Calculating Waterfall Flow Rates

Use this easy guide to help calculate the flow you require for your waterfall.

Because an accurate flow rate at the top of the waterfall or feature is critical to choosing a pump, please take you time in calculating the flow.

There are two ways to establish the answer.

1) How to measure flow rate using hose/s and a bucket

This formula can be used to measure the flow rate of the pump you require.

With a container of known volume (i.e. a 25 litre bucket), measure the period of time (in seconds) taken to fill the container by the flow that you wish to measure . Then divide 3600 by the number of seconds taken to fill the container and multiply by the volume (litres) of the container. The result will be the flow rate in litres per hour.
Example: It takes 10 seconds to fill a 25 litre bucket.
3600 divided by 10 seconds x 25 litres = 9,000 LPH

You can also use this formula to decide how much flow you would like over a waterfall. Simply place a hose at the top of the waterfall and adjust the volume of water until you find the flow that you like. Measure this flow and you will have an idea of the volume required to get the effect you desire.

2) Waterfall flow rates using water width of overflow point (weir) and thickness

The most important consideration in setting up a waterfall is it's appearance. The volume of water required to achieve different effects, from a robust waterfall to a modest trickle, will depend upon the width of the waterfall lip (weir) or stream and the material that it is constructed from. The chart below will tell you how much water is required per 100mm (10cm) of waterfall width to achieve different depths of water over the entire width of the waterfall weir. Once you have calculated the total head of the waterfall, it becomes quite easy to determine which pump to use.

Example

You have a 1-meter high waterfall (measured from the pond surface), and will have 4 meters of tubing run between the pump and the top of the waterfall. The total head  is thus 1.4 meters. To achieve a 7mm water depth over the width of a 250mm wide waterfall weir, you would require a pump producing 2,520 LPH (1008 LPH per 100mm x 2.5 (250mm) total width) at a total head of 1.4 meters.

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