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Trench Drain vs Roots

11 years ago

Water flows diagonally from the top left of my backyard to the bottom right (which is according to the natural flow for all the houses in a 2 block radius). However, the negative slope towards my foundation was not good, so I reversed the slope for the first 10 feet away from my foundation. Where the two slopes meet (10ft out), I've already dug a trench, to be filled with gravel later.

My plan was to continue the trench drain around the right side of my house, and out to the street, by extending the trench that far.

However, I've run into roots (which I've now cut out of the trench area) from a few of my neighbors' trees (see the right side of my diagram).

I figured I could change my original plan of having the trench drain continue along the right side of my house and instead just carry the backyard water to the front via a PVC pipe which won't become a victim to the roots.

However, there is still the possibility of tree roots seeking out the backyard trench drain, which would simply be gravel wrapped in landscaping fabric topped with rocks. What can I do to protect my backyard trench drain from root invasion?

Also, I'm wondering instead of trying to have a long PVC pipe to go towards the front of the house (and also trying to figure out how to get the water from the trench drain to actually flow into a PVC pipe), if maybe I should create a dry well in the middle of my backyard instead, and let the water disburse there? My soil is mostly clay, so absorption would not be fast, but I've tested it out and a bucket of water is absorbed within a few minutes.

But I guess I'd also have to protect a dry well from tree roots as well.

Thanks for whatever suggestions you can provide.

I've enclosed a diagram, because my backyard is all dug up now and a picture would not be as helpful.

Comments (20)

  • PRO
    11 years ago

    My first inclination is that you are solving this in a more complicated and expensive way than is necessary. The solution might be questionable, too. My second inclination--and don't take this personally--is not to completely trust the descriptions of drainage affairs as important details are often overlooked. While it can't always be done, it's better to keep drainage solutions above ground if at all possible. When you bury them you are likely adding maintenance and the great possibility of failure at some point in the future. Oddly, the failure usually makes itself evident when the torrential rains of the century begin pouring down. No one cares about "the mess" that your back yard is currently in so I'd reconsider if your situation couldn't be better explained with a few pictures. Your description and diagram don't give any idea of the degrees of slope involved, or how they were achieved. Was the slope away from the house created by adding fill? This project would be better in many ways if you could solve it with an above-grade swale. Are you sure it isn't possible?

  • 11 years ago

    Thank you so much for your help - I really appreciate it. I don't know how to attach 4 pictures in one post, so I'll do 4 posts.
    No, the negative slope by the foundation was already too high so I removed soil so that the grade would be at least 6 inches below the top of the foundation. From there I sloped it 2% downwards, towards the trench you see in this first picture. There is gravel in the closest section - that is where the deck will go. Then I placed sod over the further section. The house is immediately to the left.
    This picture shows the lower right corner of the backyard and the trench that has been dug. But there are mounds of earth to the left of the trench - I haven't graded this yet.

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  • 11 years ago

    This second picture is facing the right side of my backyard. I'm hoping the gradual slope downwards toward the right is visible.

  • 11 years ago

    This is looking from the bottom right of my backyard (the lowest point), towards the left (the higher side of my yard). The fence posts gets gradually lower (I'm not sure if that's easy to notice).

  • 11 years ago

    Finally, this shows the back fence, going gradually lower as you go towards the right.
    The reason I went with the trench idea was because I was not sure if I could do a swale deep enough. The width of the backyard is 58 feet, so a 2% slope on a swale would mean I'd have a swale 14 inches deep by the time I got to the far right side of my yard. Even if I opted for just 1%, I'd still have a swale that would be 7 inches deep.
    But truthfully, your reply made me realize that I have not taken into account the NATURAL existing slope of the land, so if I were somehow able to calculate how many inches difference there is between the grades at the far left and at the far right of my backyard already, I might find out that the net (final) depth for the swale might only need to be a few manageable inches. (I hope you understand what I'm trying to say).
    So, thanks for your help. Thanks for making a very logical and correct suggestion. I guess the swale is also the best course of action anyway, in order to avoid my tree root concerns. It will certainly be more delightful filling in the trench now than it was digging it.

