Backyard drainage - french drain placement, etc

[jeromedawg]

Hi all,

I'm trying to think through solutions for adding an extra drainage system in our yard and just came up with this after surveying the yard again:



- Blue lines/arrows indicate the outside of our property which is a hill and the *general* direction that water flows when it rains (there is also a drainage channel further up the hill).
- Green lines/arrows indicate where existing drainage is (3" solid triple wall I believe).
- Yellow circles indicate where current surface area drains are.
- Orange circles indicate where I might consider adding additional surface area drains (if even necessary in conjunction with a french drain)
- Red lines/arrows indicate where I might consider adding french drainage (3" perforated triple-wall surrounded by crushed rock and fabric)

The direction the arrows are pointing either indicates where the water is currently flowing OR [in the case of the red lines/arrows] the direction/grade I'd want to achieve for the water to flow 

Here is a picture 'on the ground':



There is a retaining wall as can be seen so what we're concerned about is the boundary of our yard as defined by the retaining wall/iron fence of course. I've built up a bit of a 'berm' (it's probably around 2-3' high and 50-60' long) against the long-side of the yard and was thinking it might be a good idea to install a french drain near the base to capture water/run-off from that. My intention is to plant towards or at the base of the berm (possibly even on it) and then have a french drain in front of those plants, etc. At first I was considering a swale but I want a crushed rock path instead.

The "middle" of the red lines essentially would be a high-spot in the yard. The reason for this is because the existing grade is slightly challenging and the left side of the yard (at least in the context of the picture) seems like it's a bit of a low spot currently. I can build-up dirt over there but we already have drainage there as well and I also don't want to build up so much dirt that the level is drastically higher, especially in comparison to the concrete patio. NOTE: I just attempted to get the rough grading (using stakes, line and a line-level) and to further support this, it appears the existing grade is already such that there is a bit of a high-spot mid-yard where it sort of tapers downhill and watersheds(?) on both sides. Given this, do you guys think it would make sense to follow-suit with the french drain then?

Anyway, I'm still trying to figure this out as I go but open to any suggestions.

[former player]

Not an expert, but I do have a house on a slope.

I think it is usual to put the french drain on the upper side of a wall, which for your boundary wall would seem to mean putting it on someone else's land?  Could you get permission to do that?  Are there signs that the boundary wall is having structural or damp issues?

For your garden, what makes you think that the existing drains are going to be inadequate?  They seem to be in the right location to protect your house.  Planting up the slope and using permeable surfaces for any hard landscaping will absorb rainfall and help to avert a flood risk.

[jeromedawg]

Not an expert, but I do have a house on a slope.

I think it is usual to put the french drain on the upper side of a wall, which for your boundary wall would seem to mean putting it on someone else's land?  Could you get permission to do that?  Are there signs that the boundary wall is having structural or damp issues?

For your garden, what makes you think that the existing drains are going to be inadequate?  They seem to be in the right location to protect your house.  Planting up the slope and using permeable surfaces for any hard landscaping will absorb rainfall and help to avert a flood risk.

That makes sense but that area belongs to the city. Actually, I thought it had come up somewhere that 2-3ft outside of the wall may actually be our property. The only concern is that the city has irrigation piping running in the same area. I think it could get hairy trying to add the drain on the outside of the wall either way. From what I've seen there are no structural issues. Presumably the drainage channel up the hill mitigates most of the concerns with landslides...hopefully.

One other consideration is our neighbors on the right. They are downhill of us, so it seems like placing a French drain at least near the fence on the right side would make sense?

Someone on one of the DIY forums was just suggesting having the french drain at the base of the berm as an additional means of 'security' in case there are any issues with grading. I intend to have a flagstone patio 'extending' out from a majority of the concrete patio/pergola area with the crushed rock path surrouding it. The entire area is definitely meant to be permeable as well - it was just scary during the last heavy rains but the existing drainage system seemed to work pretty well. Anyway, I'm just thinking ahead and trying to foresee and mitigate any *potential* issues that could arise. I do plan to plant on the berm although closer to the base and the types of shrubs that are native to my area but also can grow quite large to fill into a bit of a privacy hedge (hopefully). 

