Flow Capacity in Gravel Underdrain 2

[DMcGrath]

How would one go about computing the flow capacity in a gravel underdrain (Dewatering groundwater)? For an example, a constructed underdrain is 3 feet wide, 2 feet deep, has a bed slope of 1% or so, and is filled with gravel (AASHTO 57 or something like it, then topped with soil. Soil clogging aside (filter sand or geotextile considerations are separate issue), would this be related to the permeability of the gravel or some sort of quasi-open channel flow?

 

[bimr]

You can probably estimate it using the procedures used for well packing.

Q= KIA

Where K = hydraulic conductivity ~ 17,000 gpd/ft squared (from well pack materials)

I = hydraulic gradient = (Water Head)/(Distance water will move through gravel bed)

A= filter bed flow area in feet squared

 

[fattdad]

Assuming that you are just looking at lateral flow and you are asking how much flow can move laterally through a fully-saturated prism of open-graded aggregate that's 3-ft wide and 2 ft deep on a 1 percent incline, here's an approach to solving the problem:

Estimate the hydraulic conductivity - let's use 50 cm/sec (100 ft/min).

Using Q=kiA you'd get Q=100*0.01*6= 6 cfm OR 45 gpm.

Some variables -

A.) will you really have fully saturated conditions to the top of the gravel prism? If not proportion the answer accordingly.

B.) can you have pressure head driving the flow? If so, then the hydraulic gradient will exceed 0.01 and your flow will increase proportionatly.

C.) what is the real hydraulic conductivity of 57 stone? My estimate is pretty close, but you can easily do a bench test and see for yourself. I'd take a concrete test cylinder, drill a few holes in it and do a constant head test in the lab sink. Crude? Sure, but it'll be close. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

Why are you doing this? Let's say you wish to drain a high ground water situation in a road cut, or similar. My general experience with drains seems to indicate it takes one heck of a porous situation to cause even a 4 inch line to flow full. They seldom carry even a half full flow. Generally I've never seen a ditch or similar feature carry more that that from the usual site.

I once saw a metal 6 inch perforated sub-drain pipe flowing half full, but in a very unusual situation from a very porous gravel stratum.

So, the size of the drain you are discussing probably will only have an inch or so flow in the bottom for the average situation, controlled only by the stuff from which the water comes.

 

[DMcGrath]

We have a site with a known spring source that we are going to build on later. The drain will be to make sure the groundwater has a place to go and does not come up underneath our future construction efforts. The size I gave was illustrative only, I sure hope it won't have to be that big! We will estimate the anticipated flow, then we can size the drain from there.

The easy way would be to put a perforated pipe in the gravel pack and then only have to compute the flow capacity in the pipe. For some reason, however, the powers that be have scratched the pipe portion....

Thanks to all for the insight.

 

[bimr]

Not sure what units fattdad has used. This is how I would calculate it:

Q = 17,000 gpd/ft2 * 0.01 (unit less term} * 6 ft2

Q = 1020 gpd

 

[fattdad]

17,000 gpd/ft2 = 1.58 ft/min, which is low for open-graded aggregate (0.79 cm/sec). Otherwise our methods are the same.



¡papá gordo ain’t no madre flaca!

 

[BigH]

The above notwithstanding - check Cedergren's tome on
"Seepage, Drainage and Flow Nets."