runoff to french drain at approach

chaparral(Zone 9)March 25, 2012


I live in Southern California, and in our remodel project we are implementing a few things to lessen our eco-footprint including solar, gray water, and rainwater capture. One of the last things we are trying to do is to encourage runoff from our driveway to seep into the ground rather than down the street. However, we do want to use concrete (for basketball) and we don't want to use permeable concrete (too much of a specialist product).

To this end, we our thinking of putting a french drain right above our driveway approach. It would be about 30" deep and 30" wide x 23' long, and filled with3/4" gravel. The concrete of our driveway would go over this except for a channel drain the width of the driveway.

We are thinking of using something like the Fernco39.5" single storm drain channel with gray grate

Or the NDS Pro Series 5 in. x 40 in. Channel and Grate Kit

And then drill holes in the bottom of the channel so that water can drain into the french drain.

This seems like it would be a relatively low cost way to achieve what we want.

Does this make sense? Has anyone done something like this before?

I would have thought there would be more online about diverting runoff at a driveway approach but haven't found much.


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Im not familiar with these products however just be sure that the pipe is graded down (at angle away from the house) from where you begin to lay it-make sure it is not layed level. You may already be aware of this but so many people who install a french drain don't do this. The water ends up filling the pipe and possibly leaking into a basement if there is one or enough grade/angle in the pipe.


    Bookmark   March 26, 2012 at 4:31PM
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If I understand your proposed scheme, you're going to create a subsurface, gravel-filled trench across the drive and imbed a channel drain in its upper portion (which is the driveway)...? In theory it sounds reasonable, but I suppose in practice, that's a lot of hole-drilling to actually make into that channel to handle the volume of water. Drains have away of silting and clogging up as though it's their objective. Seems like unless the holes were sizable and numerous, the drain could fail. Then again, a french drain is subject to failure after a while due to silting up. Then again, if this drain fails completely, the only downside is that the effort and expense of creating it come to a premature demise.

I consider french drains a last resort because of their limitations. Being filled with rock, they're inefficient in how much material their construction requires relative to their capacity to store water. Seems to me it would be much more efficient to have a sturdy underground storage vessel that was strong enough that it didn't need to be filled with rock... then to have a conventional water collection and piping system (like a trench drain with no holes and some 4" pipe connecting to it.) I'm thinking that a buried section of standard concrete pipe could be fitted for use as a retention device...kind of like a cistern that leaks on purpose. It would not need to be near the size of a french drain to hold the same amount, or more, of water. It would not need to be located directly under the drive. And I think it would be much cheaper as the pipe itself is not expensive. They're a little heavy to move, but not impossible.

This is a little tangential but for fun, here's a link to a video that shares some secrets of manually moving very heavy objects.

Here is a link that might be useful: Guy explains moving very heavy objects

    Bookmark   March 27, 2012 at 12:00AM
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chaparral(Zone 9)

Thanks for the comments-- a few more details.

First, the "drain" (really more of a catch basin) will be about 20' from our house, so no issue of it seeping into our garage.

Most of our roof water will be collected in above ground cisterns which when overflowing will go back to groundwater. About 600 square feet of collection in total will feed water through our driveway to the street. Our soil is REALLY sandy and percolates very well. We just want to do a relatively inexpensive catch basin to divert runoff before it hits the street. Right now our driveway hasn't been poured.

Our GC suggested digging out a trench about 18" x 18" and filling it with 3/4" gravel, then forming up the concrete on our driveway so it covers all but a 6" strip o fit. We would then put a plastic grate on it. So no channel drain per se. If it overflows in a torrential downpour then it will just go to the street. But like I said the soil is really sandy. So there will be (18" + 18" + 18") x 20' of surface area for the water to permeate--about 90 square feet. The driveway itself is about 400 square feet.

I'm thinking then this is somewhat equivalent to about 90 / 400 = about 23% permeability. I had considered permeable concrete, which is 30% permeable, but does suffer from clogging.

We think this additional work will cost us about $700 to do.

Interesting idea of burying a concrete pipe--but I guess the missing piece of my original post was how permeable our soil is.

