comparing runnoff on different road surfaces

[templeton]

My question is "would the runnoff produced from a paved (asphalt) road surface be similar to the runnoff from a dirt road surface which has been overlaid with compacted 'crusher waste'. In this case the dirt road with the crusher waste appeared to be impervious and gthe underlying dirt has a high clay content. In this case the roadway surfaces have the same angle and orientation.

 

[Ron]

Probably no more than a 5% difference. One difference in the unpaved surface is that surface flow will be slower, thus increasing (however slightly) the time of concentration.

 

[templeton]

thanks Ron,

is there a formula or table which I could reference or is this dependent on factors which need to be analysed before calculating etc. Also it is generally well know that dirt (with the compacted crusher waste on top " would be similar in runnoff etc.

Templeton

 

[Ron]

The USDA Soil Conservation Service has lots of info on this. Look for a table of runoff coefficients/imperviousness coefficients. You'll find them for various soil types, gravel, asphalt, concrete, etc.

 

[cvg]

templeton - suggest you ask your local agency who will review your grading and drainage plan for the C-values they will approve. It could be a lot more than 5%. Approved values around here vary depending on the return interval for the design storm.

10-year and 100-year storms
gravel road 0.60 - 0.88
asphalt 0.75 - 0.95 (rational method)

Ron - By the way, it is no longer the SCS, they changed the name several years ago from SCS to NRCS.

 

[Ron]

cvg...yes, I knew they had changed their name, but couldn't remember what it was...thanks.

 

[beej67]

(these are CURVE NUMBERS not RATIONAL COEFFICIENTS, don't confuse the two methods)

From NRCS TR-55:

Paved: (excluding r/w)
98 (regardless of soil classification)

Paved: (including r/w, presumes open ditches)
A: 83
B: 89
C: 92
D: 93

Gravel: (includes r/w)
A: 76
B: 85
C: 89
D: 91

Dirt: (includes r/w)
A: 72
B: 82
C: 87
D: 89


Your soil type is probably B or C, depending. There's a chart in TR-55 to look it up.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[templeton]

thanks,

the soil is considered to be a clay (HbC)with runnoff being 'slow to medium' so I'm assuming that is a 'c'

It would appear the difference is no more than 5%

I suspect it may be less because the original road surface did seem impermiable because of the highly compacted 'crusher waste' overlay. The fines in that material seemed to form a cement like barrier which would contrast to a straight gravel base.

Thanks very much for pointing me to the 'tables'.

Templeton