I'd like to collect rain water and have it run through a filter and then into a pond.

The problem is, I can't simply run a pipe underground.

|\\ <-- house roof
| \\
|                                            pond
| <--house                                     |
|                                              v
|                    [_]                                                [_]
|                    | |~~~~| |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| |
| <-- 12 ft -->      | |    | | <-- overfill drain                      | | <-- wall
|                    | |    | |                                         | |
|                    | |    | |                                         | |
|=====================\ \   | |               <o||-||-<<               / /===== <-ground
           ____________\_\__/ /                                       / /
           __________________/                                       / /
                         \ \.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o/ / <-bottom of pond

Normally I would just go from the downspout, into a filter, under ground, then dump directly into the pond, but as you can see two features complicate this:

  • There is the 2-ft (60 cm) high wall. I'd rather not cut a hole in the pond, that risks leaks. The pond has a 2-ft high wall around it to prevent floods from dumping sediment into it.
  • There is also an overfill drain in the pond. That is so if the pond fills too much, it drains into the street. That helps direct the flood water away from the house. If the pond didn't have this, the water would spill over all edges of the pond, and damage the walls around the pond and perhaps flood the house.
  • The 12 ft (4 m) space between the house and the pond is a busy path, so it isn't a good place to run a pipe, unless it is buried, but then how does the water get into the higher pond?

How can I redirect rain water too the pond when the pond has a wall?

  • Can you say more about the filter requirement?
    – AllInOne
    Nov 19 '20 at 20:27
  • 1
    @AllInOne There is a product like this that gets buried in the ground under the downspout (aquascapeinc.com/products/downspout-filter), but since I don't know if a solution exists that goes underground, I can find some other means to filter the water. I think the main need is just to make sure the pipes don't get clogged with leaves, as the fish will be exposed to the rain water anyway.
    – Village
    Nov 20 '20 at 0:51
  • 6
    Be aware that some roof shingles have copper granules embedded in their surface to inhibit the growth of algae. Water taken from the roof would therefore contain some dissolved copper compounds, which may affect whatever is living in the pond. Whether that's good or bad for the ecology of your pond depends on the specifics of your pond. If in doubt, you might contact the manufacturer of your roofing material and inquire about copper content.
    – MTA
    Nov 20 '20 at 16:42
  • @MTA Thanks for the point. I am planning to change the roof next year to a metal one, so maybe that will be less harmful to the ecology.
    – Village
    Nov 20 '20 at 17:13
  • How big is the overfill drain? From the looks of it it seems like you could use it both ways. Nov 22 '20 at 15:07

I would build a decorative arbor over the path. Its purpose would be to camouflage a basic gutter and downspout arrangement that dumps into the pond. Simple as can be.

enter image description here

Image source

  • 9
    Depending on how crafty you wanna get, it doesn't even have to be a closed pipe. Maybe build some kind of decorative waterfall system where you can watch the water purling down across multiple levels. Of course keep in mind that this needs to be big enough to deal with a heavy downpour.
    – MaxD
    Nov 20 '20 at 15:08

enter image description here

You could call it an "inverted syphon"... I'm searching for a word here, but it's not coming to me.

Build a leak-tight pipe that starts at the house above the top of the pond wall. The pipe goes down, underground, laterally to the pond, up the exterior of the pond wall, then over the wall and dumps into the pond. This pipe will be pressurized so you have to use competent plumbing pipe (sewer pipe will do) and actually glue/caulk it.

enter image description here

It will need a screen at the top to keep out junk. Also you will want to "tee" in a cleanout pipe that goes downgrade, with a valve; open the valve and blast water down it with a hose to clean out crud, and also leave the valve open all winter to winterize the pipe.

The pressure in a pipe is decided by the height of the water above it, and due to how pressure works, the only factor that matters is height. So as long as the pipe intake is above pipe outlet, the water going down the intake side will apply more pressure than the water sitting on the outlet side, and water will move that direction.

This is the same concept that happens under your sink and under every toilet bowl. In that case the objective is to create a "plug" of water to stop sewer gases, but you notice this, um, what's the word, help me out here Admiral Ackbar, does not impede the flow of water at all.

