Flow Capacity in Gravel Underdrain
Flow Capacity in Gravel Underdrain
(OP)
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?





RE: Flow Capacity in Gravel Underdrain
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
RE: Flow Capacity in Gravel Underdrain
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!
RE: Flow Capacity in Gravel Underdrain
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.
RE: Flow Capacity in Gravel Underdrain
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.
RE: Flow Capacity in Gravel Underdrain
Q = 17,000 gpd/ft2 * 0.01 (unit less term} * 6 ft2
Q = 1020 gpd
RE: Flow Capacity in Gravel Underdrain
¡papá gordo ain’t no madre flaca!
RE: Flow Capacity in Gravel Underdrain
"Seepage, Drainage and Flow Nets."