[GilesMM]

Is that low wall stucco over wood?  I would be reluctant to place dirt against it. You are inviting moisture, pests and rot.  Couldn't you level out all the dirt in fashion that is slopes from the fences toward the drains?  It is going to tend to want to level itself out the next hard rain and you don't want a yard that is shifting all around and slumping into a mess.  What are you going to plant to stabilize the soil?

[jeromedawg]

Is that low wall stucco over wood?  I would be reluctant to place dirt against it. You are inviting moisture, pests and rot.  Couldn't you level out all the dirt in fashion that is slopes from the fences toward the drains?  It is going to tend to want to level itself out the next hard rain and you don't want a yard that is shifting all around and slumping into a mess.  What are you going to plant to stabilize the soil?

Its stucco over cinder block with concrete footing - no wood... At least none that I'm aware of. The problem with grading it now is that there is too much of it if I still need to lay down roadbase (or whatever stabilizing base) for te crushed rock path and especially the flagstone area.


I have some lemonade berry plants as well as a toyon and coffee berry that i was going to plant along this berm. Fortunately these should be good for erosion control per this -https://californianativeplants.com/blog/native-plantings-for-slopes/

[lthenderson]

I always put French drains at low points and up next to structures you want to protect. In this case, I would put it between the patio you plan to create and the existing concrete next to the house seen in the second picture and roughly where your green lines are drawn. You mentioned you have drainage there but it is solid piping. I guess I would just replace that with French drain materials surrounded by rock and filter fabric. I would also do some rough calculations on the capacity of everything. Assuming the French drain is feeding into an existing line that is probably already fed by gutters and other sources, you may easily exceed capacity when draining your entire back yard which can create even more problems. I would work very hard to have your yard absorb as much rain as possible BEFORE it gets to the French drain and not the other way around as you are proposing.

[snic]

it was just scary during the last heavy rains but the existing drainage system seemed to work pretty well.

Then why do anything at all? Seems like unless there's evidence of a problem, such as pooling water when it rains, there's no need to fix it.

Also, what are the legal and ethical implications of adding a drain that just ends at your neighbor's property line (on the right)? (I'm assuming that's what you're doing - unless there's somewhere for the water to go on that property, like a big drain?)

[jeromedawg]

it was just scary during the last heavy rains but the existing drainage system seemed to work pretty well.

Then why do anything at all? Seems like unless there's evidence of a problem, such as pooling water when it rains, there's no need to fix it.

Also, what are the legal and ethical implications of adding a drain that just ends at your neighbor's property line (on the right)? (I'm assuming that's what you're doing - unless there's somewhere for the water to go on that property, like a big drain?)

I'm just thinking ahead and trying to do some "preventative" maintenance in this case.

I'm not sure I understand the second question about the legal and ethical implications - my whole intention of adding a french drain parallel to the neighbor (note: this wouldn't be right up against the fence line) is to capture water and direct and feed it into our existing drainage system which will get carried into the street. I'm not trying to capture a bunch of water here and direct it onto the neighbors' property.

[jeromedawg]

I always put French drains at low points and up next to structures you want to protect. In this case, I would put it between the patio you plan to create and the existing concrete next to the house seen in the second picture and roughly where your green lines are drawn. You mentioned you have drainage there but it is solid piping. I guess I would just replace that with French drain materials surrounded by rock and filter fabric. I would also do some rough calculations on the capacity of everything. Assuming the French drain is feeding into an existing line that is probably already fed by gutters and other sources, you may easily exceed capacity when draining your entire back yard which can create even more problems. I would work very hard to have your yard absorb as much rain as possible BEFORE it gets to the French drain and not the other way around as you are proposing.

