It is generally accepted that 110mm (4 inch) bottom drains are essential and should be fitted to all ponds under construction. "If you want to maximize the performance and minimize the maintenance of your pond you absolutely need to install a bottom drain. It is not a matter of opinion, they are a fundamental must have - anyone who tells you otherwise is either uninformed or lacks objectivity." Clark Koi Ponds
The term "bottom drain" is really a misnomer since it is not intended to drain the pond Perhaps it should be more aptly referred to the "pond bottom feed" or "bottom filter intake" or "filter supply" or any suitable combination that rings true to the ear. They are a dynamic part of the functioning pond and rarely used drain the pond.
A pump is used to circulate pond water through the filtration unit and back to the pond. It carries toxic chemical fish waste products to the biological filters and in doing so transports physical waste like uneaten food, faeces and other debris for accumulation and final removal from the pond. This is why a bottom drain is where it is. A submersible pump is not aesthetically attractive in a pond and not so effective in removing larger debris. They also tend to block from leaves and algae and become a source of continual frustration as you toil to maintain your pond.
The number of bottom drains required will depend on the size and shape of the pond, the angle of the slope to the bottom drain(s) and flow rate. Yet almost always the preferences of the owner determines this based on his design or budget. However, to get a decent sweeping action, drain should be placed so as to cover a radius of 1,5-2m (4-8 feet), or one drain per 20-25,000 litre (5000-6500 gallons) pond water when expressed in volume.
Some bottom drains come with an air diffuser disk in the middle. The updraft of the column of air provides powerful circulation in the pond assisting greatly in "sweeping" the debris at the bottom of the pond to the drain intake point. Aerated bottom drains must have an air pump to work that is powerful enough to overcome the water pressure at operating depth of the pond. In addition, the continuous aeration ensures proper mixing and healthy aerobic conditions in the pond and ensures sufficient oxygen for the biological filtration.
Bottom drains are placed at the deepest part of the pond for the removal of the waste. If more than one is needed, the pond bottom must be shaped so that each drain sweeps its section efficiently. Ideally you want equal flow through each drain therefore each drain must be connected via its own pipe to the filter room, preferably via a sump/ settlement chamber.
In a large pond the pipe losses, and therefore the flow through each drain, must be calculated for the expected turnover rate. Then flow or sweep reach equalized accordingly. This means sweep area/slope or in severe cases pipe diameters, be changed. This matching can be done by inserting a pre-calculated length of 160 mm PVC pipe where appropriasie. Do not just guess if you wish to make the effort worth it.
There will always be sediment building up in the drainpipe. Therefore, a drainable sump/settlement area is as important as the drains to allow occasional flushing of drainpipes and settlement chamber to remove of the buildup of debris from the pond..
Note: When multiple drains are used they are never connected in series to the same drain pipe. They should also never combine to one pipe before the sump/settlement chamber. One straight pipe between the bottom drain and sump standpipe elbow and never any other kink or elbow.
Servaas de Kock