French Drains

You could cater to all opinions (or maybe none) by placing holes randomly all around the pipes,

Pipe comes pre-holed & unless your client has way extra expendable cash.....

GA I like your analyses and agree with the 90/270 orientation. But I'm an electrical engineer. What do I know about flow and currents?

"...In the event the bottom holes don't become fouled, there is undesirable draining and general swampiness near whatever area or structure you are trying to protect via french drain. ...."

You sure about that? Fluid doesn't just flow out because the hole is on the bottom. If the pipe has air in it that outside water can displace, that will occur. If the pipe is full then the direction of flow depends on the difference in head not the hole position.

Additionally holes on the top side of a buried pipe might not clog as easily, but holes on the top side do have a tendency to admit more dense solid material into the pipe which can limit or obstruct flow of the entire pipe....this is when swampy areas near the area intended to be protected will truly become a problem.

Differentials in head and channel flow direction are not always related , but are, when the entire length of perforated pipe is in a saturated zone .

In the case of “ catching “ water from a rising water table , perforations at 90/270 or biased towards the top will preserve channel flow and direction will be determined by pitch . However water tables vary greatly so head will determine direction and rate at varying durations .

When the water table falls , channel flow loss can occur when going from areas of high head pressure to low head if the perforations are biased towards the bottom of the pipe .

In my experience some areas that need a french drain have silty soil that can eventually foul the crushed stone and leave a streak of silt that can become a concrete like plug down the center of a drain pipe. Having the sides open allows rising water to flow in after years and years. When top holes become the only entrance for water the system is not functioning properly.

IMO of course

I think we are actually in complete agreement. The alignment of degrees was throwing me off actually..

I agree. The perforations are intended to let water into the pipe so that it can be carried away from the area. Holes oriented to the sides makes more sense. Holes on the bottom would only admit water if the water table was so high as to be awash - and only then as long as they weren't fouled. So even if the source of water was a spring (vs rainwater from above or draining down a slope) side holes would be better for the purpose of capturing water to be drained away.

That being said, the type of pipe we've used for french drains here is perforated on all sides with slits rather than holes.

https://www.homedepot.com/p/4-in-x-10-ft-Corex-Drain-Pipe-Perforated-4040010/100211970

It has worked very well. After reading your post and thinking about it, I would guess that there is little or no flow to or from the perforations in the bottom since they are narrow and readily closed by the first wash of sediments in the life of the pipe.

I agree with Artsmith in drilling small holes on all sides, but not slits for it is to slim that it will be congest too easily.

"... I prefer the perforations be placed on the pipe walls at 90 and at 270 degrees as this preserves channel flow while catching the surrounding water at a reasonably deep depth within the trench

.."

For clarification, where is zero degrees? Typically zero and 180 degrees are on horizonal with the center and 90 and 270 are top and bottom respectively, but your description suggests zero might be at top or bottom.

If zero and 180 are the sides on the horizontal through the center, holes drilled a little below those two angles would probably be a good solution in a wide variety of conditions.

Holes centered between say a range of 185° - 240° as well as 300° and 355° would provide a wide center swath for flow undisturbed by holes, leave plenty of room for holes and have all holes angled such that material finding its way into the pipe will be biased towards being transported out of the pipe upon encountering a drilled hole.

If zero degrees denotes the top of the pipe, then a good range for holes would be 95°-150° and 210°-265°.

Being only familiar with French curves used a lifetime ago in mechanical drafting, I went to Google.

The consensus of the dozen or so articles/forums I visited was 5 o'clock and 7 o'clock, and the use of a proper screen/filter to keep silt/sediment out of the pipes.

Well if the perforations locations are being criticised for premature failure, then your detractors may have a point....but, if this is an argument over aesthetics, then I would follow my intuition...I personally feel that the perforations should be in the bottom half of the pipe...

http://www.balkandraincleaning.com/about-french-drains-drain-not-french/

Yes, your 90-270 configuration is the more effective choice for conveyance of groundwater through the subgrade. (The first priority of water drainage pipes is to drain the water.) Ground water below the flowline of the pipe will remain unaffected in either case, and the impounded water above the flowline up to the 90-270 level is a relatively small quantity to be retained.

If, on the other hand, you were trying to convey only soil gas away from, say, an earthen landfill embankment, then, the openings are more effectively located in the 0-180 configuration.

