Making one side of a rain garden stable and firm

Hi,

I live in the central United States where we get almost 50 inches of rain each year.  I'm in a historic district where the houses are quite close together, and I've been working hard to redirect the water coming off my room, partly into a huge rainwater tank and partly into a rain garden.  The issue is that the side of my rain garden is about three feet from my neighbor's concrete driveway.  I need somehow to stabilize that side of rain garden and leave him a walking path between that edge and his driveway.  

My current idea, which seems inadequate, is to somehow build up a narrow berm or perhaps a row of natural stone or blocks following the curve of the raingarden and then using more concrete to try to stablize everything, even if it would need to be repaired every year or so to keep it from crumbling away.  Given how narrow this lot is, the limited incline of the land (which is mainly working against my purpose here), and the fact that I'm successfully feeding about 800 square feet of roof rainwater through a 4" schedule 40 pvc pipe (with plenty of fall to give the water momentum) into a pop up which daylights in the middle of the rain garden, moving the whole thing to another place is not really an option.  One flaw in the current design is that when we get a super rain event (as we did with the front that came through a few weeks ago), there's isn't a great way to direct the overflow.  If I could raise the entire level of the rain garden itself, it might be possible to send the water across the sidewalk into the street/storm drain.  Also, once I finish digging it out, I will plant the rain garden with native wildflowers that can withstand periodic inundation and also hot sun.  (A very helpful horticulturalist who runs a local native plant nursery was very generous in pointing me to the best plants for this situation, which I purchased from him and am ready to plant.)  

Returning to the question of stablizing the edge of the rain garden--which, incidentally, seems to perc pretty well--I realize that soils are wildly unstable inherently, so there probably isn't going to be an ideal solution.  One thought was to fold some hardware cloth or remesh and pin it against the bank somehow, and then going over that with concrete.  It's all pretty wonky, I confess.  I know that folks who do stream restoration where there is a great deal of erosion (e.g., North Texas Blackland Prairie) speak of "hard armoring" with gabions or stacked sacks of concrete vs. "soft armoring" with plants, but the area I am working with is pretty tiny, so I'm not sure any of that is really relevant to my case.  So, in summary, there are two questions:  1) Is there a way for me to make this rain garden better for my neighbor, which means that it's better demarcated, by a berm or some other structure that is at least semi-stable?  2) Is there a way that I could get the water to overflow to the street without entirely regrading the area all around the rain garden?  (I should mention that provided the water doesn't flow backwards towards his house, it not a huge problem if it simply overflows onto his driveway and then out to the street. By "entirely regrade" what I have in mind is that I don't want to bring that whole strip on his side above the level of his driveway.  It would be okay to have a narrow berm of some kind and then a narrow strip where he can walk without having to enter into his carport area.  My thinking here is that sometimes he has guests and they need to directly access his backyard when the carport is filled with vehicles.)

As you can see from the photo, on my side, I've got a row of concrete blocks and am poised also to add a couple wood fenceposts and some cables between them in order to create a fence over the top of said blocks.  Where I slopped concrete onto the soil I used a product that had some fiberglass mixed into it, but I really don't have much confidence that that won't start cracking apart soon enough.  All these issues really arise from one root cause, namely, the lack of space.  Just for info, the whole back yard is heavily planted and contains a micro food forest, a hand-dug wildlife pond, a long in-ground vegetable bed, four above ground beds, muscadine grapes, a fig tree, pawpaws, a persimmon tree, a pollinator patch, a rock garden, an outdoor room, etc.  In other words, it's a bit of a backyard permaculture operation.  In keeping with that ethos, I would prefer if the raingarden was aesthetically pleasing and not an eyesore.  Planting is going to help with that, but my wife is already looking at me with a furrowed brow whenever I talk about slopping more concrete around it.

Thanks in advance for your ideas, comments, and suggestions.

Mark

Hi Mark, welcome to Permies!

I'm no expert, but one way to make a slope stable is to build a wall from unmortared rocks, where each rock slopes down into the soil. The rocks will want to slide inwards, but are stopped by the soil. The soil will, in turn, be held in place by the rocks. The absence of mortar means that the rocks can move a bit without the whole thing cracking and crumbling. See below for a trashy five-minute Paint drawing of the building technique, and a picture of the foundations of our future cabin which was built like this and has survived a couple of winters, as well as being walked on a lot, without any issues.

I like the suggestion by Eino.

I was going to suggest using brick, though.  Maybe some wooden stacks to hold the bricks in place.

into a pop up which daylights in

what does this mean please?

'Rain gardens, also called bioretention facilities, are one of a variety of practices designed to increase rain runoff reabsorption by the soil. They can also be used to treat polluted stormwater runoff.'

I think you are over thinking it.
I have you talked with the neighbour, he may have ideas?
From. https://www.gardendesign.com/eco-friendly/rain-gardens.html
there are a few ideas.
I believe your wife is on the money, no more concrete is needed, consider river rocks or broken rubble to allow a volume of water to sit and soak.
If you go deeper use a concrete wall in the ground to stop the drive collalsing into the 'mega pit'.

Thank you to the folks who replied.  The ideas were very helpful and confirmed my thought that Permies is a deep well of practical information.  (This was my first time posting.)  My solution is now to not let the perfect become the enemy of the good; in other words, I've decided to trim back my aspirations letting safety and the interests of my neighbor effectively preclude the idea of a true rain garden that would have been planted with natives.  Thus:  I will finish digging the pit and use some of the soil to level the ground between it and the neighbor's driveway (not the one with the concrete blocks but the other one).  Next I will fill the pit with washed gravel.  Then I will use natural looking edge stones to create a ridge all around the contour of the pit.  This should enable the water level to rise high enough to flow out towards the street during a heavy rain event.  I've now added a trellis to my side and am training flowering clematis vines along it, so that well at least soften the look and provide something of interest for our insect friends.  The natives I purchased are being potted up and will eventually find their home on top of the gravel-filled trench that I was originally envisioning as being a rain garden.

Hi Mark (belated welcome!) I like what you're doing and look forwards to seeing the results. Personally I quite like the 'dry river bed' aesthetic in a garden and planted with appropriate plants think they give movement. One good thing about not using concrete is that things are easier to adjust if you need to.