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I've installed a new downspout to drain a section of gutter that was getting overloaded during heavy storms. The rest of the downspouts all terminate in corrugated black tubing that's buried in the ground, which prevents the water from simply being dumped onto the lawn where it might pool and work it's way back under the slab. For my new downspout:

  • How much pipe should I bury (i.e. how many feet)?
  • How much bigger than the pipe should be trench/hole be?
  • How much slope should the pipe/trench have?
  • Should I put gravel in the trench?

For background on my situation:

  • I live in the South. During heavy rains several inches can fall over in only a few hours.
  • Our soil has very high clay content so it drains poorly. During a heavy rain as described above the soil cannot absorb the water as fast as it falls so it just runs over top of the lawn.
  • The amount of water this downspout has to handle would is approximately a hose at full blast, or close to it.
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2 Answers 2

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This page says clay soil absorbs water at the rate of 1/2 inch per hour, 4 times slower than sandy soil or loam.

http://www.ornamentaledibles.com/tips/soil.htm

Let's assume you get 5-10 gallons per minute or 300-600 gallons per hour. A gallon is 231 cubic inches or 462 square inches 1/2 inch high. So 600 gallons/hour needs 277200 square inches or 1925 square feet. Assuming your trench is 12 inches wide, you need 1925 linear feet of pipe under steady state conditions.

Let's consider another situation where you store all the rain water and let it absorb as slowly as it wants. Assume you get 600 gallons which requires 80 cubic feet. So if your trench were 12" by 12" you would need 80 linear feet of pipe.

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I've since moved, but that's the kind of answer I was hoping for. Perhaps the new owners will stumble upon this and extend the 3' I actually laid. –  Adam Wuerl Aug 3 '12 at 1:27
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The length and size of your drain depend on several factors:

  1. How many inches of rain do you get
  2. What is the area of roof that will be draining
  3. What type of soil do you have (sand, clay)?

For items 1 and 2, this calculator will tell you how much volume you have to be able to handle: http://www.calctool.org/CALC/other/default/rainfall

Sandy soil will dissipate water faster than clay soil. Not sure how to measure this though. You should probably allow a little extra if you have clay soil.

Typical slope for a gutter is 1/4 inch for every 10 feet. I think you could do the same for your drain.

Yes, put gravel in the trench. It's a good idea to wrap your gravel in a porous landscaping cloth that will allow water to pass through, but keep most dirt out.

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I have no idea how much square footage of roof drains into this one downspout, but in terms of water that it has to handle during a heavy rain it'd say it's at least the equivalent of a hose at full blast, or at least close to it. I'm in the South, so it's common for a rainstorm to dump a inches over only a few hours. I'll amend my question above accordingly. –  Adam Wuerl Apr 16 '11 at 17:15
    
A little tricky to estimate based on that. You'll want to measure the area of the roof that catches the rain you want to drain. That can make a huge difference in the amount of water you have to handle. Here's a link to a previous question that addresses how to build it: diy.stackexchange.com/questions/2865/… –  Shane Apr 17 '11 at 1:59
    
So I used the calculator above and estimated 0.3 cubic meters. Of course I'm not sure how that number means anything as it's a volume of water, not a volume rate (i.e. cubic meters per hour). It seems like it would matter if that amount of rain fell in two hours or over the course of three days. –  Adam Wuerl Apr 17 '11 at 13:30
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