  • 17
    IT'S A DOWNSPOUT! no... that's not it... Only drawback to this is the potential for mosquitoes to breed in the standing water in the pipe. Of course, they could also be breeding in the pond, but if the fish in the pond are more alive than the ASCII skeleton would lead us to believe, they would probably eat the eggs/larvae and take care of most of the problem in the pond.
    – FreeMan
    Nov 19 '20 at 21:28
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    My dad built some sort of hydroelectric dam bypass pipe on the side of a mountain in the '80s that he liked to call an inverted siphon--the only time I've heard that term until now. I think it's a bit overstated in both cases. It's just a drain pipe with a low spot. :)
    – isherwood
    Nov 19 '20 at 22:22
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    The main caveat is that the plumbing underground is going to be a huge pain to deal with if it clogs, so you gotta make sure no debris goes in.
    – Nelson
    Nov 20 '20 at 5:07
  • 1
    It's not a siphon, nor is the plumbing for your sink. A siphon is a sealed system that draws fluid due to gravity at the bottom end and vacuum at the top.
    – isherwood
    Nov 20 '20 at 12:56
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    @Village I don't think the spout needs to be higher than the drain. It could enter the pond underground and be essentially the same as the diagram that Harper provided. The pipe on the side of your house would always be full of water to the same height as the pond, but it would still drain into the pond (by equalizing the water level) when you add water from the house side.
    – Dan A.
    Nov 20 '20 at 15:02
  • Run your down spouts into barrels.
  • Plumb these barrels together into a central feed.
    • These can run underground if you'd like, so long as the pipes have the proper slope, gravity will be your friend.
  • Have a sump pump (or other, float-activated pump) pump the water up and over the retaining wall.

Depending on how full your pond normally is and how much rain you get, the addition of all the runoff from your roof could end up overfilling the pond on a regular basis, meaning that you've spend a lot of time, effort & money moving water around, just to have it go down the overflow tube and into the storm sewers, when you could just let the runoff do that for free via gravity. You may want to do some calculations along these lines on your own before doing all the plumbing to ensure it's worth the effort.

  • 2
    That is a very good point about not wanting to fill the pond with the water directly...the problem is maybe 90% of the annual rainfall falls in 5 days in the year, so I don't know if I can make barrels big enough...earlier this year the rain fell so fast it filled a 50 gallon wheel-barrel within 2 minutes from the water coming off just a small part of the roof.
    – Village
    Nov 19 '20 at 20:04
  • @Village, you could size the pump to the "deluge" quantity rain fall. Or, you could increase the pre-pond storage capacity - maybe dig a large sump which will fill as the pump catches up with the rain fall. Or, just acknowledge that you're not going to capture it all and have overflow basins that go to the storm sewers.
    – FreeMan
    Nov 19 '20 at 21:25
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    Of course, if you're getting that much rain that quickly, the pond is probably overflowing without being fed from the roof anyway...
    – FreeMan
    Nov 19 '20 at 21:29
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    @Village if you get that much rain and you want to save as much of it as you can, then you need a cistern, not a pond. Presumably you want to even out the consumption of the collected water over the year, which means it should be covered, ideally underground. Nov 20 '20 at 5:19
  • A cistern will significantly reduce water loss through evaporation in a desert @Village.
    – FreeMan
    Nov 20 '20 at 14:06

This answer incorporates some of the info provided in comments. To simplify Harper's answer a bit, I'm pretty confident this system will work just fine:

enter image description here

During a time when the pond is low, you can dig your trench and install pipe underground, entering the pond underneath the wall.

When the pond fills (from roof runoff or other means), you'll have standing water in the pipe, but never higher than the level of the pond. Make sure to slope your underground pipe downward into the pond by about 1/4" per foot, so that if the pond goes completely dry then you don't have standing water in your pipe.

I might try putting some sort of filter in the pipe next to the house, where it is easily accessible and always above the maximum water level, for easy removal and cleaning. I would also put some sort of grill over the end of the pipe in the pond, to prevent leaves or ASCII fish from getting into the pipe.

You'll mostly have flow going from the house out to the pond, so the Bti used in the pond for mosquito control might not find its way into the pipe. But if you find that the pipe is causing mosquito problems then you could always pour a little Bti into the pipe using your filter access point.

  • OP said "I'd rather not cut a hole in the pond, that risks leaks." by which he must mean, "...cut another hole in the pond" - Perhaps he could connect the rain pipe to the overfill drain, outside the pond. Nov 20 '20 at 22:23
  • Since that quote was from a paragraph about the wall, I interpreted it to mean “I’d rather not cut a hole in the wall”, which I agree would be a poor idea.
    – Dan A.
    Nov 20 '20 at 22:54
  • 1
    @A.I.Breveleri: "Perhaps he could connect the rain pipe to the overfill drain, outside the pond." What would entice the water to go up the overflow rather than to its drain?
    – Transistor
    Nov 20 '20 at 23:01
  • @Transistor: Yeah, good point -- I feel like Groo the Wanderer "Did I err?" Nov 21 '20 at 3:16

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