Did you plan for water level fluctuations in your pond?

[combatwombat]

The pond I'm designing will eventually accept roof drainage (directly into the pond, through a pre-filter, no storage system). About 600sf of roof in the PacNW. I've been casually calculating expected evaporation and precipitation to figure out how much freeboard—how much holding capacity above normal water level—to leave myself to account for rises from rainfall before overflowing to a dry well that I have downslope of the pond.

Wondering if any of you thought much about this before digging and how you decided what to do about it.

I don't care for the aesthetic of a big berm around my pond, and I think I've come to a happy compromise for how I'll handle it, but I'd like to know how you approached it.

I think the main issue I'm going to have from draining my roof into the pond will be that I'll get tons of water when I least need it, and very little when I do. But I think that's a limitation that can't really be worked around without some sort of "out of pond" rain exchange / storage system that I don't have the desire to figure out right now.

 

[pebbleoz]

I personally think you will be replacing far too much of the water in the pond too frequently.
Also a rush of water could displace some of your fish.
The water in your pond actually has beneficial bacterial that grow and provide a stable environment for the fish and plants.
By flushing the pond every time it rains you will lose this beneficial environment.
Have you thought of putting your rain water in a tank? Then you can refill the pond as required if it evaporates on hot days. You will have to check the pH of your tank water but mine is very stable and neutral so it is a great source of water for the fish if my pond needs a top up.

An inch of rainfall on a square foot = 0.623 gallons
Your roof is 600 sq ft x 0.623 = 373.8 x 131 (pacific north west average annual rainfall) = 48,967 gallons of water a year.
So you are planning to flush 48,967 gallons of water a year through your 7,500 gallon pond.

Calculating Rooftop Rainfall | The RainCatcher Santa Fe, NM

A single quarter acre of land in Santa Fe can harvest over 100,000 gallons of water annually! Learn how to calculate the amount you can harvest from your roof.

theraincatcherinc.com

 

[combatwombat]

@pebbleoz, thanks for chiming in.

I personally think you will be replacing far too much of the water in the pond too frequently.

You know, I haven't actually thought through how frequent changes will occur. Pond will be about 7500 gal.

Also a rush of water could displace some of your fish.

I forgot to mention... no fish in the pond per wife's strict orders. Just a water garden. I bet I'll eventually wear her down, though, and we'll get a few. I will have a planned overflow route that I think I can make fish safe in any case.

By flushing the pond every time it rains you will lose this beneficial environment.

I understand that the chemical makeup of the water will be different from the rain water, but doesn't the bacteria live on surfaces, not suspended in the water itself? I don't know too much about that part.

Have you thought of putting your rain water in a tank?

I have! And I immediately dismissed it as too expensive and too much trouble! Haha. If enough people tell me I'm a fool for draining the roof into the pond, then I'll just hold off for now until I feel up to figuring out an out-of-pond storage solution.

 

[Jhn]

In my pond, it overflows through the rocks at the edge of the pond and the ground slopes away, so the water just flows away from it there. The water level can’t get above the rocks at the pond edge, so no fish can escape or get washed out into the yard. No berm around either, around my pond the ground does slope away very gently from the pond edges.

Also, @pebbleoz there is minimal bacteria in the water column in established ponds, so wouldn’t be too concerned. Beneficial bacteria in established ponds are predominantly on the surfaces underwater in the pond.
I would make sure the roofing material doesn’t have anything on it that can leach into the pond with the water though and a lot of run off from the roof could cause water quality swings in ph etc. depending on whats in the rain and how much of it flows into the pond.

 

[combatwombat]

@Jhn: Thanks. That's about what I'm thinking for overflow. Might also do a buried pipe w/ grate to keep debris out. Could be subject to clogging, though.

@pebbleoz: I just saw your edit with the rain calcs. Not sure where you got the precip data, but annual rainfall in my local area is 42"/year. I don't think anywhere in the PNW gets 131" of rain/year. That would be pretty insane. Maybe you meant 131 days of rain per year? That would seem pretty accurate, maybe even a bit low.

 

[pebbleoz]

I got it from here for the pacific north west as that's where you said you lived:

Wettest Places in United States - Current Results

Towns and places in the United States of America with the greatest rainfall, and the most rainy days.

tinyurl.com

Your roof is 600 sq ft x 0.623 = 373.8 x 42 (your average annual rainfall) = 15,666 gallons of water a year from your roof.

