General principles of swimming pondsA swimming pond is a body of water designed for people to swim or splash around in, just like a swimming pool. But unlike a swimming pool, where the water is kept nice and sterile with disinfecting chemicals, a swimming pond relies on natural filtration and biological processes to maintain the water quality. Since the goal is not to make the water uninhabitable for aquatic life, they are easier to fit into a naturalistic garden, and more consistent with eco-friendly gardening principles such as organic gardening and integrated pest management. But you needn't be a die-hard organic gardener to crave a swimming pond - the aesthetics of a pond with aquatic plant life, and without the smell of chlorine, are enough justification. Natural biological processes do a great job of maintaining water quality in creeks and lakes - as long as a proper balance is maintained between nutrient addition and consumption. I'll talk more about maintaining that balance on the (future) page about filtration and water quality. For now, let's focus on the mechanical means of keeping the water clear. All swimming ponds need water to circulate between the main body of water (the part where you swim), and a secondary zone designed for mechanical and biological filtration, where sediment and nutrients are removed from the water. So at a minimum, the swimming pond consists of the swimming area, a filtration area, and a pump to circulate water between these two systems. Choosing the locationIn most cases, it will be best to site the swimming pond close to the house. Certainly if children will be using it - so that adults can keep an ear out for trouble. I also find that it helps to take a look at the pond daily, so that I notice any problems developing, such as low water level, a clogged skimmer, or newly exposed liner. When the pond is further away, those problems may go longer without being noticed. And in the winter the pond can continue to provide visual interest when viewed from the toasty indoors. In order for the water to reach swimmable temperatures early in the season, you'll need a sunny location. But a partly shaded location may be just fine too – algae don't gain as much of a foothold when deprived of the sunlight they crave, and you'll have some shady spots near the pond to set up a lawn chair on hot sunny days. Our own pond receives full sun, although we have planted a Kwanzan cherry alongside the adjacent patio, which will hopefully provide some shade in years to come.  silver maple seedling in the filtration area Speaking of trees – you want to avoid establishing the pond in locations where large trees drop their leaves. Some leaf blow-in during the fall is inevitable (unless you install a net across the water feature in autumn), but a massive influx of leaves is likely to overload the filtration system, and cause future water quality problems. The needles from evergreens are even more difficult to deal with, because they quickly sink to the bottom and can be difficult to remove. When we installed the pond, we worked around an existing silver maple tree. Within two years, however, it was clear that the tree was going to be a problem: not only did it grow rapidly to where its leaf load was going to be excessive; it also started to shade out the filtration area, and the thousands of seeds it dropped in spring sprouted en masse in the filtration area and other shallow places. It had to go – we replaced it with a paperbark maple, which is much slower-growing, smaller tree. Keeping the water inOne of the first things to decide is what the main water barrier will be. Ponds can be constructed with concrete, with rubber liners, or even puddled clay. In recent years, many swimming pools of various constructions have been reworked into swimming ponds. Our pond uses a heavy-duty rubber liner to keep the H2O from seeking lower ground. The same liner is used for the waterfall run and the filtration area. During construction of the pond, ledges were cut into the heavy clay soil of our garden; the ledges and bottom of the pond were covered with a fabric underlayment, across which the liner was spread out. The ledges are maintained in position by large boulders set on top of the liner. So far, the boulders have done their job well - I have seen no settling of the ledges. But they have a distinct disadvantage: they steal a lot of the space that we had counted on for swimming in. Even though our pond is roughly twenty feet in diameter, the deep area is only about half that wide. Enough for a splash, but not for adult swimming. The filtration areaThe purpose of the filter is to remove small particles, and to sustain an environment where beneficial bacteria thrive, metabolizing nutrients that would otherwise build up in the pond water. Metabolized nutrients are thereby made available to plants whose roots extend into the filter. The plants in turn provide an oxygenated environment within the filter, in which the aerobic bacteria can prosper. Water is constantly circulating between the main pond and the filter. This means that the medium inside the filter must allow free flow of water; this means a relatively open consistency, such as a bed of pebbles or porous particles. At the same time, it must provide an environment conducive to bacterial growth. Ideally, it provides a high surface area for the bacteria to grow upon. Lava rock, with its high porosity level and jagged outside edges, is a good choice. Our own swimming pond is low-tech in this regard: the filter is filled with rocks (at the lower level) and pebbles (in the top layer). It does a good job of mechanical filtration, but may not be as effective at biological filtration as a more porous substrate would be. Most sources recommend that the filtration area be between a quarter and a third of the main pond. This provides a sufficient filtration capacity to provide clear water even at times of peak filtration demand. Filter depth should be between three and five feet - deep enough for the roots of the filter plants to fully extend. Real swimming ponds are designed with the pump in the bottom of the filter zone (or the pump intake, if the pump itself is maintained above-ground outside of the pond). This means the water flows from the main pond into the filter, and seeps downward through the filter, becoming depleted in both oxygen and nutrients as it gets deeper. This sets up an environment with plant roots and aerobic bacteria in the top layer of the filter, and some beneficial anaerobic bacteria deeper down. Our pond, having been designed by a company specializing in ornamental ponds, works in reverse: the water flows from the main pond into a skimmer that houses two pumps. The larger pump delivers water to the bottom of the filter (which our pond guys called a "bog", a name that has stuck with us even though the filter has little to do with bogs in the traditional sense of the word), the smaller one feeds a waterfall (actually, it's more like a creek, flowing downhill for about 12 feet). The upward movement of water through the bog, with the most oxygen-rich and nutrient-rich water at the bottom, is opposite compared to the traditional swimming pond arrangement. I don't know how this affects the effectiveness of either mechanical or biological filtration. Our filter is roughly rectangular, about twelve feet long and ten feet wide. The large pump from the skimmer sends water through a large flexible reinforced-rubber hose to a broad pipe in the bottom of the filter. The pipe is accessible through a standpipe that extends to just below the water level in the filter, with a removable lid. It is surrounded by fairly large rocks as a first layer, with smaller pebbles filling out the top half of the filter. Water overflows from the bog into the main pond, across a pebbly area we call "the ford". This is one of the best features of our pond, not least because it attracts birds who like to splash around in the shallow water.
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Last modified:
August 29, 2009 |
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