Gravel Surface 2

[bank]

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.

 

[Ussuri]

There are effectively two ways to determine your impermeable area when using the rational method. The first is based on surface type, grass, roof, lawns etc; the second is based on type of area, city centre, industrial, residential, suburbs etc. Which one you use generally depends on how big an area you are looking at.

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.

 

[LCruiser]

There would be a phenomenally wide range of permeability of a road base. Effective road base is based (pun not intended) on interlocking particles. Talking permeability of a road base is talking apples and oranges. It could be quite tight, or relatively uniformly graded and therefore extremely permeable. The only way to know is to do a permeability test on the mix - not just a single sample, but a representative sample.

 

[Ussuri]

LCruiser

Thats a fair point.

 

[TerraSouth]

I don't think its the gravel your concerned about as much as the base material beneath the gravel. The base material is going to be compacted before the gravel is applied. The rainfall is going to penetrate the gravel, hit the base layer and begin to runoff. In my opinion your "C" value is going to be on the high side, say around 0.8 to 0.9 or so. You also have to take into account the slope of the parking lot and other site conditions effecting the runoff. If your parking lot is relatively flat, your "C" value is going to be lower, and on a steeper lot the "C" value will be on the high side.

A good Engineering Judgement type question.

 

[bimr]

The rational method is recognized to have a number of weaknesses in light of modern knowledge of runoff mechanics. It is a great oversimplification of a complicated process. However, the method is considered sufficiently accurate for runoff estimation in the design of relatively inexpensive structures where the consequences of failure are limited.

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.

http://pasture.ecn.purdue.edu/~abe526/resources1/runoffdoc/runoff.html

 

[cvg]

even though the subbase is compacted beneath the gravel layer and would have a high runoff coefficient, you should also take into account the thickness and permeability or void ratio of the gravel layer. For instance, if the gravel layer is 6 inches thick and relatively permeable with a high percentage of voids - it may be able to accept and retain a significant amount of water. With a 20% void ratio, it could capture and detain approximately an inch of rainfall.

 

[bltseattle]

Because of the large number of factors, many listed above, you need to apply engineering judgement to pick a value in the aforementioned ranges, say 0.25 - 0.80 approx. Design for the future conditions after years of the expected maintenance. Factors to consider are: size of the storm analyzed, porosity and depth of gravel (e.g. storage potential relative to storm size), potential for ponding (e.g. degree of maintenance in the parking lot), slope of the parking lot, permeability of underlying base material, potential for sediment sealing/clogging the gravel surface over time, etc.

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.

 

[saruman26]

It is very common for municipalities and state DEP's to require that gravel surfaces are treated the same as a paved surface for storm water runoff computations. The intention is to design for the worst-case scenario and ignore the realm of possibilities between 0.20 and 0.90 that are open to individual interpretation and can vary with time, use, and environmental conditions.

 

[LHA]

I always count gravel as impervious, and the Twp. where I consult as Twp Engineer requires it, for the reason given by Saruman.

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.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve