Q: How do I calculate what size basin I need for a pondless waterfall?
Robert – Waverly, TN
A: With the help of a few friends, installing a pondless waterfall is an easy way to upgrade your land- or waterscape. If you are planning to design and build your own feature, here’s how to do the math to determine your basin size.
Step 1: Calculate total amount of water in your stream
Sound tricky? It’s not. To calculate the water that’s in motion in your stream, first measure your stream’s length, width and depth. Then plug those numbers into this equation: L x W x (0.25 x D of stream, generally 1 to 2 inches) x 7.48.
Here’s an example. If your stream is 25 feet long, 2 feet wide and 1 inch deep, the equation would look like this: 25 x 2 x (0.25 x 1) x 7.48 = 93.5 gallons.
Step 2: Calculate amount of water your basin needs to hold
As a general rule, you will need your basin to hold 2½ times the amount of water in your stream. To find out how much water your basin will need to hold, multiply the gallons of water in your stream by 2.5. In the above example, it would be 93.5 x 2.5 = 233.75 – so your basin would need to hold 233.75 gallons of water.
Step 3: Determine the size basin needed to hold the water
Here’s where things get a little tricky. You’ll likely use boulders, stones and other décor in your design. Well all that rockwork will take up space for water, and so you’ll need to go with a larger vessel.
Try this quick equation that will help you determine what size basin you’ll need with boulders: L x W x D x 2.2 = gallons of water the basin will need to hold. To give you an idea, a 5 foot by 10 foot basin that’s 3 feet deep and contains stone will hold 330 gallons of water.
A space- and cost-saving option is to use a basin matrix in place of some of the boulders. A basin matrix is a strong, 27-by-16-by-17½-inch hollow box that’s perforated with half-inch holes to allow water to collect and pass through. You can stack them together and landscape right over them. Each basin matrix holds 31.5 gallons, adding valuable water volume without increasing the size of the basin. Don’t forget to add a pump vault for easy access to your pump.
Go with a Kit
If you aren’t trying to customize a large waterfall, consider investing in a pond free waterfall kit, like PondBuilder™ Cascading Falls Pondless Kits. They contain all components you’ll need – from underlayment and liner to a waterfall box, pump, basin matrix and plumbing – with predetermined dimensions to help you create your water feature.
Pond Talk: What’s the largest size boulder you’ve installed in your water- or landscape?
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