Concrete Pond Construction

Frequently asked questions about Bottom Drains & Pipework

Q. I am concerned as to how to seal the drain pipe in the concrete. Is the pipe just painted with unibond before casting or just buried in concrete or what?

A. You just pour the concrete around the bottom drain and the pipe.

Q. I have 2 x 4" bottom drains and my vortex has a 6" inlet. How do I connect 2 x 4" into 6". Do I use a tee at the vortex and put the 4" into the 6" reducers? Or can I put a tee in between bot drains and then to an elbow up to 6" with a 4" reducer so that way I only have 1 4" line going to vortex? My pond is 15' long x 10' wide x 6.5' deep.

A. If your pond is rectangular then it will hold just over 6000 gallons.
You will need to pump over 3000 gals/hr and one 4" pipe will only allow a maximum of 2400 gals/hr. This rules out having the two pipes connected with a 4 ins tee piece.
The best way is to have a 6 ins tee piece and then connect each pipe from the two bottom drains using reducers. You should try to have each pipe about the same length with the same number of bends or elbows so you get the same draw from each bottom drain.
You will need to put a 4 ins valve on each pipe so you can shut off one bottom drain and purge out the other pipe at regular intervals. Check this photo.

Q. In an attempt to keep the pond dry I went out, and bought a big tarpaulin to put over the pond. Even though the rain can't fall directly into the pond, it still seeps in from the dirt that makes the walls of the pond.
Since the pond will be 6 feet deep do you think the weight of the pond water on the liner will keep water from seeping in under the liner when we have more heavy rains? I'm concerned about water seeping in under the liner, and pushing the liner up. When I dug the pond, I didn't hit the clay layer until a foot from my final digging depth (6 feet).
If you think it's a good idea. I'll encase my bottom drains in 4 inches of concrete, to help hold them in place.

A. I certainly would not bother about the water under the liner as the weight of water in the pond will keep the bottom from raising.
This happens with all pond liners except for those that are on very porous soil.
It is important to never completely empty the pond once it has been filled the first time. Partial emptying down even to half way should be OK.
Big tarpaulins are a good idea, but I find that they often have a tendency to collect pools of water and then drop the whole lot into the hole they are covering.
You will need to dig a trench for the pipes and the bottom drains. I would put in a few mounds of cement or concrete and then lay the pipes and drains
on this until you have them level and laid out the way you want.
When this has set and you are satisfied with it then fill in the trench with concrete including around the bottom drains.
When the pipes come out beyond the bottom of the liner then this part of th pipes should not be encased in concrete as you may wish to alter them in the future.

Q. My pond is approx 4.5k-5k gallons,16ft long by 10 wide by 5ft deep, the bottom is benched with a bottom drain.
The shape of the pond is rectangular with the top end curved. I have one return plus a venturi about 6 inches under the surface and a waterfall, the current goes around the pond clockwise. The pond only has 4 fish in at this time which are all about 10 inches long as I am still finishing off this pond.
The trouble I am getting is that the fish waste gathers at the bottom left hand side of the pond (under the venturi, the return and the w/fall). the pump i use turns 2,580 galls per hour. Any thoughts to why the waste is collecting like it is, and any suggestions to solve this problem?

A. For a 16 x 10 ft pond I would recommend a minimum of two bottom drains, and for anything over 15ft I usually recommend three. Even then there is no guarantee that all corners of the pond will remain clear.
One way to help the situation, and used by quite a few of our members is to have a 45 degree bend on the end of the venturi and this can help with the flow of the water where you want it.
If you have even short blanketweed growing on the bottom then this will prevent the debris from flowing towards the bottom drain, and you will have to vacuum out the pond. You have only 4 ten inch fish in a large pond. When you have more fish then they will tend to stir up the fish waste and help it on its way to the bottom drain.
You mention benching of the bottom, but do not say how much. The minimum should be 6 inches, but with a 10ft wide pond I would recommend 9 inches.
Another method used successfully on several newly built large ponds in the UK is to have several deep water returns. These are angled downwards, and so keep the fish waste and debris moving. The only problem with this is that you have to have a separate pump for each return so it becomes expensive to build and to run.

Q. I'm in the process of building a 4500 gallon pond and have been getting lots of good conflicting advice about bottom drains and pipe sizes.

1. Use one 4" bottom drain with 4" pipe
2. Use two 4" bottom drains, but use a 4" to 3" reducer at the drain and use 3" pipe.
3. Use two 4" bottom drains with 4" pipe.
4. Air diffusers on the bottom drains are good because they help to scour the bottom better.
5. Don't use air diffusers on bottom drains as they just cause turbulence and suck the debris from the bottom into the air stream and throw all the debris back into the water.

My thoughts?
I like the idea of two 3" pipes because of the increased draw.
I like the idea of two 4" pipes because if one blocks then there will be enough flow through the other.
I don't like two 4" pipes because each will only have about 1250 gph through it, so might silt up. But then all it does is turn it into a 3" pipe, and once it gets to that size will have the flow to stop it silting up even more? Who knows?
A bottom drain will draw 1500 gph with or without an air diffuser, plus blanket weed likes oxygen and the one place I don't need blanket weed is around the bottom drain!