If, you needed to convey both ground water and soil gas, then, the gas pipe network should be locately directly over the ground water pipe network at least 1 vertical foot directly above, in order for the water to lift the gas into the ''piggy-back'' gas conduit, rather than tend to trap it in a full-flowing water conduit.

It does not matter where you place them, but if placed on the top, the fine silt will block the slits. If placed on the side, the pressure will be less.
And it all depends on how much of the SUMP water you want to catch. Sometimes you want to keep the ground flooded for trees and only tap-off the high water.

I wonder why no one has designed a pipe specifically for the purpose; something like this:-

for plastic pipe, too much geometry in some areas to roll for transport/storage and not enough for strength in other areas, as well as material usage to make that?...

hmmm.. to my eye the sketch is a bit off purpose, where most of the water will drain away from the holes by dripping off the shoulder.

I also would expect it to be vulnerable to mechanical stress.

This led me to ponder why pipes are round - is it not simplicity alone, but the fact that a round pipe is more resistant to pressure from all sides.

Water typically intended to be drained would not drip off the sides of a buried pipe. Water that is descending rapidly enough to call 'dripping off' isn't so much the concern as is water level that is building up.

There are lots of reasons for round piping. Amount of material (cost) to sectional area enclosed (capacity) for a given length would be a big one.

Given the recent popularity of elliptical concrete drainage pipes installed with the long axis of the ellipse horizontal and therefore suboptimal for resisting pressure from above, it would appear resisting pressure can often trumped by other factors when deciding on shape of pipes.

"Water that is descending rapidly enough to call 'dripping off' isn't so much the concern as is water level that is building up."

We are thinking of different applications. You're right, this could be a good design where the purpose is to drain an area with a chronic high water table - boggy ground. It would keep the water table below the pipe perforation level, and the cap would minimize infiltration of particles and fouling of the pipe.

OTOH if the purpose is to prevent the ground from ever becoming boggy by carrying away rain water, then it would not be such a functional design.

We must indeed be thinking of different applications.

Seems like a great number of french drain pipes would need to be installed with very close spacing if catching predominantly water descending straight down before any buildup of water level in the soil is to be accomplished.

I'm probably misunderstanding what you are describing. I mean, seems like to catch the water as it descends straight down without buildup or displacement, would mean the only rain falling immediately above portions of holes that are open to above could fall into the drain.

Then again, if the water really isn't building up at all and just descending straight down without pause, it seems like there isn't much need for a frenchdrain and that it would be difficult to notice the effects of such an installation.

Yes.. one of the benefits of international forums is learning about conditions entirely different from ones own. (I'm thinking about what I've learned from farming and gardening forums, about the variation that exists in terrain both above and below ground).

I was thinking about the french drain we installed when my home was built. When excavating for the foundation we struck solid rock - the mother rock poking up like an underground cliff. House plans were modified somewhat instead of blasting down to the intended level. The driveway and north side of the house are higher than the west side. The possibility that water would run off from the higher grade and pool next to the house where there is solid rock at or very near the surface was the reason for installing the drain on (I mean under of course) the contour where the land slopes away from the driveway, from the corner by the house to a lower area. It has never flooded, presumably because of the foresight in installing that drain. But it didn't have the potential to be boggy ground - it's rock with scant bit of gravel above it.

A second example was a drain installed at my father's home, entirely rain related and not to do with high water table. Rain water from the slope above was running down the old road and flooding his cellar. The land continues to slope down towards the sea and is well drained not boggy whatsoever, but the runoff was holding up on the concrete wall of his cellar. A french drain was installed to catch and carry that water away from the foundation.

Maybe these issues are entirely because there is so little topsoil here, and not far to go below ground before you strike boulders if not solid rock, which can obstruct the flow of rainwater. If similar problems don't occur elsewhere, I am surprised.

I suspect the various conditions that comprise our individual experience do differ somewhat, but that is probably not the important difference here. It seems more likely our individual mental models of how drains function differ significantly.

My model suggest that even french drains built in highly permeable media with little soil on top and with an impermeable layer (bedrock, hardpan, etc) just below will not have the installed perforated pipe predominantly filled with water that is coming straight down from directly above.

The water drained by the pipe will have descended into the ground from all over then as the downward movement largely arrested by the impermeable layer, the water will begin to flow more laterally toward the perforated pipe down the gradient in water table resulting from the water being removed more readily as it flows down the pipe.

