Thread: Double bottom drain

Hello, have started my new pond build. Knocked down the small pond that was here when I bought the house and started to jackhammer out the bed rock.

The pond will have a liner but be onto bed rock at the bottom and be gravity fed. It will be slightly below ground and have concrete block walls rising about 1 meter above ground also.
Plan to have 2 bottom drains at different corners of the pond.
Problem being that there is little room for the pipes due to the bedrock, created a channel for each into the garden. One will run around 6-7 meters to the filter, the other will have to run around the pond past the other bottom drain piping and into the filter.
Due to lack of room I wonder if it’s better to connect the two bottom drains in one T junction?
Or if they both need their own piping to the filter?
The one that has to travel further will also sit slight lower in the water that the 2nd. Maybe only 10cm.
I have a filtreau combi 2 drum and moving bed filter. It has 2 inlets and one outlet.
The piping for the filters will run under my decking and into the shed with a rise at the end to reach the filter. There will be some slight changes in height before that to run with the bedrock.
Alternatively, could I step down the 110mm pipe to 40/50mm without issues for the bottom drains.?

It will be 15-20000 litres, around 4m by 3m and 1-2 meters depth, varying because of the rock bottom.

Okay so forgoing the t junction and having 2 separate runs to the filter of different lengths, will that mean one bottom drain pulls more than the other?

Given one must go round the pond under the ground, it will have a 90 degree turn, is that not recommended? It will be difficult to do anything but have a 90 degree turn at the corner. Unless using tubing instead of pipe.

Due to the lack of room, could I step down the bottom drain pipes from a 110mm to a 40 or 50mm without causing too many problems?

I wouldn't say having 2 bottom drains is absolutely essential, especially if you can centralise a single drain and slope the pond floor towards it. But this is on the assumption of a square or uniform pond.

If for example the pond is very long and thin, or has a number of curves then 2 drains remains preferable. For example, my pond is just over 15,000 litres but is 7 metres long and keyhole shaped. one drain in the middle of the key hole would leave debris to build up in the long thin part for example, so in this scenario a drain at each end is much better. To purge the BD pipework I alternate closing one drain off to get the necessary flow, which would be difficult to achieve if they were joined by a T-piece.

I'd not entertain the T-piece idea for a list of reasons that I probably don't need to explain, uneven flow and the inability to purge the pipework being the main ones.

90 degree bends should be fine on 110mm / 4" pipe.

Are you not able to break out a channel in the rock for a BD pipe to run directly under the pond? Not essential but would reduce pipework length and remove the need for the 90 degree elbows, though as I say, having a 90 degree bend on one of the pipes is not the end of the world, though the less hidden joins the better for sure.

It won’t be possible to take out a track in the rock big enough to run the pipe directly. At least not with hand held machinery.
There is some natural channels that I have taken deeper and wider to fit the drains that naturally sit at the bottom of the pond.

I’ll try to use a 90 degree sweep to get around the corner. I’ll dig some more to see if I can fit 2 separate runs for the drains. If it’s not possible I’ll use a sweeping T and maybe a valve just before it.