For now, could you focus on explaining the problem and let the solution go until later? Illustrate with photos the things you are talking about: retaining wall ..... eroded it's way under his wall. ..... created a channel where water flows underground ...... boils up in my backyard out of a sinkhole ..... settles right next to my house. One must grasp the ENTIRE PATH OF DRAINAGE in order to advise.

In your existing photo I can't read any of the print at the back. The picture only illustrates a general drainage swale but does not explain (show) the details that are spoken about.

OK, then, more extensive explanation:

My uphill neighbor has a retaining wall made of poured concrete, pictured below. The wall is very close to grade, so you can't really see it too well in this picture. You will notice some straw about in the middle. There is a deep eroded out channel that gets clear under the neighbor's retaining wall, flows underground, and boils up in my backyard. My house is in the background. Ignore the labels if you can't read them, I can't make new pics right now with bigger labels. Just now that the brush in the middle conceals said eroded channel.

Water travels under the neighbor's retaining wall and boils up out of sinkhole in my yard. This is a picture in my yard, uphill of the sinkhole, with my retaining wall, the fence, and then my neighbor's retaining wall to my back.

Here is another picture from the side of sinkhole, looking down my backyard's drainage swale. Again, ignore the labels if you can't read them, I'm not where I can make them any bigger at the moment. Most water that falls in my backyard drains down the drainage swale. Water boiling up out of the sinkhole does not. Water that falls between my retaining wall and the neighbor's retaining wall is drained off by an existing drain behind my retaining wall:

It is practically impossible, or near impossible, to plug the eroded channel from my neighbor to me. Therefore, the water must be diverted. My current plan is to run a french drain (the kind with perforated PVC pipe) through the sinkhole itself, and drain it off downhill into my drainage swale (seen in third picture). Then I plan to regrade the yard to discourage water from pooling near the house. When the water does boil up from the sinkhole, it does so energetically. I wouldn't call it a fountain or a gusher, but it comes out with some authority. I don't want to rely on only regrading there, as I'm somewhat limited in what I can do given the current slope of the yard. Plus, for aesthetic reasons, I don't want to keep looking at that nasty sinkhole either.

I want to know if a french drain through the sinkhole is a reasonable course of action for diverting the water that comes out of it. To put it into perspective, my neighbor uphill is a two story office building that's about 80 feet long with an asphalt parking lot. I'm getting about half the runoff from that parking lot and the runoff from at least two of that building's downspouts.

When it comes to drainage problems, it must be determined, without any vagueness, what the actual cause of the problem(s) is. That's why I'm being picky about your explanation, and not concerned for the time being about your proposal for a solution.

As background, has this problem always existed since you owned the house or is it a problem that developed, or has been developing, recently?

We must understand the neighbor's retaining wall and exactly how it eroded. A good, well lit picture of that and an explanation would help.

We must also understand what you are calling the "sinkhole." Is there piping of any kind draining your neighbor's yard that runs through your yard? Or is there piping belonging to you that is collecting the neighbor's water? Can you see any piping in the ground that connects to this "sinkhole?" ... which I shall hereafter refer to as the hole. If you don't know, it would be time to do some exploratory digging, starting at the hole and trace its path until you understand where it is coming from.

Also, I want to mention that municipalities have regulations preventing uphill neighbors from concentrating water and dumping on their downhill neighbors. We need to find out if it's what's happening here. If it is, it would be a mistake for you to "fix" it with a half-baked scheme that will end up causing problems down the road.

I think this problem was impending when I bought the home, one year ago. It didn't come to my attention until I started getting water in my basement. That started around October.

It will be next to impossible to get a picture of the eroded hole on the neighbor's side. If I can spare time I'll try again tomorrow, but we're not likely to get much out of this. There is a good bit of brush over the eroded spot. I do know that a city code inspector tried running his probe into the eroded hole and it was deeper than his probe (his probe was about 3.5 feet). The sewer agency also pumped water into the hole on the neighbor's side, water which promptly came out of the hole on my side. The neighbor's retaining wall is poured concrete, about 2 to 3 feet tall.

There is no piping that drains the neighbor's yard through my yard. I have a french drain around the back of my retaining wall, which would catch any water that tops over the neighbor's retaining wall.

