Now that you have a general idea of which components you will need and know approximately where they will go, you need to ensure that the pump, waterfall, stream, filtration or skimmers are sized correctly.
Sizing the Pump(s): Initially, we are sizing the pump to turn your water over 1-2 times an hour a pump to turn over based upon the estimated gallon capacity of the system. The previous article discussed pump and component placement. This will give you an idea of the distance the water will travel through plumbing and how high above the surface you are moving the water in feet (head height). As a starting estimate take the number of feet above the pond and every 10’ of distance in plumbing and make that your starting static head amount (round up on any partial footage). Most pumps have a head chart. The rating of the pump is at full capacity, but you need to look what it can pump at a given head height. Now consider if a pump fails or is taken offline, what will happen to your system while it is off? Do you have expensive Koi? Will they be okay? This is why you may want to divide the work between multiple pumps for redundancy. This is your baseline size of the pump(s). Here is an example of my pumps: I have an external pump (rated at 3900GPH) that is turning over my entire system (2600 gallons) about once an hour (~2575GPH at an 11’ head). I have a submersible pump (4490GPH) for skimming but the water acts as a backup to the main pump should it go offline (~2400GPH at an 8’ head). This would be enough to keep my pond relatively clean while the main pump is off. Together they make a sizable waterfall and filter lots of pond water per hour too (~5000GPH).
Sizing Waterfall or Stream Flow Aesthetics: While sizing the pump must meet the gallon capacity filtration needs, you may want to upsize the pumps further. Why? For every 1’ width of waterfall or streambed, the feature needs 500 GPH for each increment of flow aesthetics as follows: 500 GPH: Trickling, 1000 GPH: Gentle, 1500 GPH: Average, 2000 GPH: Rigorous, and 2500 GPH: Turbulent. My waterfall is nearly 2’ wide, with my combined pump flow of nearly 5000 GPH; I have achieved a turbulent waterfall. With only one pump running, I achieve between a gentle and average waterfall flowing down to the pond.
Sizing Filtration: Most bio-filters will give a range of water they are able to treat an hour OR list it by pond capacity. Instead of meeting this, I would double the size of the filter; or use a pair of them. Why? This is because filters are most effective if the water moves SLOWLY through them. If the water rushes through, the beneficial bacteria will not have a chance to change harmful compounds into safe ones. Further, if you are incorporating a settling area under your filters, the slower the water moves, the more inorganic matter will drop out of solution (sludge pit). This will clean your water much faster. The same goes for other components like UV clarifiers – size them so that the water divides up and passes slowly in front of multiple lamps. This will be much more effective than water speeding by one. For large pond systems, custom filters can be fabricated. In my pond, I have a 150 gallon stock tank. The pumps both discharge into the bottom 4” of the tank (the sludge pit). From there, the water has to travel up through 14” of course and medium filter pads. Further the large, deep tank is 5’x2’x2’ – this slows the water down while it travels upwards. This filter can handle approximately 4500 gallons (I have a 2600 gallon system).
Sizing Skimmers: Skimmers are size similarly to filtration by what they treat per hour, system capacity or some might be by surface area. Ensure that you have a skimmer big enough (if not more) to meet this. If you have dead zones where debris eddies or floats, like in a side channel, consider putting multiple units by dividing their coverage. You always want the debris to flow towards a skimmer. A skimmer is just the housing, filter and weir. You will need to size the pump to go with it (or hook to an existing pump system).
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