  • 11 years ago

    I commend that you are wanting to fix the grade, but I think your solution will be annoying to maintain over time. In order for it to work, you'll need to keep leaves and debris off of the gravel. It might be hard to do during a multi-day rain at certain times of the year. And you'll need to keep gravel out of the grass. I think it will become tiresome to keep up with it. During the heaviest rains, water may not be able to enter the gravel fast enough and gloss over the top of it instead, at which point it will pool until it drains out some other way. That your yard is forced to accept a rigid appearing (as if done by an engineer instead of an artist) gravel-filled trench as a dominant design element forever after is not exactly a plus. I guess you can figure that I do not like your plan. It seems that it would be much better to develop swales that carry the water above ground to the exit point. They do not need to be deep and pronounced in order to work. Mild, gentle--almost imperceptible--swales would be fine. (Cannot comment on them too much as I can't see any of the side yards.) An experienced person could do this with a Bobcat in a short period of time ... less than 1/2 day for what you need. I hate to tell someone this when they're well along in their project, but I think you will be unhappy in the future to follow the current plan.

    Since the grade is relatively modest, just in case, I'll warn of one thing you cannot do... have the point of termination of the pipe at a higher elevation than it's lowest point. Many people think it's okay to have standing water sit in the end of the pipe, but it may produce unending clogs.

    A "dry well in the middle of [the] backyard" is not a good solution for general drainage of your lot. These things have capacity limits.

    Water would enter a pvc collector pipe through mfg. holes spaced at intervals. The pipe would be surrounded by gravel and the gravel by filter fabric. This would fit in the trench and be covered by a layer of gravel mulch.

  • 11 years ago

    Thanks Yardvaark and Strumhead, I appreciate your comments and agree with you both that a swale would be the best solution for my backyard.
    However, when Strumhead mentioned "side yards" it dawned on me (again - I had thought of this originally before I started this whole project) that I'd have difficulty creating a reasonable swale on the side of my house.
    Let's do the math for a minute. If I create a swale across my backyard with just a 1% slope (even though I'd prefer 2%), I'd end up with a swale that was 7 inches deep when it ended on the far right side of my yard. In reality, it might be half that, if we adjust for the natural downwards slope that is in the land already.
    Then what would I do with the swale once it got to the far right side of my backyard? It would have to turn 90 degrees, to go down my side yard towards the street. My side yard is 6ft wide, so that would mean I'd have a swale right down the middle of it. By the time this swale made it to the street, it would be another 5 inches deeper (a continual 1% slope). At 9 inches deep this swale would get pretty noticeable. It would be more like a gully than a swale.
    And I'd have to rip up my front yard to create this gully. I just created a prototype of it with some blankets in my living room - and yes, 9 inches deep by 3 ft wide is definitely noticeable, it's definitely a gully. Not something I'd want on my front lawn.
    I've attached a picture where you can see a bit of my side yard. If you look closely in the picture, you'll see the white outline of my air conditioning unit that protrudes about 3 feet out into my 6ft side yard. I don't think it's a good idea to loosen the soil under the heavy air conditioner to create a swale under it.
    So, these things are some of what originally moved me towards the idea of creating a trench underground instead of trying to mess with digging up my front lawn and removing some of the support for my air conditioning unit.
    But I agree with you that the swale would look nicer and be much easier to maintain.
    The irony is that the far right border of my yard, where it meets my neighbors' yards, is perfect - the grade allows the water to flow it's natural course. The only problem I have is getting the water from my swale/trench to go somewhere - anywhere. My new grading from my foundation 10ft outwards is great. The old grading in the rest of the yard is great. My problem is where those 2 grades meet (10 ft out) that creates a swale/trench. Once the water leaves the swale/trench I'd have to manage to direct it somewhere. But how? Where?

  • PRO
    11 years ago

    "...9 inches deep by 3 ft wide is definitely noticeable, it's definitely a gully." You wouldn't chisel out a channel in such a profile. It would be blended into the surrounding grade so as to look natural. Another factor is that there is existing slope on your lot and presumably the fore, right corner (as viewed from the street) is the lowest point. So the 9" drop you've calculated from the back yard to this corner is not necessarily a 9" grade differential at any point along the swale. It could be much less or even 0. We'd need to know how much elevation change there actually is in order to predict how things would work and what they would look like. If you haven't measured the elevation change yet, I suggest you do it (very inexpensively with a taught line and line level.)

