Bottom Drains in larger ponds

[Alarakalama]

Hello All,

(New to the forum. Long-time listener, first-time caller as they say in radio. I tried to post this once but I think the internet ate it)

I am constructing a 25,000 gallon koi pond in my back yard. It is difficult to find guidance for a BD system on ponds this size. I am shooting for a 12,500 gph pump and filter system for a two hour turnover time. As you can see in the alternatives shown in the image below (not to scale!) the pond is irregular and is 38 feet by 30 feet at its longest/widest points.

I originally planned to slope everything down to one or two 4" Bottom Drains (BDs) with the balance of flow going to a skimmer. However, after doing some research it isn't clear to me if the 5 to 6 foot sweep radius of a BD should maximize the area including slopes or if it should simply cover the lowest point in the pond where everything is funneled and collected. A pond this size could easily have 6 BDs and still have lack of coverage along the slopes. Obviously the more BDs you add, the more complex your TPR arrangement becomes.

That said, am I OK on a 1:1 or 2:1 slope down to just two BDs centered in the floor with the majority of the pond not swept by BDs? or should I maximum BD coverage as much as possible even on slopes?

Any guidance is welcome!

Regards,
Brent

[icu2]

Welcome and thanks for joining Brent!
All new members first few posts are automatically moderated due to spammers, but they'll be approved asap
and it's a short probationary period.
Enjoy the forum!

[icu2]

What are the dimensions of the pond floor?
Does your filtration plans have the ability for each of the BD's to flow 3500-4500 gph each?
Everything has to match... all pond egress flow, filtration max flow rates, and the flow back to the
pond. 4" BD flowing at the above rate is normally considered to be good for about 6' radius, 12'
diameter.

Looks like a terrific project!

[kimini]

Agree that four 4" bottom drains would be better. A couple of other things to consider for such a large pond include:
1. Being unable to net the fish (they'll swim to the far side, out of reach).

2. Skimmers. If the bottom drains are aerated, they push floating debris away from them in a circle. Any two drains/aerators will push floating debris to the side of the pond right where the two radiating currents meet. If there's no skimmer there, the waste will hang around that spot a really long time until something forces it away, or it sinks.

2a. Even a single skimmer/aerator can trap floating debris in a corner if said debris can't float on its own out of a corner trap. I marked up your drawing in the likely areas where stuff will get stuck. Your options are to ignore it, as the waste will eventually sink; put a couple skimmers in the worst areas; make the pond have less "bays" to trap floating debris in; or some combination. Having jets to circulate the entire pond volume doesn't work all that well since truly floating debris is affected by only the very top layer of water, and that current is predominately from the aeration current, and the wind.

[montwila]

I would take this one reeaaaallllllll ssslllllooooowwwwww.

First off turn over is not linier. IF you have a 25000 gallon pond: a single 12500 gallon pump will not turn it over every two hours. The larger the pond the slower the ratio is of turn over verses time based upon pump size. Yes 12500 does divide into 25000 twice but that does not work in fluid dynamics. Also with the irregular shape, I would say that you are looking at closer to 3-4 hours before "most" of the water would see the filters.

Just for fun: 30 X 38 X 6 X 7.48 = about 51000 gallons. Now let say due to the irregular shape we will use 70% and we are looking at closer to 36000 gallons with straight side walls at 6 feet.

OR : 30 X 38 X 4 X 7.48 = 34100 X 70% = about 24000 for 4'deep straight wall. This is without the conical shape of the floor for bottom drains. Which at a 45* slope for 2' (basically another foot in depth over all) will come to another 8500 gallons. 24000 + 8500 is back to 32500.

Both these calculations put you well above 30000 gallons and IMHO with a single 12500 gallon pump will be much longer than 3 hours for a "complete" turn over.

The reason there is not much information on "Large" ponds and how many bottom drains they use or are required is: not many people try this. What is your knowledge of building ponds and experience running them? This is not meant to be derogatory. But if I am going to give advice I think it is a fair question.

[kimini]

Since we're all piling on

As was said, dividing the advertised pump volume into the pond volume doesn't work for two reasons. One's mentioned above, that it's a logarithmic volume replacement curve and it'll take probably six hours for a true exchange. Second, the pump specs are always at zero head, which is never true. If you're lifting it any amount, and running the water through any pipe, it's going to be a lot less flow than claimed in the ads. Lastly is energy costs - do you care? Regardless of the pond's construction costs, the electrical expense is forever.