Thanks for the pointers. Yes the other option was to replace the existing section (green line) with a french drain so that the excess water will have somewhere to go. Thats still a possibility I think. I definitely need to grade from the fence line back toward the concrete area anyway. As far as water absorbing into the ground the crushed rock path and flagstone patio area will have class II roadbase under which I believe is water permeable so i think that would promote drainage 'naturally'

Btw if i replace the green line section with perforated triple wall would it still be a good idea to keep those surface drains (e.g. in case i might want to clean them out in case if a clog or something?)

[former player]

Surface drains and french drains might be doing different jobs - one taking surface run-off, the other preventing water in the soil and waterlogged earth from reaching the foundations.  Having both would depend on the particular circumstances you are dealing with.

I think you are being very wise to take your backyard drainage so seriously: it is clear that heavier rainfall events and cloudbursts are going to be an increasing problem everywhere, even in previously dry areas..

[jeromedawg]

Surface drains and french drains might be doing different jobs - one taking surface run-off, the other preventing water in the soil and waterlogged earth from reaching the foundations.  Having both would depend on the particular circumstances you are dealing with.

I think you are being very wise to take your backyard drainage so seriously: it is clear that heavier rainfall events and cloudbursts are going to be an increasing problem everywhere, even in previously dry areas..

That makes sense. I'm thinking it might be easier just to replace the existing lines then with perforated. Of course, now I'll have to add in crushed rock and all that, which means additional depth/excavation and having to make sure the grading all lines up. This is a 'small scale' and shallow French drain as well, and it won't be as deep as the standard specs per google want it to be (8" minimum depth - at the shallowest spot, I'll probably be looking at a depth of 4-6" minimum)

[jeromedawg]

I always put French drains at low points and up next to structures you want to protect. In this case, I would put it between the patio you plan to create and the existing concrete next to the house seen in the second picture and roughly where your green lines are drawn. You mentioned you have drainage there but it is solid piping. I guess I would just replace that with French drain materials surrounded by rock and filter fabric. I would also do some rough calculations on the capacity of everything. Assuming the French drain is feeding into an existing line that is probably already fed by gutters and other sources, you may easily exceed capacity when draining your entire back yard which can create even more problems. I would work very hard to have your yard absorb as much rain as possible BEFORE it gets to the French drain and not the other way around as you are proposing.

BTW: regarding fabric - I've been seeing a number of videos where they warn against using fabric as any clay soil (which we have) will slowly end up blocking it. In my case though, I'm excavating much fo the clay out (4" or so, up to the patio) where I would intend to backfill with class II roadbase, which is more permeable. If we were to remove the existing 3" drainage and replace with the 3" perforated counterpart, at the same elevation, as well as adding at least a 1" layer of 3/4" rock under the drain, I'm wondering if I might end up running into the same problems with clay mucking things up if I were to wrap the trench. One other thought that came to mind as I was thinking through this might be to excavate out the trench for the french drain deep enough to where I could get 1-2" of the class II road base into it and compacted in very well, then installing the fabric before layering 1" of rock adding in the drain and covering back up with more rock. This way the road base would act as somewhat of a barrier of separation to keep the clay away from the fabric.

[Finances_With_Purpose]

I had a similar problem with one of my buildings.  I built a small berm on one end and it solved all of my problems.  I simply plopped down a layer of rocks and covered it up with dirt, then let some grass/vines grow on it. 

It diverted the strong water flow away from one side of the building and towards better places.  As an added bonus, the berm now grows an edible berry patch which extended my prior berry patch by about 10-15 feet. 

It was cheap, easy, functional, and works great.  I spent all of $0 and a few hours on it, max. 

I also thought about a french drain down one side, but what I like about my berm is that it's really obvious to see at a glance if it's having any issue (is it still there or not?) and, so 2.5 years in, it seems to be 100% effective with no erosion.  It also diverts far more water than a french drain ever would have - the water would have to get higher than my berm (which is only probably 8 inches high--but a tremendous height relative to water flowing over a slope).  The berries/grass/roots help prevent erosion, too.  A win-win-win.  Definitely recommend: 10/10. 

