Anchor trenches, with or without curbs, are a simple, reliable, and pretty standard option for flexible pond liners like RPE. There are a few differences you’ll encounter if you’ve chosen a different material:
- You must determine if your project will require underlayment. If needed, the underlayment is laid out first, allowing even more excess material so the underlayment extends slightly further than the liner itself. It’s anchored together with the liner in the anchor trench, but the geotextile should continue beyond the liner, up to the top of the trench’s far side.
- PVC liners must be carefully buried under at least 12” of soil to protect them from any exposure to sunlight. A modified version of the anchor trench can still be used, but either the liner and maximum waterline will be as much as a foot below the surrounding grade, or a hump of soil will mark the outline of the pond where it covers the liner.
- An EPDM liner must also be protected from UV and ozone exposure, but it typically doesn’t require as much as 12”. It’s a good idea to check on the manufacturer’s warranty to be clear on exactly how they define “full coverage” since that may affect the details of your anchor design.
Preparing your Order
If you measured a pre-dug pond, it's a simple matter to add four feet to the total length and again to the total width. That will give you the dimensions of the sheet you should order.
If you're working from a drawing, here's the calculation you'll use. Our example assumes a roughly oval shape, with the widest points being 8' by 6' and a maximum depth of 4'.
- Start with the width (8') plus twice the maximum depth (4' + 4') for a total of 16'.
- Calculate the amount needed for the anchor trench: 2 feet on each side in our example, then multiply times 2 to account for each side for a total of 4 feet.
- Add the two (16' + 4') for a total of 20' wide.
- Do the same calculations using the length: 6' + (4' + 4') = 14' plus 4' for anchoring for a total of 18' long
For this example, then, you'll need to order a sheet that's 20' x 18'. If you're ordering a custom fabricated RPE liner, this won't be a big deal, and you'll have very little waste. If you're ordering standard cuts of some other material off the shelf at a pond liner store, you may need to play around some to figure out the layout that requires the fewest seams and is most cost-effective.