[Alarakalama]

Thanks everyone for the great feedback so far - it seems like more BDs is the generally accepted path so far. I did not consider pockets for floating debris to be caught but I think there is not much I can do about it with such an irregular shape!

Montwila,

I am a licensed Civil Engineer with experience working in water treatment and I also have a Class II Waterworks license. That said, I fully recognize that there is a big difference in treating water/wastewater industrially and building a koi pond (that looks good in your backyard!) using qualitative information drawn from the internet.

This will be a 40 or 45 mil liner. The approximate area is 800 Square feet based on my drawing in Sketchup. I assumed an average depth of four feet. The left hand side and bottom (blue lines) are retaining walls (about 30 inches water depth depending on pond side backfill depth). I would slope down at a 2:1 or 1:1 slope from the retaining wall footer another few feet. The remaining earthen walls will be built up with spoils at a similar slope. I may go with a deeper average depth but even at 1:1 I am very limited in how deep I can go. It may ultimately be an average depth of 5 feet but I didn't want to draw it up without knowing how many BDs I would have. But yes, the average depth may go to 5 feet. The depths indicated in my last image was meant as a peak depth since I am sloping so much.

I really don't want to go too much higher in pump capacity due to operating costs. I see your point on turnover due to lack of mixing in larger ponds. Is 3-4 hours acceptable on a pond this size?

My biggest concern is the dense population of herons in our area. I've only lost one fish so far in our existing smaller pond but that's because I keep it fenced now. I am hoping that the retaining wall and dense vegetation/fencing on the sloped side will help manage herons. The retaining wall footer is already poured and I'm going to work on laying block as I have time and weather permits. I don't really intend to excavate the pond until mid or late spring due to a seasonally high ground water table.

Thanks again everyone. As I get more feedback/confidence I can start drawing it in sketchup with more detail.

Brent

[Alarakalama]

Kimini,

I care a lot about electrical costs! Thankfully there are some decent low-head pumps in this flow range. Everything on this pump will go back to TPR's or potentially a waterfall perhaps 12 inches above still water level. It really depends on the friction losses in the pipes feeding the TPR's. That said - what is an optimal design flow for a 2" TPR? I have ready different things on this subject.

Brent

[audioenvy]

My pond is a 20000 gallon rectangle with six 4" aerated BDs, ZERO TPRs, and almost no slope to the BDs. We do NOT get any build-up on our pond floor.

The problem with having lots of bottom drains is that if you don't get enough flow through each drain then you end up with stuff that settles in the drains. We have eight pumps and turn over at least 1.5x per hour.

[Alarakalama]

Audioenvy,

Very beautiful pond and fish! I am encouraged to go with a 5th BD for even coverage. What type of filtration do you use?

[coolwon]

Once a big volume of water starts turning around the pond, does it really want keep getting interrupted, bumping against curves and being diverted into the centre of

the pond.

Would it not bounce off each curve and head down the nearest bottom drain to create a dead area.

Looks pretty on paper, but is it functional with 25000 gallons of water trying to go smoothly around a big circumference.

No brakes for the corners

Garfield

[audioenvy]

Audioenvy,

Very beautiful pond and fish! I am encouraged to go with a 5th BD for even coverage. What type of filtration do you use?

RDFs and showers.

[batman]

[audioenvy]

I am shooting for a 12,500 gph pump.

What pumps have you looked at? Into what will the water from the bottom drains empty?

[Alarakalama]

Audioenvy,

The Artesian Pro AP1-120 pumps at 10,800 GPH @ 10' of head. https://www.performancepropumps.com/...low-rpm-curve/

Goulds (Top notch brand) GCP series - GCP15 for example performs at 12,600 GPH - https://www.underwaterwarehouse.com/...ow-head-pumps/

Goulds doesn't embellish their stats and is a reliable brand - it is probable the direction I will go. If I go up a foot or two of depth and require more flow, higher flow pumps will have trouble with the low head conditions so I have to be careful. I may have to throttle the pump effluent with a valve to prevent cavitation (ultimately making it slightly less energy efficient) or just make the waterfall higher.

I am pumping into TPR's and a waterfall perhaps 12 inches or so above still water elevation. Head loss will almost entirely derive from friction loss in the pipe which I will size accordingly.

My current plan is to flow through the BDs and into a sedimentaiton basin with a sump on a timer. I have a home-made RDF that works fairly well but it won't be able to handle this flow alone - thus a sed basin ahead of it. After the sed basin and RDF, i'll run it through barrel(s) of aerated K2. I am certainly all ears if anyone has thoughts on this layout

I am hoping someone knows of an min/ideal/max 2" TPR flow rate.