A. There has been a lot of discussion recently on using three inch pipe for bottom drains, but I am all in favour of using two four inch pipes.
Quite a few of our members have ponds of 4500 to 6000 gallon ponds with two and three bottom drains. They have been using them for several years without any problems with silting up.
I think the most important thing is to flush out each pipe separately at least once a month, and this should dispose of any silt that may be in the pipe. It is also important that the pipes have as few bends as possible, and that you use bends and not elbows.

Air diffusers on the bottom drains seem very popular now and I would certainly recommend them. Three of our members have had them in for a year and they are working well.
The old way was to run the airline down the four inch pipe, but several dealers are now selling bottom drains with an additional connection so that the airline is not in the 4" pipe which seems sensible as this could be another cause for silting up, and this should be seriously considered.
One well known pond in the UK had 1/2 inch hose in the 4 inch pipework and this has choked up and all pipework has had to be replaced.

Q. I'm just wondering if the one foot drop on the base of the pond will actually do anything, or should I just have a flat bottom and go for the increased volume?

A. The one foot drop is absolutely essential as it is important to bench the bottom so that the debris is directed towards the bottom drain. You should try and make it in the shape of a saucer with the bottom drain in the centre.
Increased volume is a good idea so why not go six or twelve inches deeper.

Q. I would like to pick your brain regarding the position of the bottom drain to the concrete finish of the base.Do I finish the concrete level to the top of the drain and the rendering finish on top of that, or allow for the rendering to finish level to the top of the drain i.e. the concrete finishing say 0.5 inch from the top.
I would appreciate your thoughts on this problem as it isn't clear on any drawings I have seen so far.

A. This is a very good point and I agree it has not been made clear, and something I will add.
The concrete should be say 0.5 inches below the level of the top of the bottom drain, and then the final rendering will be on top of that and finish level with the top of the flange on the bottom drain.
You can just see this on the last photo on this page but it is not explained.

Q. I am in the course of planning my second pond - a Koi pond of between 2,000 to 2,500 gallons.
I envisage about 8 by 12 and average of 4ft deep. (I'll try to go to nearer 5 ft for drains). I am intending to install bottom drains and I have been told that it would be beneficial to install 2 of these.
Whilst I feel this is common sense, I am getting conflicting advice on the pipe diameter that I should use.
Two established Koi dealers involved in construction say they use 4 inch for all jobs, but I have read that with only roughly 600 gallons an hour through each drain, this could cause settlement in them and not provide enough pressure to the filter. Should I use smaller pipework of say two inches?

A. I agree that with a 8 x 12 pond that two bottom drains would be beneficial and suggest you do that.
Barry Goodwin in his book "The enigima of Koi" does state that with 4" inch pipe the minimum flow should be 1,500 to 1,800 gallons per hour. The maximum that can be drawn through a 4" pipe is 2,400 gallons per hour so it si very difficult to keep within this margin.
I would certainly not like to use two inch pipe as this can easily choke up and consider 3" to be the minimum diameter. You could install two bottom drains and connect one to the filter, and the other would go direct to waste. This may be the best solution. It is essential to flush through the pipework at regular intervals and methods of doing this is shown here and another method here. In both cases a vortex chamber is shown, but it will work with any type of settlement chamber.

Q. I've had another change of plan. Instead of two bottom drains I've decide to install one 10inch bottom drain plus a 4inch connection around two feet off the bottom. I will then attach a pipe at right angles which will end 0.5inches from the pond floor and will vacuum the area around it and feed the second vortex unit. In winter I can remove or rotate the pipe so it draws water from a higher level (I've done this before and it works a treat).

A. If you wanted a mid water feed I would have still have had two bottom drains as well. The mid water feed would then be connected to one of the pipes from the bottom drain to the vortex chamber. With two slide valves you can then decide where you want to draw the water from in the winter.

Q. Are two bottom drains sufficient for my new pond? I plan each to feed a vortex + two other vortex units (32"wide, 38" deep) with matting or Flocor for a total of 2 vortex + 4 biological units? Does this sound OK?

A. Two bottom drains would probably just be enough as each one would serve an area of 12ft x 11ft. The advantage of two in this case would be that one would serve each of the vortex chambers, however most people seem to wish they had more bottom drains.
The Vortex chambers should be purchased before you start any digging as then you can put them in place first, and all your heights and levels are then taken from that point.
I would suggest that when you purchse the vortex units you ask the manufacturer or person you buy it from as to their suggestion of number of drains etc. If you have problems with green water or something similar later on then you can go back to them for advice. You should also ask them for advice on size of vortex chambers and size of pumps, as these people should be able to make recommendations as they know exactly how there systems work. You cannot apply one rule to all vortex chambers as they do vary regarding entrance/ exit points, and depth.