    You would not remove any support under the AC unit. A swale or any drainage channel would skirt it.

    One statement is unclear ... grade at the right lot line you say is perfect. I'm not clear where exactly the water would be trapped by this neighbor's property. (Maybe at the blue square?) In order to gain understanding, it would probably be necessary for you to measure spot elevations at critical points, labeling them relative to one another. The pic shows the locations I'd start with ... red squares. The one circled is presumably the highest so that's where you'd want to attach a line (to a stake you install or you could use the existing fence post if it's close enough) in order to take the other measurements. (If you don't know how to do it ... fix the line to a "stake" at the highest point. Run the line to a stake driven at the lower point. Leaving the line at the highest point fixed, adjust it at the lower point until the line reads level [place the level at center of taught line.] Measure the vertical distance from the line to the earth at each end. The difference in measurement is the elevation change. For each location to be measured, relate the elevation difference to the same original highest point. It is vertical difference from this point you are interested in.)

    Hypothetically, let's say that continuing a swale to the front yard is impractical. Then the second choice would be to install a catch basin at a low point in the back yard and run a drain pipe from it to an even lower point in the front yard where the water will discharge. You would still need to know critical elevations in order to create this. A catch basin would be preferable to a linear channel (slot type) drain like you are doing now so as to minimize impact to the rest of the yard and lessen maintenance.

    With either a swale or pipe, you would want to avoid sharp turns (90*) and opt instead to make gradual radius turns. Sharp turns invite clogs.

  • 11 years ago

    I would like to second what others have said; keep the water on the surface! You wont know what is possible for a drainage design until you have accurate elevations throughout your property shown on a scaled drawing. Help is here if you don't know how to go about this task.

    If it turns out no surface grading will work, use surface inlets to solid pipe. Put the surface inlets as far downstream on your property as can be done because they wont carry the water of a major storm event. Only surface drainage protects against flooding.

  • 11 years ago

    Thanks for your suggestions.
    Running the swale to the front really won't work. The ground on the side of my house retains grade till the last 2 feet or so by the sidewalk (so my front yard has a mound/hill and then drops at the last minute. This mound/hill also drops about 3 ft sideways towards the neighbor's property, so putting a 9" swale in the middle of the mound/hill would make it really noticeable.
    But, I think I have a better solution.
    If I can't dig down, then why not build up?
    How about creating a berm?
    The berm (not too high, just a ridge) would be just in front of where the trench/swale is on the drawing. So the water running across my backyard (which is the bulk of the water because it starts on the street behind me and comes in from the top left corner), this water would continue to flow through my yard and into my neighbor's yard (like it has for the past 40 years).
    Because I changed the negative grade which used to exist for the 10 ft next to my foundation, the "join" between those 10 feet and the rest of the yard (the trench/swale thing) would have ended up catching the water. But if I divert the water with a small berm before it gets to the "join", then that solves 90% of the problem (the backyard is 100ft deep, so literally a berm solves 90% of the problem).
    Then, to facilitate the flow within the 10 ft of my foundation, I would have to make sure the far right side of the swale stayed the same grade as the beginning of my neighbor's property. I mentioned previously that there is going to be a deck on the left side of my house, so instead of starting to grade the swale from the far left side of the property (like I had drawn (and dug) originally), I could reduce the depth of the swale by starting to slope it from the CENTER of my backyard (where the deck ends and the open area begins). Previously I was going to start the swale from the far left side of my backyard to direct water flow, but with the berm as a diversion, I won't have to. So, being able to have a shallower swale should mean that it should match the natural diagonal grade that I've got going on now.
    The problem came when it looked like my swale/trench/join would be lower than my neighbors' properties (and hence there would be nowhere for it to empty). But this way, the grade on the right side of my yard could continue to be flush with my neighbors' properties and the water would continue to flow as it always has.

  • 11 years ago

    Most US states recognize some form of the Civil Law of Drainage adopted from English common law. I'm not sure what applies in your area but the swale onto your neighbor would be an unlawful discharge here because it consentrates water that entered your neighbors property across a wide area into a single location of flow.

