I did a soil percolation/infiltration test to determine how quickly water drains through the soil:

  1. Dug a hole approximately 12" in diameter to the planned depth of the dry well.
  2. Filled with about 2 feet of water
  3. Let it drain completely to saturate the soil
  4. Filled it again with another 2 feet of water
  5. Measured total drainage after 1 hour
  6. Result was 6.5" of drainage after 1 hour, or a little more than 1" per 10 minutes

Assuming I've calculated the right number of dry wells and permeable gravel backfill to surround the wells (that is total storage volume is appropriate for rainfall rates and roof area), is that soil drainage rate adequate for this application?

  • Depending on where you live, that test might appear great for “end of summer” test, but not so great late in the rainy season. – Lee Sam Sep 24 '18 at 18:11

I’d say yes. That’s a pretty good absorption rate. I work with a city program building residential rain gardens for storm water management and the guidelines for passing infiltration testing for those is 1” per hour to run 600 sq ft of roof drainage from a downspout to a designated garden site.

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