[jeromedawg]

I had a similar problem with one of my buildings.  I built a small berm on one end and it solved all of my problems.  I simply plopped down a layer of rocks and covered it up with dirt, then let some grass/vines grow on it. 

It diverted the strong water flow away from one side of the building and towards better places.  As an added bonus, the berm now grows an edible berry patch which extended my prior berry patch by about 10-15 feet. 

It was cheap, easy, functional, and works great.  I spent all of $0 and a few hours on it, max. 

I also thought about a french drain down one side, but what I like about my berm is that it's really obvious to see at a glance if it's having any issue (is it still there or not?) and, so 2.5 years in, it seems to be 100% effective with no erosion.  It also diverts far more water than a french drain ever would have - the water would have to get higher than my berm (which is only probably 8 inches high--but a tremendous height relative to water flowing over a slope).  The berries/grass/roots help prevent erosion, too.  A win-win-win.  Definitely recommend: 10/10.

I'm not sure how well it's going to work in my case with the type of soil I have - it's clay soil with lots of roots and I think it's expansive as well (so it shrinks when dry but swells/expands when wet). I'd have to pay good $$ to remove it and then pay more $$ to bring in new material to replace it, so trying to avoid all of that. I'm thinking maybe I ought to consider building a retaining wall of sorts. Though I wonder if using the more durable and thick 4" bender board with stakes to hold it in place might be sufficient in keeping the clay soil on the berm from spilling out and getting all over the place. After the current heavier rains, I've noticed that the soil has slowly made its way down from the berm but not in an amount of overwhelming concern... I figure if you plant stuff on it though, that should help with preventing it from eroding. The biggest mistake I made was probably not compacting it down as I was dumping the excavated clay soil along the fenceline - I was on such a roll with digging and dumping the dirt with the wheelbarrow that I failed to take the time to go over and at least walk over the soil. So I'm thinking there might be some air pockets in there. Maybe a short(er) retaining wall is in order at this point.

[Finances_With_Purpose]

It sounds like you could improvise and buy a truckload of dirt.  I can't imagine that a truckload of non-clay dirt is going to cost you much at all.  Still the cheapest and most effective option, I suspect. 

On the upside, too, you want more rocks in it anyway.  The dirt is just a top covering/layer to ensure that the water is completely diverted, but you really don't need a ton of it.  The rocks were by far the more tedious and more important part of the job. 

[jeromedawg]

It sounds like you could improvise and buy a truckload of dirt.  I can't imagine that a truckload of non-clay dirt is going to cost you much at all.  Still the cheapest and most effective option, I suspect. 

On the upside, too, you want more rocks in it anyway.  The dirt is just a top covering/layer to ensure that the water is completely diverted, but you really don't need a ton of it.  The rocks were by far the more tedious and more important part of the job.

If I were to remove the clay dirt, I probably wouldn't even bother bringing more dirt back in to add a berm at that point as I'm not sure it's necessary. The *only* reason I've created this "berm" is because I was trying to avoiding paying the expensive fees to haul the excavated clay soil away hahaha.

Anyway, I think bigger question I'm currently faced with, as it has rained quite a bit since posting, is if keeping the berm there really makes sense or if it's going to be a bad idea long-term (since I didn't do it right and just dumped loads of it against the side etc). With the rains, I think it'll just slowly slide back out and make a mess across the yard. At this point I think the next best option besides removing it is to build a retaining wall or figure out some other way of reinforcing it to prevent potential erosion and sliding. On that note: I posted a "Free Dirt" ad up on FB marketplace and broadcast it across multiple groups in my area. A couple people have reached out so far... ideally, it would be nice to have neighbors take most or all of it away and not have to pay hahaha. Of course, this is risky because you never know who it is you're dealing with on FB either (like if someone were trying to case our house for instance). I'm not super concerned about it either way - we're not wealthy and don't own luxurious items and jewelry that anyone would want lol.