There is no piping in the ground that connects to the hole. I know for a fact that the hole at the edge of the neighbor's retaining wall connects to my hole because water has been pumped through the hole on the neighbor's side, which then came out the hole in my yard. For further proof, the neighbor filled dirt in the hole to try to stop. Said dirt promptly washed out during the next rain storm and that dirt was deposited in my yard, which you can see from the picture.

As for municipal regulations, there are none that compel the neighbor to fix this problem on his side. The water I'm getting here is entirely from natural surface runoff that found a channel under the neighbor's retaining wall that is steadily eroding. The state I live in, on these matters, follows the Common Enemy rule where water drainage is concerned. Short version is that my neighbor uphill is not under any legal obligation to do anything if he doesn't feel like it. The city code inspector has already made that crystal clear and I can't afford a lawyer to challenge it. And even if I could, I doubt I could win.

You say there is no conduit carrying water underground from your neighbor's yard to the "hole." But there has to be a cavity, acting like a pipe, for water to run through as you say it does. One of the common problems of underground pipe is that it clogs and why can't this principle be used to seal up this underground tunnel? You've put loose dirt in and it's washed right through. But what about putting things in it that CAN'T wash through and which have the propensity to trap silt and clog it further ... permanently? Problem for me is I can't see what we're dealing with ... the opening.

Regarding what you've been told by the city ... often people say what absolves themselves of any involvement and what is most expedient. No guaranty it's correct information and I doubt it is. But if you can't involve a lawyer, so be it.

I agree with where yardvaark is going with the conversation: look at the root of the problem. I may be misunderstanding something here because like yaardvark said we could use some more details, but I'll try to read in to what I think you are describing:

From what I can tell from your description, there are two problems:

1) water can't get out from behind the neighbors side of the wall in a "normal" way.

2) your yard can't deal with the water, especially when it comes "forcefully" out of the sinkhole in that location.

I'd be worried about silt clogging your proposed french drain, and/or the "inlet" of the sinkhole to your propsed drain system moving/collapsing. If it's currently a dirt tunnel, it will continue to change in size and direction as the water carves it. BTW: I'd put money on it that this was happening before you bought the house too- holes like this don't arrive in a year (unless the wall was put in within the year).

More on #1: By "normal" I mean that a sinkhole is not normal, and as you've said it concentrates the water from both left and right sides of their lawn into this one hole. Walls that have this much water from other yard areas draining into the back of them should not be a solid "dam" of concrete: Do a google images search of "drainage behind retaining wall" and you'll see lots of photos showing a clear rock layer behind the wall, and tubing in the rock layer to have water come out the face of the wall or thru a tube to an appropriate exit location.

The city may be right that you can't enforce a rule that says "water can't drain downhill"' because gravity will always move water downhill. But you should be allowed to prevent concentrated outpourings of water from sinkholes in your yard that are flooding your basement.

Who pays for it may be an issue, but here's a proposed improvement:

I see two options for drainage behind the wall: add drainage behind the full width (or nearly full width) of the wall and have the tubes connect to either a) multiple spaced face drains cut in the wall. This should distribute the water to have it come out more evenly and on the property line instead of so close to your house, then you stand a chance of managing it on your side. And/or b). Put drainage behind the wall and route a tube to the side/front of your property where it won't cause problems (and put small rock bed and/or distribute the water at the exit to spread out the water and to slow erosion at the exit).

Even with drainage behind the wall, you should correct grade problems on your yard: surface grade away from your house and make sure the swale drains to the side/front as you mentioned.

Good luck with your project.

This reminds me of another question that should have been asked, and that is, "what was the normal way that water was draining off of the neighbor's property before the breach of his retaining wall?" Was is working well ... or can it work well if repaired to that state?

Yardvarrk:

I had a landscaper come out and look at my situation yesterday afternoon. He thinks that 1) the tunnel was likely dug by an animal at some point (he thinks ground hog) and that 2) the tunnel can be plugged by digging into it from above, packing tight with fill dirt, and doing a *proper* repair on the neighbor's retaining wall. So far this is all coming at a price that I can stomach so I'll probably try this first.

As you say, the hole is actually a cavity, or small channel that the water uses to move from point A (neighbor's retaining wall) to point B (my yard).

As for whose paying for this repair, I'll do it if I have to, though I might see if the neighbor is willing to contribute. This will be likely saving his retaining wall from eventual destruction after all.