Water movement through infiltration basin

[JWHITE73]

I need to find the drain down rate of an infiltration basin with an underdrain.
If I have a filter media that is 30 inches deep, and I have a sudden rainfall that places a rapid 2" of water over my media. The media has a design infiltration rate of 2 in/hr. How will the water be routed through the media? And at what rate will the water drain to the underdrain?

Obviously, if the the rain continued at a rate that there was ponding over the media, then it will take 15 hours for the water to reach the underdrain and it would drain at a rate of 2 in/hr. But what about a small rain event?

 

[bimr]

The maximum rate that water can enter a soil in a given condition is the infiltration capacity. If the arrival of the water at the soil surface is less than the infiltration capacity, all of the water will infiltrate. If rainfall intensity at the soil surface occurs at a rate that exceeds the infiltration capacity, ponding begins and is followed by runoff over the ground surface, once depression storage is filled. This runoff is called Horton overland flow.

Robert E. Horton suggested that infiltration capacity rapidly declines during the early part of a storm and then tends towards an approximately constant value after a couple of hours for the remainder of the event. Previously infiltrated water fills the available storage spaces and reduces the capillary forces drawing water into the pores. Clay particles in the soil may swell as they become wet and thereby reduce the size of the pores. In areas where the ground is not protected by a layer of forest litter, raindrops can detach soil particles from the surface and wash fine particles into surface pores where they can impede the infiltration process.

http://en.wikipedia.org/wiki/Infiltration_(hydrology)

 

[JWHITE73]

I have got a handle on how the water gets into the filter media. I need more information on how to calculate the flow coming out of the media to the underdrain.

If the media is dry, or at a normal moisture conent, then water is applied to the surface. The upper level then becomes saturated then water will move down the media veritcally.

If there is water above the media and the soil infiltrates from the top at 2 in/hr. After 1 hour the water will 2" deep in the soil. What happens after another hour? Will the water then be in a 2" band that is 2" below the previous level? And then incrementally dropping 2" over every 1 hour time interval.

 

[fattdad]

refer to figure 4-1 (see link below):

http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_818_5.pdf

Look at "gravity flow" and the equation for "Q."

These sorts of problems are frought with misinterpretation. Folks think that the water can enter the media at the infiltration rate and to some extent the water can. Now the problem is how to get the water out of the media. The infiltration rate does not govern, as it is based on a vertical gradient of unity. The movement of water toward the underdrain is governed by the horizontal gradient and the media's permeability (i.e., permeability is not the same as infiltration rate). Your drain spacing can help control the horizontal gradient.

Please note when you use equations on figure 4-1, they are for flow into one side of the slot only. If you have flow into both sides of the slot you need to double it.

Please also note that these equations assume the term "H" is uniform. "H" would be your initial saturated height of 30 inches. You'd want to look at the flow for "H" of varying amounts to gauge how the flow attenuates with time.

You can use figure 4-23 to estimate the value of "L" (R as shown in figure 4-23) and use the value of "C" for a single line of well points.

You can use this graph to estimate permeability of your media once you know the D10 or D20 size of the media. This is a good graph!

http://home.comcast.net/~fatt-dad/permeability_correlation.pdf

Good luck. You can also setup a numerical model in "Visual Modflow" and it's pretty easy, but you first have to learn the software and that's pretty complicated (well, I mean to understand the program and such).

f-d

¡papá gordo ain’t no madre flaca!

 

[JWHITE73]

Thanks for the information. Very insightful. I only have one other question. I guess I would use this only until the field capacity of the soil is reached, right. Some of the water will not ever drain down.
My soil has a approx. field capacity of 2 in/ft. So I can assume that if I have 30" of media, then I can assume that it would hold 5" of water before it starts to drain.

 

[fattdad]

Yes. You should expect the media to have some typical moisture content and some available void space for new water. If you want to assign a value of 17 percent that may be correct. I guess it depends on where you are and such (i.e., Iraq v. southeast Alaska).

f-d

¡papá gordo ain’t no madre flaca!

 

[beej67]

The Florida Development Manual has a design standard for a retention basin with a sandy bottom and a perforated underdrain set a given distance below the pond bottom.

That sounds like your case, is that correct? If so, pick that up. It's got some simple design calculations to follow that are proven in Florida soils and rainfall amounts.



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