    As to what is possible for your grading, without acurate mapping I don't have a clue ... and neither do you. That may be a harsh way of putting it. Skip the mapping if you want, but I've made a living dealing with these drainage problems for 50 years and the mapping is the first thing I would do.

  • PRO
    11 years ago

    The existing conditions are not becoming easier to understand. Succeed, you're there and can see everything, but for us it's like wearing blinders. If the explanations, photos and diagrams are not sufficient, clear, accurate and correlate well to one another, it's impossible to understand the situation. I still don't have confirmation of where water would be trapped by the neighbor's property. Where would a pond start to grow in a heavy rain?

    Most people doing this kind of fix put water in a 4" pipe and sink it several inches below grade. A 2-inch depth swale--whose gently and imperceptably tapering sides would blend into the surrounding grade all within a 4' span--can carry 4 times the amound of water ... without being subject to clogging. If a problem occurs it will be obvious and easy to fix. In a pipe, one must sometimes dig to diagnose. I can't help think that you are envisioning a swale as being of Grand Canyon proportions, but I think something like that is probably entirely avoidable. We don't yet know if it needs to continue all the way to the front sidewalk. We still can't know what's possible without comprehending the site.

    Admitting that I'm unclear on several factors, a berm sounds on its face like a compounding of the problem. And where would it divert the water to?

    You're faced with a fair amount of work to complete your project. My instinct tells me that the next owners of the property are not going to appreciate it like you think you will... and that in a year or two, you won't like it either. Elevation mapping of the site--at least the critical portions of it--sounds like the thing to do. It's not difficult or terribly time consuming. For one thing, it will allow us to comprehend the conditions without mystery and that will be the basis of good advice.

  • 11 years ago

    Just shoot me now and get it over with.
    I went to Home Depot, bought a laser level, bought plastic stakes, bought some new mason string for my line level, bought some shims to pound into the ground - I had good intentions.
    However, once I got out in the yard, I discovered that taking accurate measurements was a challenge.
    The trench is already filled in (I did that immediately after you nixed it the other day). So the first 10ft next to my house is sod, and the previous trench is just dirt that has not been graded yet so there are lumps and bumps, and sodded and non-sodded areas.
    Point A to point B (10 ft) seems to drop 1 1/8", A to C seems to drop 1 1/4", A to D seems to drop 1 1/2", area E seems to drop about 1/4" in 10ft, but E to F gains about 2 inches cause of underground root growth. And as mentioned before, F to G drops about 3ft to my neighbor's property.
    The weather is against me. If I manage to finish grading the yard in the next few days, perhaps I can return and give you more accurate measurements.
    Pls8xx, thank you for your comments. I will continue the swale (which is now barely noticeable) around the corner, and let it disburse across a larger area, the way it always has. There is no need for it to feed water in a concentrated way.
    Yardvaark, thank you for continuing to encourage a swale - that is definitely the best solution. Now that the trench is filled in, the "swale" is barely that, barely noticeable. If I continue it around the corner, and down the side of my house, it can go as far as the front of my house - the water should have disbursed by there because of the sharp drop to my neighbor's property - and this is the way it has always been. So there is no need to take it onto the front lawn. Yes, the "tree roots bulge" on the side of the house will have to be fixed.
    Finally, I'm not sure what you mean by your "water being trapped by the neighbor's property" question. In previous heavy rains, the water used to pool on the left side of my backyard and a bit by A. But both of those areas have been re-graded this summer, so it shouldn't pool in either of those places now.
    Finally, thanks for all your helpful comments and suggestions. I appreciate all the time and effort of each of you.

  • PRO
    11 years ago

    "If I manage to finish grading the yard in the next few days, perhaps I can return and give you more accurate measurements." It should be that you take measurements, plan, then grade ... so as not to do wrong work that must be corrected.

    It seems that you have no drop from point B to point C. You will not want a swale to make a right angle turn at C. You'll want it to curve gradually around the corner.

    And that's what I'm thinking ... that water will spill off into your neighbor's yard along the length of your side yard, eliminating the need to a trench into the front yard.

    For the tree root issue, you might add a photo that shows it and the surroundings.

