Permeability 2

[bank]

I need to design a temporary parking lot using a permeable material on top of a crushed stone bed. I am using underdrains since the subgrade is clay, and there will be virtually no infiltration.

My question is, how do I calculate the velocity or volume of water passing through the stone, assuming the surface material is 100% permeable? I have to prove to the City that the runoff from the site will not increase if we use this design.

 

[ishvaaag]

You seem to be designing something to catch the full rainfall, is it that is to be drained through your drainage in full to rain sewers and that worries the City? Or just that making the surface more impermeable -or by whatever you do- you excess the rainfall allowance in the public sewers for your lot? In that case you will need some regulatory deposit or whatever the city allows for such cases. Sometimes it is like in Alice's: it is difficult to change something and not to change it.

 

[cvg]

you could use Darcy's law to calculate the velocity. This may not be a good idea with a clay subgrade which will expand and contract as it wets and dries.

 

[bank]

Thanks cvg. Darcy's law is exactly what I needed.

Ishvaaag, the City will require storm water detention unless we can prove that the installation will not increase runoff from the site. My client does not want detention. I'll run the numbers and let the chips fall where they may.

 

[fattdad]

Fundamentally, the rate that water flows from a layer of gravel sitting on top of a clay is governed by Darcy's law. The problem is what's the gradient? If you take a 12-in thick layer with parallel subdrains that are 20 ft apart, one instant after zero seconds the gradient will be some big number. After a few minutes, that gradient will be come increasingly small. Even if you approximate a gradient by a grade line that's 12 inches in 10 ft (i.e., one-half the drain spacing), that's only good for a short while as in time that will become 10 inches in 10 ft and then 8 inches in 10 ft, etc.

I have just recently made a MODFLOW model to solve this problem. If you use the actual permeability for 57 stone, you can't believe just how fast this takes place. That said, there is no "simple" Darcy solution in the form of Q=kia that'll get it done. There's some integril to worry over, that's one thing I'm sure about!

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[emmgjld]

At first blush, your problem appears to be slightly to moderately difficult until you run go through the thought process given by fattdad. The problem is actually very complicated.

A method I used to approximate a small parking area at my office, using brick pavers as a surface over sand, over gravel over clayey soil was to provide enough 'immediate storage capacity' by filling the sand & gravel voids with the precipitation. I adjusted the sand & gravel thickness to account for the precipitation volume. After running out a numeric analysis, above method provided a somewhat conservative but reasonable approximation FOR A SINGLE STORM EVENT.

The only reasonable numeric analysis I am familiar with is SEEP/W or a similar program specifically designed for water movement in soil. The very permeable stone will complicate the analysis. I have used SEEP/W to size Storm Water Retention Ponds to achieve partial or complete drainage and to approximate drains behind MSE retaining structures while checking for global stability. Can be very complicated.

 

[Ron]

Bank...consider runoff mitigation based on storage capacity of the voids in the pervious pavement. Assuming a void ratio of 0.60 to 0.70 or so, you can prove the storage capacity. At least, you can match peak runoff for pre-development and post-development conditions, keeping in mind that you only need to attenuate peak runoff, not total runoff.

 

[BigH]

Give Cedergren's book a look - he has good stuff on pavement drainage and passing water . . .