swale or outside french drain for wet basement

[jay2000]

Hi, I’m receiving water in my basement after a heavy rain. My property borders up to a
large hill. I’ve read several places that a swale or outside French drain could help my
problem. Here are my questions?

1. Which is best swale or outside French drain?

2. I’ve read that a swale should be approximately 10 inches wide and 2 inches deep
with a 1/8 to 1/4 inch drop per linear foot. The length of area I would need to
cover is approximately 70 feet which would mean the drop would be 8.75 for the
1/8 value and 17,5 for the 1/4 inch drop, which I believe would be quite
noticeable in the yard.

3. If I decide to construct a French drain I’ve read that there is a 1 inch bed of
gravel, 4 inch drain tile and another 1 inch bed of gravel followed by a 1 inch
layer of top soil. I’m not sure if I would need some type of surface drains tied
in to the drain tile, or does the water naturally soak through the ground into the
drain tile. I’m also assuming that the ditch depth should be approx. 76 inches
deep. 1 inch for gravel, 4 inches for drain tile, 1 inch for gravel and one nch for
top soil with the same pitch as above 1/8 or 1/4 per foot.

So there’s the situation. Water in basement, swale or french drain,which is best.
Thanks ahead for any advice regarding this isssue.

 

[cvg]

first of all, you need to determine the amount of runoff from the "large hill" 10 inch wide by 2 inch deep swale may or may not convey the water running off this hill. if you divert it in a swale, you need to return it to the original flow path, or you may be directing this water at some other unsuspecting property owner downstream.

your french drain design is also a bit scant - 1 inch of gravel top and bottom is not much. I suggest you do some basic hydrology calcs - use the rational method - and come up with a peak flow rate and volume first before going any further with this

 

[jay2000]

Thanks for the advice. As far as the discharge area of the swale, it would discharge into a public street and should not affect any neighbors. The hill that will be feeding the swale is approx 500 squre feet on a 30 degree incline.

 

[cvg]

The small size of the hill indicates that peak flow rate is not a problem, but volume of water may be. You might try a shallow swale, following the natural slope of the yard. This will intercept the water at the bottom of the hill. then construct your french drain below the swale. Don't skimp on the trench dimensions, a larger trench with more gravel will allow the water to soak in faster. Put the 1/8 or 1/4 inch per foot slope on the french drain pipe.

try the following thread for more info on french drains:

thread162-48077

 

[jay2000]

thanks for the advice. I'll keep everyone posted on my progress

 

[jay2000]

when you say shallow swale what dimensions are you suggesting? Also, when constructing a french drain, is it best to leave gravel exposed or would putting top soil over the gravel affect the rate the water soaks? I am concerned about apearance as well as funtion. One last thing, considering that the length of the swale woulbe be approx 70 feet or more the drop from start to finiosh would be 18 inches. Quite noticable.

 

[cvg]

slope of swale can be flat, or follow the natural slope of the yard. It only needs to intercept the water from the hill and allow it to percolate into the french drain. Swale needs to be large enough to do this without overflowing and allowing the water to reach your basement. Put the slope on the pipe in the trench. It is best to leave gravel exposed - you will get better percolation. However, you could cover with a thin layer of sand and grass may grow in the sand.