Dry Wells, Retaining Wall Drainage and French Drains

by Sylvia Redmond
(Naugatuck, Connecticut)

Here are questions about dry wells, retaining wall drainage and french drains.


Good morning.
My yard has a hill that comes towards the back of the house.There is about 20/25 feet of flat before the house. It becomes a soup after a heavy and continuous rain and of course after the winter. (I live in Connecticut.) I do have a small stonewall, but need piping in back of it to redirect the water. What is the best way of installation? (There are no base perimeter drains around the house so I expect water in the basement under certain conditions) Once the piping is put in, the man who is going to do the digging, recommended a dry well. How far away from the house should it be? How big? How many barrels? Should I put in french drains also? I would prefer to overkill than to continue with this as it is.

My lot is 125 deep, 30 in front, 25 house, 20/25 flat and balance hill. There are trees and wild things up there with a small shade garden I'm trying to maintain. Between the slugs and all the rain this year, oh well.

Thank you for your time.
Sylvia Redmond
I don't know how to do the picture thing.

ANSWER

Hi Sylvia,

Regarding the Retaining Wall

Take a look at this page.
Retaining Wall Drainage

Also, there are lots of grading Q&A at the bottom of this page.
Landscape Grading

It's different than your scenario since the water is flowing away from the house in the photo. However, the way to lay the pipe behind the wall is the same.

The pipe behind the wall would be a perforated pipe with holes in it to allow water in. The pipe is pitched so that the water travels along the pipe and then connect to a solid pipe. The water in this pipe and the pipe itself must lead somewhere to bring the water away from the wall.

If there is no optimal place, then a dry well would be used. I would consult with an engineer as to size, placement, etc., as this is all determined by the amount of water that will be entering the dry well. It should be pretty far away to handle water for that 100 year storm (the amount of water that would accumulate in a huge storm that happens once every 100 years, although it seems that 100 year storm is coming more often!!!).

The dry well might be a concrete drum, or more simply it can be filled with stone. It will be quite deep...8 feet or more is likely, and quite wide...8 feet wide or so. Again, I would always run it by an engineer.

Regarding French Drains

You have two options here.
1. Create a swale between your house and the retaining wall. See Lawn Drainage

2. Install french drains at the foundation/perimeter of your house. These can be led directly into the dry well. However, if you do this, the size of the dry well will have to accommodate more water and the dimensions will be different than if it was needed only for the retaining wall piping.

Which method you choose will depend on ease of installation and costs. With the swale, your property will be dug up and new lawn will have to be installed. With the piping, the disturbance is less, but may or may not be more expensive, depending on how much pipe is needed.

I have both on my property...a swale for one area and a french drain for another. The swale leads water away from your house which is good. The french drain method allows the water to drain towards the house but collects it before it does any damage. I'd have to see the lay of your land and property before I could recommend one rather than the other. But as you mentioned, you don't mind some overkill, so do one or the other to fix your drainage problems once and for all...peace of mind is fabulous!

Susan

Comments for Dry Wells, Retaining Wall Drainage and French Drains

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Similar Concern NEW
by: Lindsay

I have a similar situation in which our backyard slopes down toward the house. The house is essentially sitting on the middle of a hillside with a very tiny flat area that makes up the backyard. We are planning to dig into the slope to expand our backyard and put up a single 3' retaining wall that will run about 58' across (parallel to the back of the house). I have read the recommended pages and have a few follow-up questions:

1: "Over the wall" flow. Rainwater typically flows down our sloping yard like a heavy stream before it puddles in the flat area behind our house. How will the retaining wall drainage mitigate this? i.e. will the water drain behind the wall and out through the drainage pipe? If that is the plan, does this imply that the gravel needs to be visible all the way up to the surface or should a catch basin grate be used? I am concerned we will build a lovely wall with ground-water drainage but still have the unresolved issue of our swamp.

2: Drainage outlets. We plan to dig alongside our house (still sloping down toward the road)in order to provide a proper outlet pipe for the water in the wall's drainage pipe. However, I'm confused about whether we need one outlet or 2. Should we plan to "cap" one end of the pipe and provide an outlet on the other? Or should we anticipate digging up both sides of our yard?

3: "Tying in" the gutters. There is one gutter spout in particular that also flows water from the roof into the flat backyard area. We would like to tie this spout into our drainage system. Can you provide any website/image resources for how to do this?

Thanks in advance!

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