Alan’s Book

Chapter 2 Pond Construction

Once the planning stage is finished the build stage may commence. Some people may prefer to use an experienced builder to construct their ponds whilst others may prefer D.I.Y.

The following paragraphs are not intended to give detailed pond construction details but they make provoke some thoughts and offer a few ideas which may help overall.

Generally speaking the ground will determine the construction to a large extent. I have always been lucky to have had heavy clay beneath the ponds that I have built and a base of 4″ concrete has been strong enough. Those who have to construct their bases on sand or similar ground types should take expert advice on construction.

The pond base

Before pouring the concrete base, the drains should be in position and connected such that the 4″ pipework extends beyond the pond walls. It is good practice to include a damp proof membrane under the pond floor which extends well beyond the base and can be used as a barrier to ground water between the outer pond walls and the backfill. This is particularly important in areas which have high water tables. In such circumstances it is wise to leave a section of the backfill open such that if rain or ground water is encountered a pump can be used to prevent water entering the pond via the walls before it is filled. If this happens it is very difficult to get pond linings such as fibreglass to adhere to the walls. Alternatively a chamber can be constructed when the pond walls are being built into which a pump with an automatic float switch may be placed permanently and any ground water will be removed to a convenient drain. An access lid could be arranged at ground level for maintenance and inspection purposes. If ground water is allowed to build up around the pond, this can push any lining from the pond walls if the pond needs to be drained down for any reason.

The pond walls

Solid concrete blocks 4″ thick are acceptable for below ground work as long as these are backfilled well to counter the weight of the water from the other side. Above ground 6″ blocks are usually satisfactory but some pond builders prefer to use 4″ blocks on their sides making the walls 9″ thick; this allows for wider coping stones and confers a more robust image of the pond structure.

Two inch pond returns should be built into the walls during construction; top and bottom returns are good practice and these should be installed about 40 cm from the bottom of the pond and about 50 cm from the pond surface. Arrange them in a corner such that the flow is close to and directed down the long length of the pond wall to create a circular clockwise rotation of water within the pond.

I always include an overflow in a pond design preferring to have the facility where water can be trickled through the pond as required to waste via the overflow. This can be build into the walls or can be arranged with a 11/2″ pipe through the pond wall and a 90 degree bend with a short length of 11/2″ pipe which can be adjusted to the correct level for overflowing. The 90 degree bend should be left unglued so that it will swivel when overflow levels are being set.

The overflow should be placed in the centre at the same end of the pond as the returns.

The pond floor

If the pond returns are correctly placed and the design follows the principles detailed in the planning stage, the solid waste will be collected via the bottom drain even with a flat bottomed pond. In practice algae will grow on the pond floor and tend to resist the solid waste moving towards the bottom drains. To counter this, the design could incorporate a slight fall towards the drains. If each corner is rounded off and the join between the pond walls and floor includes a small fillet of about 100 mm (45 degrees) this will improve the circular flows and prevent detritus from lodging along the pond perimeters.

Preparation for waterproofing

The inner surface of the pond should be rendered to give smooth finish and the chosen method of waterproofing the pond can begin once the render is dry.

There are several means of waterproofing the pond walls and floor such as fibreglass; proprietary brands of sealers can be painted onto the surface or a flexible liner can be sprayed on the inner pond surface. The first choice where budget allows is the spray on flexible liner followed by the fibreglass option. Both of these methods will allow some structural movement without causing major leaking problems.

The coping stone or surface finish will depend upon individual choice but I prefer to see the pond liner such as spray on liner or fibreglass taken over the top rim of the pond such that whatever coping arrangement is used covers this edge.

One of the benefits of the spray on liner is that the pond may be filled immediately whereas fibreglass requires time to set and “cure”. Fibreglass also needs close examination to ensure that no rough edges remain upon which the fish can damage themselves.

Once the pond is completed, filling can commence. Regular shapes can easily be measured to determine their volume but more irregular shapes are best dealt with by filling via a water meter. It may be necessary to treat the pond water for parasites or bacterial problems at some stage so it is essential that an accurate record is made of the pond volume/gallonage to allow correct doses of treatments.

Suitable materials for ponds.

Care should be exercised with materials which come into contact with the pond/ water as well as materials such as coping stones and pergolas which may have contact with rain water which runs into the pond. Any exposed cement and wood preservatives should be sealed to reduce the problems of toxins entering the pond from these sources. Some wood preservatives use copper, cadmium and arsenic all of which are toxic to koi! Exposed cement and certain low grade fibreglass products also contain toxic substances.

Copper and galvanized steel pipes should not be used in the system on the supply side of the water purifier which feeds the pond.

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