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We are redoing our front yard as part of a city rebate program to transform turf to drought-tolerant landscape. Part of the project is a Rain Garden / Swale that will be installed in the middle of the Front Yard. Another project I am trying to do is create a French Drain for a small section (about 8 feet) around a bay window in the front of the house that keeps accumulating water. Given that there is no way to connect the drain to the sewer I thought I could take advantage of the rain garden. Here's how the area looks with the french drain drawn in:
enter image description here

Currently, the area slopes towards the foundation of the house, like the sketch below, which is a problem as I always get water standing there:
enter image description here

French drain and some regrading hopefully will address that issue, like on the next sketch:
enter image description here

The French drain itself is only around 8 feet long and the pipe to the center of the rain garden will be about 10 to 12 feet. What minimum inclination will it need? I'm thinking about burying the French drain pipe about 12 to 14 in below surface that should still give me about 4 or 5 inch over the course of the 10 feet. Is that enough?

The one worry that I have is that if it rains a lot and it fills up the rain garden, water will back up and accumulate around the foundation. But I live in SoCal, so those days are not very frequent. And it certainly won't be worse than how it is currently. Any comments/suggestions much appreciated if what I'm thinking about make any sense.

Here's a birds-eye-view sketch. enter image description here

EDIT:
Thanks for all the comments, I have one additional question. For my use case, would it make more sense to have an open French drain or a closed one? A closed one would allow me already to redirect some of the surface water via grading away from the house.

enter image description here

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    Just a note for the first paragraph: In most places, draining storm water to the sanitary sewer (if that's what you went by sewer) is illegal anyway, so you do need an alternative.
    – blarg
    Dec 12, 2023 at 10:40
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    Yes, that would work, as long as the "brim" of the rain garden is lower than the top of your french drain. Having said that, is there any reason you can't simply put a small catch basin at the low point of the grade by the bay window, and run a pipe without having to do a full french drain? It'd save some money. Or are you trying to make the project excavation net zero?
    – Huesmann
    Dec 12, 2023 at 13:15
  • @Huesmann Since it's not super long anyways, in terms of excavation there won't be a big difference between catch basin and french drain. I was just thinking French drain as it will cover the full length around the foundation and thus give me more protection for the house.
    – jan
    Dec 13, 2023 at 0:13

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Slope should be 1-1/4" to 2-1/2" in 10 feet (1/8-1/4 inch per foot) - not more, not less.

More leads to build up of infiltrated soils in the pipe (water runs away quickly and heavier stuff gets left behind) less does not drain water reasonably. There will always be some infiltration into the pipe, even if you use filter fabric and stone around the pipe. Over time an excessively sloped pipe will end up full of dirt.

As for "can you drain it into your rain garden" I see no reason against it, but you could ask your city program that's sponsoring the work. I assume they'd like the additional water going into the ground, though they might require more size/capacity so the rain garden accepts the water rather than overflowing for whatever the "design rain event" is.

Typically a properly designed rain garden/swale has "relief" for water beyond the "design rain event" - so after it's filled to capacity, water drains in a designated direction, rather than backing up willy-nilly. By correcting the grade, you may not need the pipe at all. In any case, you can run it into the rain garden at a somewhat higher level by not excessively sloping it.

If the water in your bay window area is primarily coming from rain events, you can put the French drain (if used) shallower and further away from the building by laying the waterproof membrane more out than down. A deep French drain is more typically about managing sub-surface water. Surface water can be managed entirely with grading, or with a combination of grading and catchments/shallow drain pipes.

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  • This is helpful. But I guess, I wasn't clear, the pipe going into the rain garden over those 10ft. won't be perforated, that's just going to transport the water from the French drain to the rain garden. So the slope won't matter all that much.
    – jan
    Dec 13, 2023 at 0:33
  • Yes, it will. Dirt will infiltrate the French Drain, so it will build up in the solid pipe if it's sloped wrong. You won't gain anything by using solid rather than perforated there, anyway. If it's sloped wrong the other way, water does not flow uphill. If you need to drop, properly sloped to vertical drop to properly sloped works. I have experience digging out pipes laid by people who thought "some slope good, more slope must be better" and they were chock full of dirt/mud.
    – Ecnerwal
    Dec 13, 2023 at 0:38
  • OK, I see, so I guess with a drop of roughly 2inch over 10 feet should be fine. Then the outlet will also be one or two inches above the lowest point of the rain garden, so the water should have a lot of space to fill before it builds up. Rain garden is about 4ft wide and 10ft long.
    – jan
    Dec 13, 2023 at 0:44
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    How exactly does a steeper pipe end up full of dirt?
    – Huesmann
    Dec 13, 2023 at 13:44
  • Quote: "More leads to build up of infiltrated soils in the pipe (water runs away quickly and heavier stuff gets left behind)"
    – Ecnerwal
    Dec 13, 2023 at 13:47

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