|Langston University Aquaculture|
Pond Types and Liners
By Conrad Kleinholz
Ornamental ponds may be constructed by simply digging a hole and filling it with water. Ornamental ponds constructed in native soils are commonly called mud ponds. Mud ponds seldom maintain clear water, because of suspended soil particles or growth of planktonic algae. Mud ponds are favored by the Japanese for growing large koi and for development of coloration and patterns of the fish. Mud ponds are excellent for growing aquatic vegetation and develop more stable ecosystems than ponds with liners, but mud ponds seldom maintain clear water. They do not require filter systems, although supplemental aeration is recommended during summer to avoid fish kills from low dissolved oxygen. Mud ponds should contain a drain. At some time, the pond will require draining to conduct maintenance such as sediment and debris removal. An overflow drain will also minimize pond levee damage from storm water during large precipitation events.
Most ornamental ponds contain a liner to separate the soil surrounding the pond from the enclosed water. Lined ponds can also be placed above the soil surface. Lined ponds are favored for water clarity, for locations where steep sided mud ponds can not be maintained, and to reduce water loss from seepage. Pond liners are either rigid or flexible. Rigid liners are made from fiberglass or concrete. Concrete liners are reinforced with steel mesh or rods and either poured into a form or sprayed on the pond bottom and sidewalls (gunite). Fiberglass pond liners are purchased preformed, rather than assembled or fabricated on site. Spray or roller applied polyurethane coatings can be used to permanently seal leaks in rigid liners, or can be used as the primary liner, using a geotextile membrane as the substrate. An advantage to rigid liners is that penetrations for drains and fill lines can be molded into the liner during construction. Premolded fittings greatly reduce the chance for future leaks. There are two principal disadvantages to rigid pond liners, both associated with construction. The first is that rigid liner installation usually requires heavy construction equipment and access to the pond site by the equipment. The second disadvantage is that rigid liners are more expensive than flexible liners.
Flexible liners are made from polyvinyl chloride (PVC), polyethylene or EPDM (Ethylene Propylene Diene Monomer) rubber. Flexible liners have several advantages. They are much easier for homeowners to install since they do not require specialized or expensive equipment, much less expensive to purchase, and less susceptible to damage from shifting soil. However, flexible liners are more susceptible to damage from ultraviolet radiation and puncture than rigid liner materials. Vertical pond margins greater than 12” deep are difficult to maintain with flexible liners unless the walls are stabilized or reinforced.
Above Ground Ponds
Free standing ornamental ponds can be placed on level soil, preexisting concrete surfaces or yard decks. These ponds can be installed completely within a day. You can use small, preformed kits, plastic or wooden barrels or tanks, or lumber or masonry walls. Outside walls can be hidden with landscaping or carpentry. If the ponds are placed on yard decks, make sure there is adequate support for the added weight of the pond. Water weighs 8.3 pounds per gallon, or 62 pounds per cubic foot. Calculate the volume of the finished pond (before you start) to find the weight bearing load on the structure. If the deck is not strong enough to carry the weight, choose another site, or add additional piers and beams under the deck to support the weight.
Wood framed above ground ponds are another alternative. If you want a temporary pond as an entertaining accent, to hold fish or plants for quarantine, or while building or repairing an existing pond, free standing ponds can be constructed from plywood and dimension lumber. These ponds can be placed on level soil or concrete surfaces and can be constructed within a single day. The ponds are built as wall units connected with wood screws and short blocks of lumber. A flexible liner is placed inside, draped over the top of the wall units and secured in place. However, wood-framed ponds are height–limited to about 24 inches unless you add cross bracing or stiffen the sidewalls to keep the pond sides from bulging or collapse.
Lined Ponds, Rigid Liners and Preformed Kits
Preformed pond liner shells made from fiberglass, polyethylene or polyvinyl chloride (PVC) are available in many discount, home improvement, or retail garden outlets. They are usually sold in ‘kit’ form and include a pump for water circulation. The pump may also power a fountain, or a small waterfall. These kit ponds are 12-18” deep. The resulting site for these ponds can be provided by constructing a berm around the kit edge, similar to a raised garden bed or by a shallow excavation.
Concrete, or gunite (a form of spray applied concrete), was once the material of choice for ornamental pond liners, as it had a proven background in the swimming pool industry. Plumbing and electrical conduits can be incorporated into the concrete during construction to minimize the potential for leaks. For ponds more than four feet deep or for ponds on slopes, concrete may still be the material of choice. Concrete or gunite ponds are seldom owner built. Concrete ponds contain reinforcing materials, either steel rods or mesh, which makes them both strong and stiff. Concrete must be cured for up to 60 days before used to house aquatic organisms, as hydroxides from fresh concrete make the water too alkaline when they leach into the water. Unfortunately, the concrete surface decays over time from weakly acidic waste produced by fish and bacteria in the pond and may require sealants or repair after a few years. Concrete also has a tendency to crack when exposed to extreme wet/dry climatic cycles that cause soil movement, or when used in unstable soils such as clay. Concrete ponds in clay soils must have a sand liner between the soil and the concrete. Installation requires access by concrete trucks, so plan for the access. Concrete trucks are very heavy and may crush subsurface plumbing or sewer lines, which may also complicate access to the pond site.
Some ponds have fiberglass liners. Fiberglass is inert and can be formulated with various colors. Plumbing fittings and electrical conduits can be molded into the form to ensure leak-free installation. The primary shortcomings of fiberglass liners are that the liner must be built in one location, and moved to the pond site. The liners are built on a form, so obtaining a custom design may be very expensive if the company is required to make a new form. The excavated pond site must be carefully leveled and back-filled with sand to prevent deformation and eventual cracking of the liner in the event of soil movement. Fiberglass repair requires a clean, dry surface, so cracks or holes in fiberglass liners require the pond to be drained below the repair site. Fiberglass pond installation requires site access by sand trucks, and possible a crane to lift the form from the transport truck and lower it into the prepared hole.
The most widely used material for lining ornamental ponds is EPDM sheeting. EPDM is highly flexile and conforms to irregular surfaces. It is chemically inert, very puncture resistant, and UV resistant. The liner material is usually guaranteed against UV damage for up to 30 years. EPDM can be easily cut to shape and hot or cold glued. EPDM liner material is available in thicknesses from 20 to 45 mil (thousandths of an inch). Thinner liners are easier to manipulate, while thicker liners are more durable. EPDM is black, which reduces the visibility of dark colored fish. However, EPDM also becomes colonized by periphyton. While still dark, the periphyton may aid visibility of darker fish. Plumbing and electrical fittings that penetrate the liner are secured with caulking and bulkhead fittings or bolted flanges.
Polyethylene sheet liners are less expensive than EPDM. They are available in either black or blue. Unfortunately, polyethylene liners are less flexible and puncture resistant than EPDM. However, polyethylene liners are very UV resistant and can last as long as 20 years. Poly liners are best suited for ponds larger than 5,000 square feet.
Polyvinyl Chloride (PVC)
PVC liners are more flexible than polyethylene, but they are also more easily damaged by UV radiation. Protect all surfaces above the waterline from direct sunlight. Cover the surfaces with rocks, a deck, creeping vegetation, or latex paint. PVC should be considered a temporary liner, with a reasonable life span of 2-3 years. PVC is not a first choice if you plan for the pond to be a permanent fixture.
Table X. Attributes and Shortcomings of various pond liner materials.
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