Brent

[batman]

[batman]

[batman]

[kimini]

After going around and around on picking a water pump myself, I reluctantly bought a Flowfriend Pro, a variation of the standard Flowfriend pump and intended for very low-head high-flow situations. I say reluctant because of its eye-watering price of $2,700. BUT, if you do the math, and depending on your energy costs, it may well end up being cheaper over time. In my case, it paid for itself after four years when compared to alternate pumps. I bought mine from Koi Acres (in the US) but there are other distributors. Be sure to look very closely at the head/flow charts with whatever pump(s) you're considering, because as was said, they're all notorious for stating zero head/zero resistance flow as "normal." With this pump, it's moving an honest 6,000 gph @ 18" dynamic head, at an honest 110 watts.

[montwila]

Hi Brent:

As a waste water civil engineer I am sure you have a great handle on this stuff. So I can see where you want to cut to the chase.

I think the answer to your question is: a 2" TPR should be provided with approx. 1500 to 2500 gallons per hour per TPR. Basically if you are using 55 gallon barrels for your MB filters (or sand and gravel). Each barrel can supply one TPR at 2000 GPM for a MB and 1800 for S&G. The S&G will usually fluidize above this rate. Some people push their MB's to 2400 GPM but mine become static around the outlet screens at this rate. So I returned to approx. 2000 GPM and the media slowly rolls nicely.

At this rate of flow, with the TPR 12-18 " from the bottom and pointed slightly down ward. Its influence is only about 5-6 feet from the "nozzle". So on your pond it may help with those potential dead spots that Kimini has highlighted in post #4 but cannot be relied upon for circulation in the middle of the pond. Bear in mind the longer that the TPR's piping runs below the water level when returning to the pond, the more "push it requires" to achieve this 5-6' of "nozzle effect". You might be able to appreciate this already and I do not have time to explain it. Besides most people poop poop this concept until their TPR's do not provide the nozzle effect they thought they might have.

A 4" bottom drain will only draw about 18" from the edge of the BD. An aerated bottom drain (ABD) that is able to create the mushroom effect does help up to 4-6 feet out BUT to get this plume it must have a wall to create this effect. In a large pond such as you are planning an ABD will only assist to bring water for about 180* to 270* around the circumfrence of each bottom drain towards the nearest wall. The rest will be greatly reduced as far as debris being captured by each BD. The fish will have to do the rest. Yes, I believe in "fishy swishy motion" of debris movement along the bottom of the pond.

The other thing to consider since you seem to be leaning towards more bottom drains is that a 4" BD needs to flow at least 3600 GPH through the piping. Mine still get some debris but I think it might be sediment within the bottom drain appliance. Make sure that you "home run" each bottom drain to the filtration area (sedimentation tank in your case) with it own isolation valve. Each week you should purge each line. On occasion I have had drain field size stones appear after doing this. Water logged pine cones and such also come out. So if you do get debris that get trapped in the middle of the pond and eventually sink, you can at least purge each line and stand a chance of getting them out. Even with this, about once a year I have to disassemble some bends in the filter pit as small branches accumulate and restrict the flow. Most important though is that 4000 GPH would be better and at 4 bottom drains you are at 16000 gallons. adding an additional BD puts you at 20000 gallons. As Steve points out in post #3 you have to match this BD outflow with enough pump capacity. AND if you are going to pump water, you might as well filter it. I am not sure but you seem to think your home made RDF may not have a very large capacity. The other thing is wattage to move this amount of water which you also seem to be concerned about. Yes there are some good pumps out there now. My Flowfriends are not quite that economical as I needed to over come about 12' of head pressure. But I think the "Pro" is a wonderful pump but consider: the Periha pumps are very impressive for a few hundred dollars each, if you have no head to over come. Advantage has a variable speed pump that I am also fond of and have been running now for the last season at about $600 per pump. Performance pro is a very good line of pumps and usually last a long time.

As far as your turn over rate question: I would certainly still use "simple math" and shoot for less than a two hour turn over rate and hope it actually occurs in less than 4 hours. I believe you are going to need a couple pumps running 24/7 for this size pond. Audioenvy has 8 pumps for 6 BD's and that is why he has such great looking water: turn over rate through an RDF. I hope your home made one performs as well.

By the way: Herons use vegetation for cover as they stalk the pond.

Many great points already:
Slopes on the pond's bottom do not really do much as debris and fish poop are almost weightless.
Wind direction is a major concern on a pond this size for debris being kept from floating to the skimmers.

Best of luck,
Monte