The Art of Calculating Waterfall Flow Rate

Example: How to determine your pump, biofilter & ultraviolet size as well as adjusting the flow rates for the given components. This is a hypothetical pond based on the basic plumbing schematic at the top of this page.
Take the 20 minutes or so to calculate this information for your plan, and save thousands of dollars over the life of your pond by selecting suitable components & the appropriate pump to minimize maintenance & energy cost. 


We generally recommend the complete volume of water in the pond cycle through the filter system between once per hour and once every three hours. Since this pond will be a fish pond, we will calculate for a turnover rate in the filter of once every two hours or about 1500 GPH.
As is often the case, the biofilter flow rate requirements & the waterfall flow rate requirements don't always coincide. (In this size pond, an aesthetically pleasing flow rate is 2000GPH  2500 GPH). 

Using the friction loss chart, we determine friction head loss:
50 ft. of 2" pipe will give us a friction head loss of approximately 1 1/2 ft. at the desired flow rate of 45 GPM (2700 GPH). 
1 1/2 ft. 
Number & type of fittings = (3) 2" 90°s, (4) 2" 45°s, (2) 2" Wye's, (excluding gravity return lines) = total equivalent friction head loss of approximately 40 ft. or slightly less than 1 1/2 ft. of friction head loss. 
1 1/2 ft. 
Miscellaneous other fittings such as couplings, reducers, valves, fittings (90°s, & 45°s in pumphouse), bulkhead fittings, swing check valve & strainer basket add the equivalent of 40 ft. of pipe or slightly less than 1 1/2 ft. of friction head loss. 
1 1/2 ft. 
This totals to 4 1/2 ft. of friction head loss for all the pipe & fittings.
Add to this the height of the waterfall at 4 ft. 
4 ft. 
For a total of 8 1/2 ft. of friction head loss for the system. 
8 1/2 ft. 


Using the performance chart, we find the Sequence pump at approximately 8 ft. of head yields a flow rate of 45 GPM or 2700 GPH. Allowing for a margin of error, we will achieve a waterfall flow rate of approximately 2500 GPH. 

The valves (see drawing) for the filter & the filter bypass will be adjusted to yield a flow rate into the filter of 1500 GPH. The balance of 1200 GPH will go through the filter bypass to the waterfall.
The valves that control the flow of water through the skimmer & the bottom drain would be adjusted to 60% through the bottom drain & 40% through the skimmer.
We'll choose an Aqua Ultraviolet filter: 80 watt UV filter for a 22004400 Gal Pond.

A weir is a water spillway. In the case of a waterfall, it is a rock or rocks, typically flat, over which the water cascades.
Using the waterfall weir chart below, one can predetermine, (after the above calculations are made) how wide to make your waterfall weir & at what depth the water will pool before cascading.
For this pond a flow rate of approximately 2500 GPH over an 11" wide weir stone will create a depth of approximately 3/4". 
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