So no where near as bad where you actually live. Still it is a lot to flush out your pond excluding the water that actually falls on your pond. For the price of a small water tank I wouldn't do it but then that's just me.

 

[Lisak1]

I think you hit the nail on the head - your roof water will come when you least need it. Hence the reason for a rain exchange. I don't think it will hurt anything to take the water from the roof, but if you have no way to save it for a "non-rainy day" (haha!) then I'm not sure where the gain is. Unless you build your pond with the ability to hold a LOT of extra water... doable I'm sure, but not sure it would make for a great looking garden pond.

 

[brokensword]

I bermed around the pond and when I expanded, left one corner low with an over flow pipe going through the berm and out beyond my pondhouse, where the natural slope will take it away. Though, I haven't had any rain issues and could easily absorb 4 inches of rain (which I've never seen where I live).

And re water; I have a well, so...no need to set up rainbarrels! I generally lose maybe an inch every other day in the hottest of months and only just topped up yesterday after probably a 3 week lull. It helps to have plants and barriers to wind, which keeps evaporation minimal, imo.

 

[combatwombat]

I think you hit the nail on the head - your roof water will come when you least need it. Hence the reason for a rain exchange. I don't think it will hurt anything to take the water from the roof, but if you have no way to save it for a "non-rainy day" (haha!) then I'm not sure where the gain is. Unless you build your pond with the ability to hold a LOT of extra water... doable I'm sure, but not sure it would make for a great looking garden pond.

Yes, perhaps very little benefit to the pond. I guess the real benefit is that I have one downspout in my backyard with no drywall, and it makes a mess. Dumping it into the pond would basically transport it to the overflow, which would then transport it to a dry well.

So better for overall drainage, but not really any benefit to the pond.

 

[Lisak1]

Well, I wouldn't say no benefit. You'd get some flow through, which may not be the worst thing. Water change courtesy of Mother Nature.

 

[poconojoe]

I'd be afraid of what the roofing materials might leach into the water. Maybe I'm just over cautious.

You can easily make a rain barrel. I made one from one of those big plastic drums. I was able to get a white one for $25. I think it was food grade. I installed a small threaded bulkhead fitting toward the bottom and screwed a hose bib into that. I have the barrel sitting up on concrete blocks. I can see the water level through the plastic.

I bought a diverter sort of tee fitting for my downspout. It has a lever that you move to direct the water into the barrel or back to the original downspout. So when my barrel is full, I switch it back.

You can easily do this to store your rain water, then use it as you need it. You can setup a couple barrels in tandem. When one gets full, it can flow into the next one via a piece of PVC pipe toward the top of the first one.

 

[Lisak1]

What do you use the water for @poconojoe ?

Oddly I am on a gardening Facebook group and someone was sharing that they were capturing rainwater from a roof to water their vegetables and there was concern expressed about what was on the roof that might be getting into the garden. Someone suggested that they used their roof water to fill their garden pond instead of watering with it... so I guess it just might depend where your focus is!

 

[addy1]

I have been running rain water into our pond for over 10 years now. From a 1250 sq feet of roof (approximate) and a 900 sq foot (approximate) metal shop roof. I do have a over flow for when we do rain buckets. Our average is 40 inches a year.

I have never seen a adverse issue due to the rain water.

 

[jgrandmont]

For my small 700 gal pond, I planned for avoiding a potential overflow by building a small dry well using a large plastic kitty litter bucket. I'd estimate it holds about 5 gal. I drilled holes all around it, then buried it after putting about a 2-inch layer of gravel on the bottom and sides. I then drilled a hole in the back of the skimmer box (estimating the overflow level), installed a bulkhead, and connected the dry well to the bulkhead with flexible PVC hose. Will it work? I don't know, but it seemed like a good idea at the time.

 

[combatwombat]

@poconojoe: If I were planning to keep expensive fish, I might give more thought to that, but since I'm not, I'm pretty unconcerned about it. May come to regret it! But easy to disconnect if I change my mind.

RE: Rain barrels. I can see their appeal for certain applications, but they wouldn't work too well in my space. If I'm going to do storage here, I think it's going to have to be some type of tank system under my deck or a below grade solution. Could put it under a patio with a little rock bubbler in the middle for circulation. That would be fun.

@addy1: What roofing material do you have on your main roof?

@jgrandmont: Sounds like any other dry well, just a lot smaller. If you don't have much to drain, I imagine it will work just fine.