The function is the same whether the soil is low permeabity clay or high permeability gravel, it just takes more time for it to seep through the clay.

If water wasn't building up to the level of the pipe at some point (so merely falling downward past) there would be no reason to install the pipe.

.

....or so says my model, at least.

Yes. Our mental models as well as experience must differ. But you mistook me if you thought I meant that the water is strictly "coming straight down from directly above". I know how water flows following the path of least resistance after it has entered the soil/substrate so lateral and downward flow is expected, I didn't intend to say otherwise.

Without a descriptive example of how it does work, I find it difficult to visualize how a french drain would be of any use at all where the water table has risen to the level of the pipe. It stands to reason that you intend water to enter the pipe at its highest end and thereby flow downward to an outlet or catchment area. If the water table is up around the pipe at the higher end, then the outflow will be below the water level, and it will not flow out.

At least, that's how water behaves in this country.

When you wrote...

"...hmmm.. to my eye the sketch is a bit off purpose, where most of the water will drain away from the holes by dripping off the shoulder...."

....and then followed that with...

"... You're right, this could be a good design where the purpose is to drain an area with a chronic high water table - boggy ground. It would keep the water table below the pipe perforation level, and the cap would minimize infiltration of particles and fouling of the pipe.

OTOH if the purpose is to prevent the ground from ever becoming boggy by carrying away rain water, then it would not be such a functional design...."

.

...it read it as a claim that in your application, the water table is not rising to the level of the drain pipe. If the water table is not rising to level of some perforations in the pipe, i.e. if 'dripping off' is a possibility, then water entering the pipe would be predominantly that draining down from immediately above and just lucky enough to descend through one of the perforations.

I'm glad to now understand that wasn't the idea you were actually carrying around. I am a little unsure how what was written could be interpreted in a significantly different way, but that is inconsequential so long as ypur understanding isn't accurately described by my original interpretation.

"...it read it as a claim that in your application,..."

Just to be perfectly clear, I'm not intending to "claim" anything here. Just trying to work my way through it in my mind, with the expectation of learning something. Feel free to think of my comments as water slowly percolating through the gravel between my ears.

I thought the proper use of gradient was required to support the pipe as well as filter out large debris.

If someone does decide to use your sketch, cr4 may have to pay you royalties.

Machia0705,

No one seems to address another point. Assume the holes are on the sides (not directly on the bottom), and the french drain runs through areas with varying levels of ground water. Where the water level is above the french drain water will enter it and be conveyed down-slope, but where the water level is below the french drain the water already inside it will be kept there and discharged down-slope to where the french drain ends at the surface or in some other designated location. Therefore, to ensure that the water the french drain intercepts is discharged at its intended spot, you need to have no holes in the bottom of the french drain pipe.

--JMM

Agree .

And this sort of discussion is exactly why I keep coming back. I know enough about French drains to get myself in trouble... or deep water if you prefer. But following this discussion is quite an education in itself.

I would still not attempt to install one, but now I can listen and have a better understanding of what the contractors are talking about.

Agree !

But place my inlet openings @ 90/270 degrees .

I would have thought it best to avoid slots or holes in the bottom third of the circumference of the pipe this so not to impede the flow of water. Of course, if the trench is dug with sufficient fall, the water remaining beneath the openings in the pipe will, in the event, drain through the pea shingle, albeit slowly. I recall French drains without collector pipes at all, discharging to an open channel, I think through gratings,

I posted my opinions about this design and read all others . Do you realize that we are discussing in depth the difference of inlet placement of just 2 inches in a 4 inch diameter pipe ?

Fascinating .

I’m still learning ..

1. In my work the application of a sufficient sock -filter material is by far the most important to the long life of the installation. The last comment about a 2" difference in elevation is well placed in this discussion. Silting up over time is the issue that clogs the drain or requires significant effort to attempt to clear.
2. The brick drains in europe had a variable shape to them to allow scouring of debris in an attempt to reduce fouling at the bottom.

Enjoy.

I would suggest that the perforations be placed initially at 90/270 for prevention of silt getting into the drain and also to prevent fouling of the holes at the bottom. But eventually the water in the bottom half has to be drained out also. So at the end of the drain, the perforations should be at the bottom.

eau, n'eau, not a french drain question- just how l'eau is the water?

How l'eau do you want it to g'eau?...