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Old 02-29-2016, 10:18 AM   #1
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French Drain design


Hi folks,

I'm planning in install a french drain near the back of my house due to excessive moisture retention from a flat grading (clay soil). Unfortunately, regrading isn't ideal due to the presence of a retaining wall, so I'm thinking a french drain will be the best option alleviate the issue. My plan is to use 4" pipe surround by gravel and encased in landscape cloth. However, I have a few questions about this.

1) (It won't let me link the diagram, so I'll try with a reply to my own message) My initial thoughts are to go ahead and add three of the downspouts to this drain since currently the downspouts flow into buried corrugated pipes with no protection/landscape cloth/etc. and our land is full of trees which have obviously found the pipes. It's only a matter of time before they stop draining completely, so I want to add these into the drain I'm installing. My question centers around this: if I connect to the downspouts, would I still need a grade-level inlet near the "?" for proper flow, or would I get enough airflow from the downspouts to keep the water moving? My assumption is that as long as the drain (4" pipe) is operating at half capacity or less, flow should be fine, but I'm both unsure if that estimate is accurate or how often the drain will operate at that level.

2) If I do need an inlet, what is the best way to keep it clean so the pipes don't clog from dirt/debris/etc.?

3) As previously mentioned, my land full of trees, which have already eaten into the previously (poorly installed) downspout drains. Will landscape cloth wrapping the pipes and gravel be effective at keep out tree roots?

4) If tree roots are inevitable, I'm also considering using PVC pipe instead of slotted corrugated pipe so that I can clean out the drain in the future. However, given the extra cost (and effort to drill holes), I don't want to over-solve the problem if the corrugated pipe and other measures will effectively stay free of tree roots/etc.

Thanks!
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Old 02-29-2016, 10:19 AM   #2
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Diagram link


See simplistic diagram
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Old 02-29-2016, 10:27 AM   #3
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Your drain will only function if the water can flow out to daylight (onto the ground) at a lower level than the water comes in, or if you have highly permeable soil that can absorb water coming out of the perforated pipe. So you need to start by doing a reasonably accurate topographic survey of your property to verify the elevations. Remember, water flows from higher water level to lower water level, so you need to verify the actual level of the WATER at the pipe outlet. If you are planning to drain into a ditch, a pond, a storm sewer, or similar, the water level at the drain can vary, and in the worst case can back up into your pipe. Without elevations it is not possible to evaluate whether your project will work.
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Old 02-29-2016, 10:31 AM   #4
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I haven't quantified the elevation, but my house sits on a big hill, which slopes from my house towards front yard/street/etc. The area immediately behind the house is the only area that is flat (the area for the house was leveled out of the hill and a retaining wall put in behind the house). There is plenty of elevation change once I get to the side of the house, so it shouldn't be hard to shape the back trench to get adequate flow to that side section, where I can work with the hill to drain the water away down the hill.
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Old 02-29-2016, 11:58 AM   #5
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I would suggest studying swale construction to alleviate or partially solve the problem. French drains and swales are worlds apart on the amount of water to be carried. Of course there are several articles if one googles swale(s) but I would recommend an engineering book or two to get professional advice.
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Old 02-29-2016, 04:30 PM   #6
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Quote:
Originally Posted by SeniorSitizen View Post
I would suggest studying swale construction to alleviate or partially solve the problem. French drains and swales are worlds apart on the amount of water to be carried. Of course there are several articles if one googles swale(s) but I would recommend an engineering book or two to get professional advice.

Ayuh,.... Agreed,.... A simple swale along front of the retainin' wall, daylightin' beyond the house fixes All the problems stated in the Op,.....

A Swale never gets clogged with tree roots,....

Any pipe, with Any hole in it is an invitation to tree roots, sock or no sock,...

Donno where yer at, but swales also don't tend to freeze up, like thinly buried pipes either,....
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Old 02-29-2016, 07:51 PM   #7
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Swales


I've considered a swale, but a few things had me leaning towards french drain:

1) There is a section of the brick patio near the back downspout (see previous diagram) where the grade comes all the way up to the bottom of the siding. I want to lower that a few inches to prevent bugs/water/etc. from getting behind it by removing a couple rows of bricks and digging down. I had thought a having that area then be part of the drain would be easier than having to lower/reshape everything else even lower since this section is farthest away from the side of the house with the hill.

2) Part of my perceived benefit of the drain was also being able to join in the downspouts into a new drainage system. To do that with swales, I assume I need kickouts on the downspouts into the swales? Is that a good solution, or will that cause me more other problems later with erosion or something?

3) We hoped to expand the back patio straight back to the retaining wall using flagstone. Since I would want that area to be level, a french drain seemed a better option than a swale. If I go with a swale instead, would I still want a drain under that section or is there something else I can do to let the water drain away easily from that area into the swale without compromising on the expanded patio area being level?

I'll try to take and post some pictures tomorrow. The advice is appreciate as I try to find the best solution to minimizing the moisture retention in that area.
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Old 03-01-2016, 05:16 AM   #8
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Quote:
3) We hoped to expand the back patio straight back to the retaining wall using flagstone. Since I would want that area to be level, a french drain seemed a better option than a swale. If I go with a swale instead, would I still want a drain under that section or is there something else I can do to let the water drain away easily from that area into the swale without compromising on the expanded patio area being level?
Ayuh,.... The pitch of a swale can be slight enough, that it's unnoticeable under foot, yet drains,.....

We're talkin' a slight but drainable swale, not a standard road-side drainage ditch,.....

Does it freeze where you live,..??
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Old 03-01-2016, 01:49 PM   #9
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In North Carolina, so yes it does freeze in the winter. Lows usually fall to upper 20s in the middle of winter, but sometimes lower.

Linked are some pictures of the back area I took this morning:

Picture 1
Picture 2

The first shows the area I want to improve drainage; note the downspout that goes directly into a buried pipe that (given the age of the house and wooded lot) has probably experienced problems with roots/debris. We also want to expand the patio back to the retaining wall, so any drainage system needs to be able to accommodate that.

The second shows a closeup of the brick patio that comes up to the bottom of the siding. I want to remove 1-2 rows of bricks so I can lower the grade there and also allow it to drain into whatever swale/french drain I install.

The far side of the house, not shown, slopes downhill and has two more similar downspouts into buried pipes.
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Old 03-02-2016, 12:39 PM   #10
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Ayuh,.... The tree in pic 1 is gonna destroy yer retain'in' wall,....

Ya need a laser or sight level to determine the drop in elevation ya got to work with, from the top of the swale, to wherever it daylights at the bottom,...
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Old 03-02-2016, 12:56 PM   #11
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Assuming you are talking about the tree close to where I was standing, that's actually a tall crepe myrtle; it actually looks bigger in the picture than it really is. I wouldn't have a problem getting rid of it, though, should it pose a problem.
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Old 03-02-2016, 01:26 PM   #12
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Quote:
Originally Posted by Bondo View Post
Ya need a laser or sight level to determine the drop in elevation ya got to work with, from the top of the swale, to wherever it daylights at the bottom,...
Alternatively, there's also sting levels and water levels.

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