I came up with a plan to use a single pipe for overflow AND filling the pond with rain water.

[combatwombat]

Here's what I came up with:

I would place a valve on the lateral run to the drywall and put a valve box over it so that it remains accessible. A pipe coming from the pond would have it’s inlet elevation set where I want water level to be maintained.

During the rainy season, the valve would remain open and rising water in the pond would drain through this pipe into the dry well. There is also a secondary overflow elsewhere in case of a VERY large storm where the dry well is overwhelmed.

But in the dry season (I live in the PNW where we get occasional rain showers all summer) water flow would be reversed. I would close the valve, and water from the downspout would flow into the pond.

The downside I see to this plan is that the downspout and overflow pipe would remain full of water all summer because rain water would have to fill it to the pond's water level before it would exit into the pond.

That doesn't seem like a big deal to me, but maybe it is? Can you see any ways this ends in catastrophe?

Other thoughts:

1. I plan to use a 90º fitting on the end of the pipe that I will rotate to set water level perfectly. During summer, I can rotate it down to minimize the amount of standing water in the pipe system.

2. The end of the pipe in the pond will be inside of a large, sealed culvert pipe, so the standing water in the pipe will not be accessible to mosquitoes.

3. Would it make sense to install the overflow pipe with no slope? Make it perfectly level so that it functions the same in both directions of flow? Initial plan was to install with standard quarter bubble slope downhill towards drywell.

 

[GBBUDD]

If you have too much pipe where water is trapped and you have a back flow getting any organics the water can go stagnant thus introducing hydrogen cyanide. isn't the cistern filling up and just leaching across the 12 or so feet sufficient? i'd think that's more surface distribution then would be a small drywell if that concerned dig along the end of cistern fill with stone

 

[combatwombat]

If you have too much pipe where water is trapped and you have a back flow getting any organics the water can go stagnant thus introducing hydrogen cyanide. isn't the cistern filling up and just leaching across the 12 or so feet sufficient? i'd think that's more surface distribution then would be a small drywell if that concerned dig along the end of cistern fill with stone

Ah, that's a good consideration. I think you're right about the far side of the cistern being sufficient for drainage. The value of this design is that I can:

1. Drain 99% of any overflow to the dry well, which I think opens up more plant options on the far side of the pond? If all overflow went there, I think it would have to be a rain garden?

2. I can feed 3 gutters off the house to the cistern completely underground to capture summer rain. Don't think elevations work out to do it underground otherwise.

Thinking more about #2, I guess I could also use a downspout diverter and place a dry creek bed w/ a liner off the gutter to feed the cistern. That dry creek be could also capture the other two gutters. So I guess it could still work. Would just have to re-do my landscaping plan a bit.

I could also move my active creek over there instead of coming from under the deck. Or just add another active creek. But then we're getting a little crazy...

 

[GBBUDD]

But then we're getting a little crazy...

As you said before that ship has already sailed. We wouldn't be building ponds in out back yards if we weren't a little touched

 

[GBBUDD]

He'll I just dumped 6 k on a filter that has no top a filter that has no filter pads a filter that is not known to be the best . But one I know works extremely well for me