    Not clear about your measurements E & F. Are those exact points or general areas? It would be better to have specific point measurements at the lot line (and assurance that there is sufficient fall away from the house.) Such as this...

  • 11 years ago

    Thanks for your help.

    I've taken measurements of the "swale", path the water will take, which is basically the previous trench that has been filled in. When I said I wanted to "finish grading the yard in the next few days", I was referring to smoothing the peaks and valleys out of the swale (filled in trench) and making sure it still aligned with the slope from the first 10 feet from the house. Maybe I can get that done when the temp goes up to 52 again next week. I've already adjusted the measurements below to account for those peaks and valleys.

    K->J - 10ft - drops 2 3/8"
    J->B - 10ft - level
    B->C - 5ft - rises 5/8"

    So to me (hopefully to you too) it looks like I would have 3/4" drop from K to C, over the 25ft, which is not enough. So to me it looks like I have to raise K by 1 1/4" (and adjust over the 25ft) if I want a 1% slope.

    The measurements for C to H were all taken along the property line.

    C->L - 5ft - drops 5/8"
    L->D - 5ft - rises 3/8"
    D->E - 5ft - rises 1"
    E->F - 4ft - rises 7/8"
    F->M - 4ft - drops 3/4"
    M->N - 10ft - drops 3"
    N->O - 10ft - drops 3/4"
    O->P - 10ft - drops 6"
    P->G - 10ft - drops 5 1/2"
    G->H - 10ft - drops 11"

    O->H is the front of the house, and I don't want to touch that.
    So to me, it looks like I have 2 7/8" drop over 43 feet (from C to O). And I would really need 5 1/8" if I wanted a 1% slope. But C->O doesn't really matter much, as long as I don't have a negative slope anywhere, because, the intent is for the water to go to the side over the property line, not down to the front. So, if I evenly distribute the 2 7/8" from C to O, it should be fine.

    Does this sound right to you?

    This post was edited by succeed on Thu, Nov 29, 12 at 13:44

  • PRO
    11 years ago

    While 1% slope theoretically will drain, in reality it won't do a very good job unless it's a hard paved area, which this isn't. It would be to your benefit if you can increase the overall slope. Some situations aren't perfect so we make them the best we can. If the side yard gets a little soggy after a long rain it's not the end of the world, but you should make sure that there is sufficient slope away from the structure so that there is no chance of any pooling water reaching it. If it will be spilling over the lot line along the way, that will help. IF you can add a little elevation at your starting point (near the deck as you mentioned) that will help your result down the line as well.

    I converted your measurements into elevations starting with 0" at point H (so we won't need to deal with negative numbers.) Point K is 28" above this. I added proposed elevations (both 1% and 2% ... I see in the preview that the dark red--2%--looks almost black, but I think you can figure out which one it is) at point C and point O. This makes it easy to see what your goals could be. While it might be difficult because of obstructions, to achieve a 2% slope swale, it would be better if you could. Because of the greater slope in the front yard, you wouldn't need to encroach too far into it. You might end up being able to create a compromise and achieve more than 1%; the more slope you can incorporate, the better.

    Rather than starting at one end of the swale and dig its entire width all the way to the other end, it would be better if (once you determine the route) you carve out only the center of the swale. You could do this with a pick/mattock. It only needs to be about 3" wide ... just enough so you can observe how its grade will relate to its surroundings. After the center line is carved you can easily appraise whether the swale grade will be ideal or if it needs to be altered. Make it to your liking and then, once it is, you can excavate from the center outward to each edge, blending it into the flanking grade. While creating the center line you'll need to double check that the elevations create a satisfactory slope. But for the rest of the excavation, you'll be able to eyeball that the swale profile is adequate. Creating the center line first like this may prevent a lot of wasted digging if alterations need to occur.

    As far as cutting tree roots, I'd be inclined to cut through them without much reservation up to about 5" dia. (This is pretty easily done with a 10" folding pruning saw once you remove most of the soil from around the root. Just cut out a section.) For larger roots, you might post a photo of the root and the surroundings, including the tree.

    My math is not double checked and certified so could have errors. But it's just to give the general idea. If I've overlooked something key, hopefully pls8xx or someone will jump in and bring it up.

  • PRO
    11 years ago

    A couple more thoughts ...

