Gravel Surface
Gravel Surface
(OP)
I'm attempting to do a drainage analysis on a gravel/crushed rock parking lot, using the Rational Method. Could someone tell me what C value would be appropriate for that kind of surface. I've been unable to find it in manuals, etc.





RE: Gravel Surface
I can't find anything as specific as crushed gravel parking lot but you can make an assessment. I would consider a crushed gravel parking lot as permeable (unless it has a geotextile underlay). The British Standard BS EN 752-4 gives a run off coefficient for permeable areas as 0.0-0.3.
I would size the pipes capacity based on the higher flow and then check to ensure it has sufficient fall to be self cleansing at a lower flow.
RE: Gravel Surface
RE: Gravel Surface
Thats a fair point. :)
RE: Gravel Surface
A good Engineering Judgement type question.
RE: Gravel Surface
To begin with, you should check with the local regulatory agency to see if they have developed an acceptable standard. Many agencies have their own standards.
If no standard is available, use a C value of:
For gravel or macadam pavements = .35 to .70
With the higher number being used for more compacted materials and the lower number being used for less compacted materials.
Since the rational method is just a simplified estimate, there is no reason to be more precise.
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RE: Gravel Surface
RE: Gravel Surface
If you pick a number higher than reality - so what? You have excess capacity in your conveyance system, albeit maybe some extra cost (but not likely to be much).
If you pick a number lower than reality, runoff may overwhelm the designed conveyances and cause flooding.
Another consideration, if you are sizing conveyances, is that the owner could come back and pave over this lot in the future without an engineer's involvement to re-analyze drainage, so bumping up pipe sizes to account for the ultimate surface condition may be desirable.
RE: Gravel Surface
RE: Gravel Surface
But if you really need a coefficient, use the SCS Curve Number for that HSG and Rossmiller's conversion. The equation is a monster:
C = (7.2*(10)^-7)*(CN^3)*(RI^0.05)*{[(0.01*CN)^0.6]^-S^0.2}*[(0.001*CN^1.48)^(0.15-0.1*I)]*{[(IMP+1)/2]^0.7},
but you should be able to find the nomograph. You need Return Interval, Slope, Intensity and fraction of IMPervious coverage.
On HSG B, during the 25-yr storm, on 3% slope, with 5 in/hr, with 0.5 impervious, you get a C = 0.54.
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-Steve