Q. Would two bottom drains be adequate for a 14ft x 12ft pond, or more the merrier? Or more difficult to fit?

A. Two bottom drains should be sufficient with the bottom well benched so that the debris goes towards the drains. This is assuming the pond is rectangular. If it has large rounded corners you would probably require three.

Q. Is pressure pipe 'really' necessary for under the pond? What I mean to ask is the chance of failure of terrain pipe 1% or 75%?, do you have experience of problems when terrain has been used.

A. I have not seen any failures when terrain pipe has been used. If you look at the 30 ponds featured in our section only two have pressure pipe. With cost an important item then I consider terrain pipe satisfactory.
Terrain pipe is designed for main drains and as such remains empty for most of the time. The ideal pipe is the pressure pipe, but when you come to the 4" diameter it is very expensive.

Q. Bearing in mind my pond would not be heated what is the set up recommended for winter? Drawing water from the deepest part would in effect produce colder water all round yes?

A. In theory you are right and this is stated in many books. With a large pond being constantly pumped and the fish occasionally moving I find it difficult to agree that there are significant layers of water at different temperatures. However many koi keepers do use smaller pumps in the winter, or turn down the capacity, and in this case mid water feed is a good idea and reduces the disturbance of the water.

Q. Is a mid water feed necessary if you fit a skimmer?( I read that one of your members bypassed/disconnected his mid water feed after fitting a skimmer).I don't want to be in a position of having to drill into brick/fibreglass by leaving something off.

A.The mid water feed is usually used only in the winter. As you say you do not wish to draw the water from the bottom drain in the winter when the fish are resting down there. The problem with using a mid water feed is that the type of floating debris it picks up will not be removed by the Vortex chamber. There can be a lot of discussion over mid water feed, but I would be inclined to fit one. I would ignore the comment made by our member as this is taken out of context. If you change a lot of water in the pond the skimmer will have to be shut off or it will run dry. Skimmers are usually used with a separate pump and a time switch.

Q. Thanks for the tip regarding pressure testing of pipe work prior to fitting which is a great idea as it will put my mind at ease as I feel sure it will be along time before I actually fill the pond with water, Instead of using an expandable rubber plug I have just blanked off the bottom drain and will test it before cutting the vertical pipe thereby testing it to twice the head of water.

A. This is just as good but many koi keepers do not have the materials to blank off the bottom drain, and our members usually have a rubber plug as they are very useful if you want to blank off any four inch pipe which does not have a valve.

Q. I am still a bit confused with regard to the drainage. As we are on a tight budget, is it possible to just use the drains to remove soiled water manually and then replenish the pond with fresh water.

A.We do have quite a few members that are on a tight budget (most of us are). Yes, it is possible to have drains and discharge them direct to waste. In fact, until a few years ago it was normal to do this, and use a feed at mid water to feed the filter. The idea was that you discharged the dirtier water from the bottom to waste and so used the cleaner water at half depth to feed the filter. It should also be borne in mind that with the current water restrictions, and higher charges in the future including water meters, then we must all aim to cut down on the amount of tap water that we use.

Q. Does the main drain being 40ft or more,and possibly higher than the pond mean major problems? I remember reading you had mentioned a soakaway but I really don't know if this is feasible.

A. The entry of the pipe into the main sewer must be lower than the level of the pond surface. If you cannot do this then have a sump chamber alongside the pond and use a submersible pump such as a Dab Nova with float switch.
A soakaway is only really feasible if you live in a remote area. 

Q. One problem I had with the koi even when they appeared healthy was, they all had a tendency to swim up the bottom drain and into the vortex chamber. I stopped this by fitting a mesh over the inlet bend ,I was able to stop them getting into the vortex chamber, but once they were trapped beside the mesh they did not seem able to find their way back down into the pond, I then regularly had to put them back into the pond myself. have you come across this problem before, and if so is there a way around it?

A. If you have the proper bottom drains designed for a koi pond then this cannot happen. The curved top is positioned so that there is about a half inch (one finger width) between the cover and the base of the pond. Besides keeping the fish out it draws the water from a large radius from the pond bottom.

Q. I've left a gap of around 2-3 inches between the blocks and the surrounding earth. Do the blocks need fixing (in the way wall ties work) into the earth or does the concrete backfill suffice? I am planning to make a really runny concrete mix and literally pour it in.

A.Definitely do not tie in the concrete to the surrounding earth. The mix and method seem fine.

Q. My dealer suggests that I fill the 110mm bottom drain pipes with water before the concrete pour, how do I retain the water in the pipework?

A. To retain the water in the bottom drain pipes you can use a stopper as shown at the bottom of this page.
These stoppers are available at most large building suppliers and are very useful to have around.

Q. I laid my bottom drain and then noticed when I put the spirit level on the pipe it was not level. I raised the end with the swept bend on a three inch brick and got my level reading. Question, my husband thinks that the pipe should slope downwards at the bend, I think the pipe should be level.

A. The pipe should be level. If the pipe slopes downwards at the bend there is always a possibility that silt will build up at this point especially if the flow rate is slow.