As far as bringing new materials in, regardless of what you order from many material yards I've been getting quotes from, the baseline is a $175~ delivery fee and that's for places that are much closer but where the price per yard is more. Places further away charge upwards of $200-300 per delivery while the materials per yard are generally cheaper.

[GilesMM]

Maybe you could just hurl the excess dirt over the wall to the common area and form a berm to divert water coming down the hill away from your wall?

[jeromedawg]

Maybe you could just hurl the excess dirt over the wall to the common area and form a berm to divert water coming down the hill away from your wall?

Haha, I thought about this and deliberated with my wife. It's a possibility except A) I'll probably throw my back out heaving that much heavy clay soil over/through the fence and B) the city owns that land so they may not exactly be happy to see all this dirt that appeared out of nowhere, or worse yet that off-chance that someone from the city catches me in the act of tossing/dumping dirt onto the other side hahaha.
On the other hand, the previous owners put buckets of bamboo back there, which subsequently have grown into the ground and are kind of a mess now. So with that in mind, we very well may be able to clear all that out and "backfill" it with some of the excess dirt ;) The thought of having to deal with removing bamboo rhizomes is giving me PTSD, however.

[GilesMM]

How about a home made catapult? You could blast buckets of soil into oblivion in the wee hours of the night?

[Finances_With_Purpose]

How about a home made catapult? You could blast buckets of soil into oblivion in the wee hours of the night?

Drone + bucket.

[Finances_With_Purpose]

You could always use my mother's time-tested solution for dealing with any excess natural object: trees, rocks, branches, you name it. 

Casually pile some of it in your dumpster/trash each week.  You'd be surprised how much that adds up over time, especially if you're retired and can do it religiously. 

[jeromedawg]

You could always use my mother's time-tested solution for dealing with any excess natural object: trees, rocks, branches, you name it. 

Casually pile some of it in your dumpster/trash each week.  You'd be surprised how much that adds up over time, especially if you're retired and can do it religiously.

Haha, that might be the solution. Mix a bunch of dirt in, but not too much, with the rest of our compost and trash. It would probably take up to a couple dozen times before it disappears. Between that, catapulting it over our fence, and drone+bucket over our neighbors' roofs, we should be able to get rid of all this pesky dirt in no time! lol

[jeromedawg]

Anyone have opinions on perforated corrugated vs perforated smooth triple wall pipe? This seems to be a highly debated topic. One person mentioned this, and logically it makes sense to me: you want smoothwall piping (perforated or not) for low-fall applications - if you were to attempt using corrugated in low-fall, the probability of the pipe clogging up from debris/silt/mud increases. I would think preventing the pipes from clogging up in the first place would be more important though. That said, I'm thinking about using 3" diameter perforated smooth triple wall pipe.

As far as the installation of it: 9-12" deep and wide trench with non-woven fabric laid down and 3/4" drain rock poured in to the correct elevation. Drain place in and more 3/4" poured in around and on top of it. Sounds easier than I think it probably will be....

Another factor is the clay soil - I hear using any fabric actually can cause issues with clay soil, where the clay eventually blocks and impedes the flow of water. I'm wondering if I should just apply a layer of roadbase inside the trench before using fabric (if at all) and backfilling crushed rock to try to prevent any such issues.

[GilesMM]

Smooth PVC is almost always preferred, corrugated is just easier to run, especially if you curves and whatnot, and want a shortcut.

[jeromedawg]

Smooth PVC is almost always preferred, corrugated is just easier to run, especially if you curves and whatnot, and want a shortcut.

Makes sense. What about the use of non-woven fabric particularly when it comes to clay? I keep seeing videos of failed installations where the clay gets gunked up and eventually clogs the fabric.

[GilesMM]

Fabric is a good idea just make sure it is high quality so it won’t degrade over time. You can also skip it like people did for ages before it was available.

[jeromedawg]

Fabric is a good idea just make sure it is high quality so it won’t degrade over time. You can also skip it like people did for ages before it was available.

How do you prevent the 3/4 rock from sinking into the clay mud though?