construction note no.5002
subject: how to place a flexible liner for a natural pond.
For related notes and articles, see Natural Ponds Thread
Having read Note 5001 “Types of water feature”, I'll assume that you have decided you want to create a small natural pond for your garden. There are many guides “out there” on the web or in books that will tell you to dig your hole, lay your liner in and fill with water; but I think we can and should be a bit more subtle than that.
The most usual choice of liner is either a flexible PVC or Butyl liner, a pre-formed fibreglass shell or concrete. The latter is outdated and a uses an uneccessary amount of cement (see concrete alternatives article), although concrete has its uses, as we will see. Preformed Fibreglass shells should be avoided like the plague. Why? Because the shapes are all wrong for ponds and you can't alter them. They are also prone to settle unevenly, leaving you with a lip of fibreglass protruding above the waterline on one side. and the edges just can't be disguised effectively.
So it's flexible liners for us then, either butyl rubber, EPDM or a good grade PVC. Whilst liners and felt are a result of the petrochemical industry and so based upon the use of fossil fuels and all that implies, there just is no sound alternative to them at present. I believe the long term improvement of the immediate environment and the wildlife potential, plus the healing powers of water, are more than enough to offset their use.
Looking at the diagram at left, we see a gently sloping side down from a lawn edge and at the rear (where we hope it is not accessible) a vertical face using, in this instance, a log edging. I'm not going to deal with the vertical face in this Note, but will concentrate on basic lining techniques.
I'm going to tell you about the professional, belt and braces approach to pond lining. You can choose to do a lesser job, at your risk; but a hole in the liner is a pain in the...
A 50mm sand lining goes in first. Any sand will do, although a sharp sand binds well. The sand needs some moisture in it to bind. Some say that sand is no longer necessary as we are using felt - if your soil is at all stony, use both.
Throw the sand around with a shovel. With practice, much will stick to the sides. Level out with the back of a rake and pat smooth with the back of the shovel, or your hands in small or awkward areas.
The liner is sandwiched between two layers of felt geomembrane. The felt is overlaid and the edges taped down. A heat gun can be used to “weld” the felt into one sheet but beware of damaging the liner.
Once the under-layer of felt is in place, remove your shoes whilst dealing with the main liner. carefully unroll or fold this down the length of the pond and pull out the edges. Work all creases out into neat folds in the corners and curved areas. The less folds the better. Tape folds down if necessary for easier working. Where folds emerge over the edge of the pond is a vulnerable point for leakage, so make sure they are well above the final water level.
Finally, place the internal felt layer. Watch that stones etc. don't get trapped between the layers. An industrial vacuum is good for removing soil and stones.
A ledge runs around the rear half of the pond (right), which will contain soil for marginals and rocks. A concrete pad is placed for strength and protection.
Once all permanent features (rock, pump base, etc) have been installed, the inner felt needs to be covered over. On a largish pond with moderately sloping sides, this can be covered with subsoil from the pond excavation, to a depth of 200 - 300 mm (left - note: only experienced operators should track an excavator over the liner like this!). Avoid using nutrient-rich topsoil.
Smaller ponds and steeper sides can have pebbles set into concrete as a solid base. Loose pebbles should not be used as they soon get misplaced, exposing the felt and liner. This method will be covered under stream construction.
Rocks have to be placed on concrete pads to spread the weight and prevent liner damage.
How to deal with the pond edge is crucial. A level lip must be prepared all around the pond (except in marginal shelf areas), over which the liner folds and tucks down. This will be the subject of another construction note.
Left, middle: The submersible pump in this pond is hidden under a cluster of rocks, topped with driftwood which came from the local foreshore. Note the pond edge detail - no liner will ever show!
Left, bottom: The pond one year on. Marginals are growing nicely!
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