    IF you could raise the grade along the R side of house (without creating any undesirable consequences) the swale at the R side yard could "evaporate" sooner, spilling water over the lot line rather than carrying it in the swale farther toward the front of your yard.

    While the points measured are along the lot line, the actual center line of the swale will be set somewhat away from the lot line and a little closer toward the house. You won't be able to gauge the width needed for the swale until you see it's depth relative to the surrounding grade. If deeper it will be wider. If shallow it can be narrow. BLENDING grade so that none of it seems abrupt would be the goal.

  • 11 years ago

    Thank you so much, Yardvaark, I really appreciate your analysis of this. I think things are getting much clearer.

    Looks like I made a typo previously. Looks like I really have a drop of 1 3/4" from K to C (not 3/4" like I stated).
    Yes, I agree that 1% slope is not much, but I can't add 4 1/4" to K to achieve a 2% slope (between K and C), because the drop from the house (A) to the swale is only 2 1/2" (2%), and raising K by 1 1/4" is already a compromise.

    There really is no digging for me to do - right now it's more of a case of filling in different areas. The trench/swale had been dug before, then I filled most of it in, and now there's still a bit more at the surface that will need filling in, to achieve the right grade.

    My plan is to start at C (which cannot be changed because it needs to be flush with my neighbor (or higher)), and from there work backwards to K, distributing the added height of 1 1/4" evenly from C to K. Then once the swale is at the correct height, I would fill in the dips from the house to the swale (10ft) to make sure that that slope was even and consistent. Then finally smooth out the grade in the rest of the yard as it approaches the swale. Then seed the areas in the spring. Will that work?

    I'm assuming that to build up the grade between the house and the swale I can just add soil over the new sod and then seed it in the spring? I would need more soil added closer to the swale, and then smaller amounts as I approached the foundation (I can't bring the grade any higher at the foundation).

    In my view, the slope from top to bottom (from C to O) is a non-issue, as long as it's consistently smooth with no negative slope anywhere, it should be fine, because the water from the backyard is not meant to flow all the way to the sidewalk - it should flow over the side instead onto the neighbor's property (see diagram below).
    So, there's really not a need for an actual swale on the right side of the house - I just have to make sure the soil beside the property line is not lower than the neighbor's and that my slope from house to property line is positive.

    Again, I really appreciate all the time and effort you've put into this and it's helped me immensely.

  • PRO
    11 years ago

    "(from C to O) is a non-issue, as long as it's consistently smooth with no negative slope anywhere, it should be fine,..." Agreed.

    As I mentioned you may not be able to create 2% swale slope, but be ever watchful of being able to create as much as you can (within reason!) 1 1/2% is better than 1%

    "My plan is to start at C (which cannot be changed because it needs to be flush with my neighbor (or higher)..." It can be lower if needed. Where you choose to have the swale center meet the property line is a determination you will make. See my illustration. (And remember, you are going to create a path that curves around the corner, not one that makes an abrupt turn.)

    "...(I can't bring the grade any higher at the foundation)." If the high points can't go higher, this means that you can only solve the drainage problems by lowering the elevation of the lower points ... the drainage path. And it sound like you have a good grasp of this.

    If I had nice sod and needed to raise it's grade, I'd add soil gradually, in shallow layers (1/2" - 3/4")--clipping the grass short first-- and allowing the grass to grow though each layer before adding more soil. (If possible.) But this requires the growing season. It depends on how deep and large the areas and what kind of grass and the climate. Depending on all those factors, you may want to delay this part of the operation. Then again, maybe you don't want to or can't. Likely, whatever you do will be fine in the end result.

    I'm glad my efforts have helped. It's been nice that you seem to have a good understanding of the basic concepts and some practical sense about this type of operation. Most people would not know what a percent of slope is! For some reason, you do. :-)

  • PRO
    11 years ago

    It might be helpful if I actually post the illustration!

    Where it's convenient and works out best for your project is where you'll shift the swale center line toward the property line so the water can spill over the edge. Remember, you don't want to concentrate it. The swale would flare and flatten so that sheet flow is spilling out of it. In a heavy rain the swale would swell its banks and water would spill over the property line BEFORE it encroaches too closely to your foundation.