MAINTAINING YOUR POND

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Outlets

A well-designed outlet can
mean years of trouble-free
operation. It has to be
sturdy enough to funnel pond
overflow year-round, and
withstand the upheavals of
winter frost and ice.

A well-designed outlet can mean years of trouble-free pond operation. Whether it's a natural-earth spillway, a sluice gate, or a pipe, the outlet must be sturdy enough to funnel pond overflow year-round, and withstand the upheavals of winter frost and ice. Natural spillways are favored by pond designers who want to eliminate pipe costs and risks of leakage. The natural look of a pond overflow stream and its potential for landscaping also appeals to many pond owners. Excavated ponds often feature natural spill because of the impracticality and expense of installing lengthy drain pipes in valley terrain. On the other hand, a pipe outlet may seem attractive to some pond owners who wish to: create a seamless shore; prevent erosion; or have the ability to quickly drain the pond, vary the water level, or divert the overflow for irrigation, hydropower, or aquaculture.

The simplest pond outlet is a natural-earth spillway. It may be prone to erosion, but usually this is overcome with stone, ledge, or other reinforcing material. Particularly in new ponds, earth spillways can erode dramatically if they are not adequately lined. Field or brook stone, or rock riprap, is generally used. Often a natural spillway will utilize a large rock or ledge at the crest of the spillway to prevent erosion. On-site ledge may be incorporated into a design to function as the spillway. I've seen spillways covered with a protective layer of concrete, but the concrete is often undermined by runoff and ice.

It's not unusual for a new pond to require an extra load of stone to reinforce a natural spillway after the pond overflows the first season. Ponds with especially steep overflows will be most susceptible to erosion. A spillway that descends in a curve will not wash out as readily as one with a straight drop. Sod can be used to help stabilize the spillway banks. Once vegetation is reestablished and the channel stabilizes, a natural spillway may need little attention. One pond owner commented to me that she liked a natural spillway because she could see what was happening. Because of leakage, she had replaced the original horizontal drain pipe with a natural spillway. She added flat stones in steps to create a waterfall effect.

Make sure that leaves and other debris don't clog the channel, especially if wire mesh is used to keep in fish. Such fencing can plug up. Sometimes a spillway will draw floating leaves and debris to the outlet, where it can be skimmed or raked out. This is especially true if the spillway is sited leeward of prevailing winds.

If a spillway does need repairs or additional stone, be sure not to raise the water level of the pond to a point where it might inadvertently flood the dam. In some cases, a small amount of erosion might be tolerable in one pond but not another. I've seen farms where fencing that crossed the spillway became ineffective because of erosion. In a case like that, it may be necessary to pipe out the overflow. Frequent traffic across a spillway might also destabilize the channel and require a pipe outlet.

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