How much drainage pipe should I bury for a new downspout


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.

Best Answer

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

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.