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I need to create some drainage in the form of some dry wells and french drains. What kind of stone will maximize the porosity to get good water flow?

I originally thought just ordinary gravel, but a builder told me that "river stone" whatever that is, would be way more porous and have much greater flow.

  • So ask the builder what he meant by river stone,... – Solar Mike Mar 18 at 17:13
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    River stones are roughly inch-sized stones that are rounded on all sides and found in river beds. I think this was probably suggested over crushed gravel as that gravel tends to compact over time. – Sam Mar 18 at 17:27
  • There's no size associated with river rock. I can be pea gravel or small boulders. – isherwood Mar 18 at 17:27
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You are looking at 'void fraction' Anything of mixed sizes has a smaller void fraction because the little guys snuggle into the spaces left by the big ones.

If all the rocks passed through a 2" screen and not a 1.5" screen they are close to the same size. Doesn't matter much if they are round or jagged.

It also doesn't matter much what size they are, at least until you get down to sand size. Sand runs about 30% void. So does big rock. But a mix of sand and big rock will end up with sand filling 70% of the voids in the big rock. so you get 30% of 30% void = 9%.

You will lose some volume over the years from debris infiltrating in. If you can come up with a scheme to trap debris at the surface, this is easy to fix. E.g. Stop your fill 1 foot from the surface, put down filter fabric, and put a more rock on top of it. Now, if it starts acting slow, take off the top foot, wash the gravel, put down new filter fabric, replace.

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Washed round (river) rock leaves large air gaps between each stone, as opposed to crushed rock which fits more tightly together when settled. A 2" screen might be appropriate for your needs, though smaller sizes still drain well.

Be aware that organic soil or sand will readily silt into it, so you'll need to protect it with suitable fabric or membrane.

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When you order a load of rock and ask for River Stone, you will get a gravel that is made of all rounded stones. They are supposedly rounded by tumbling in a fast running river so they have smooth edges.

When you want to lay pavers or similar, you get a totally crushed stone. The jagged, sharp edges on the crushed stone lock together to give you a solid foundation. This gravel packs more tightly and won't drain as well.

If you order just plain ol' gravel, there's no real telling what you will get but it will probably have some crushed component and some rounded. For the best drainage, specify exactly what you want (rounded stones), and that is river stone.

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