CN values are dependent on the HSG. Determining a CN value based solely on a regional land use map would not produce accurate results. I typically use detailed survey information combined with NRCS data to calculate a CN.

Additionally, if you are simply calling all "open space" grass, you are probably greatly overlooking meadows, agriculture, pasture, etc ...

 

[gbam]

i have some questions, Why not:
1. measure the land use areas?
2. measure the soil types
3. measure areas based on the zoning for current and future development
4. measure vegetation density

We take the time to measure the area, flow path, slope and centroid path, why not measure the other parameters. This is the way I was taught a long time ago, delineate -> measure parameters -> compute R/O Coeff or CN. Now with the use of CADD and GIS it is fairly quick to do so.

 

[PMAB32]

I agree with you gbam, I think just making sure you add in the factor of "could this be developed in the future" to the sections that might be grass now if the area's land use maps suggest anything with more impervious area than what you see to date.

 

[LincolnPE]

When thinking about weighting CN for an overall area, you can use a weighted-area CN if the overall area is homogenous in CN. If the individual areas differ significantly in CN, then you should use the Weighted-Volume approach.