Thanks again for chiming in!

Some day I'll post about our graywater system that hasn't quite been finished...

    Bookmark   March 27, 2012 at 1:09AM
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In climates where irrigation is needed, routing roof, drive, and patio water to the ground surface needing irrigation makes perfect sense. But any plan that increases soil moisture under or near the house, drive, patio, or retaining walls is plain stupid.

If possible, pitch the drive surface so that water runs off to the side onto lawn or garden areas.

Give us your location and climate along with a plan view drawing of all your property with elevation data and grading. It can help us to recommend better ways reduce runoff and make use of the rainfall.

    Bookmark   March 27, 2012 at 10:13AM
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chaparral(Zone 9)

Here is a picture of the rough driveway (morning after a rare heavy rain here)

Here is the overall plan. The green dashed line shows the rough location of a branched drain gray water irrigation system.

Here is a picture of the steps being formed up, to show the grade:

One of our options was to just use permeable concrete in the driveway. Not sure how this current plan is that much different conceptually. The house itself sits on top of the garage approx 10' or so at a higher elevation than the driveway.

Location is in the foothills just north of Pasadena CA. Our front driveway has a slight grade from the garage down to the street. We are leaving a small strip of driveway against block wall unpaved for a vine planting of some sort.

    Bookmark   March 27, 2012 at 11:38AM
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From your photos, it appears that it is way late in the game for suggestions for design changes. There is one issue that gives me concern; it's the area between the steps and the retaining wall.

High walls of CMU construction need attention to detail to prevent failure. Very few such walls are built to withstand the hydrostatic pressure of saturated soils. The area looks to be planned as a planting bed and also shows subsurface pipes discharging grey water into the area. What is not shown is any drainage facility near the bottom of the wall to prevent soil saturation, such as a french drain piped to discharge over the curb or weep holes through the wall. I have to wonder, if this wall was design by an engineer, was he told about the grey water and the planned use of the area behind the wall.

On the basis of the photos and the plan, I would not be surprised to see this wall fail in the first five years.

    Bookmark   March 27, 2012 at 2:45PM
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deviant-deziner(Oh zone)

Your system is basically a stylized septic leach line with a linear surface channel drain. Too bad you can't attach the top channel drain plate to the 4 inch perf pipe.

It's a bummer that NDS ( our CA distributor for drainage solutions ) doesn't offer a perforated channel drain.
I like the ease of installation of their Pro series systems - easy to snap in and screw down the top grate . Perhaps instead of drilling the drain holes you can stagger the modular section ? Not sure if that would allow enough passage of run off though. ??
We've installed systems somewhat like yours in the past but have always had our soils or civil engineer do the calc's because we can't get the building and inspection permit unless a signed report is submitted with our plans- ( SF. Bay area ) .
Bottom line w/ these systems always seems to be 'point of refusal' in conjunction with perc. rate. At 23 % permeability rate your definitely in the ball park .

and that permeable concrete - ugly. practical , but ugly, even when it is grinded to a finish. Looks like a rice crispy driveway.

    Bookmark   March 27, 2012 at 7:53PM
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chaparral(Zone 9)

pls8xx -- yes there is a french drain along the retaining wall; so I'm not concerned with wall failure.

deviant-deziner -- GC found something he thinks will work from NDS; about $280 in total.

We had done a soils report earlier before putting in the wall and foundations. The operative word is "sandy". We had to put one section of the wall in at a time--4 sections total to minimize cave-in risk from neighbor.

If this were hard clay I don't think our scheme would work.

    Bookmark   March 28, 2012 at 11:20AM
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bahia(SF Bay Area)

Be sure to wrap the backfilled gravel trench with filter fabric to minimize fines getting in and clogging it. If you aren't opposed to the texture of the permeable concrete, (which can also be colored), then the larger area of collection and dispersal would seem preferable in the long run. If your soil is that sandy, there is the potential for settling and concrete cracking. I'd be concerned about the concrete dipping over time, so at least insure that the driveway is well reinforced.

    Bookmark   March 28, 2012 at 3:50PM
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