news from the waterfront by Tim Matson
DAMMED IF YOU DO
The leaky pond count is unusually high this season. I've seen half a dozen ponds with disappointing water levels, and while some of the problems stem from the dry weather we're having, most of the trouble is due to leaky dams. What follows is a rundown on why dams leak, and how to fix them.
Poor dam maintenance can be the cause of low water levels. Earthen spillways need to be maintained to prevent erosion that could lower the discharge waterlevel, or even lead to dam failure. Earthen spillways also have to be kept free of debris or vegetation that could block the spillway and lead to water overtopping the dam, and possible dam failure.
Pipe spillways can often be the cause of leaks. Horizontal water level pipes can leak around the outside of the pipe, "piping" water alongside the pipe, and causing erosion and damage to the dam. Horizontal pipes are vulnerable to frost heaving which often causes this exterior leakage. Poor installation is another cause of trouble.
Standpipes or risers are vertical discharge pipes that are often located near the pond shoreline. They are most often found in embankment ponds. They descend several feet and then turn and run through the bottom of the dam, to discharge beyond the backslope of the dam. Like horizontal pipes, they can leak around the outside of the pipe if not properly installed. Good installation usually includes careful backfilling and fitting the pipe with anti-seep collars. Leaks can also occur in couplings that don't connect, either due to bad installation or a break. If the system includes a drain, it's important that the drain doesn't leak, whether it's a cap, valve, or gate.
Pipe leaks, whether exterior or through flaws in the pipe or drain, can be tricky to detect. Sometimes dye is used around the underwater drain to see if it discharges. Visual inspection can help detect exterior leaks, especially when the pipe is not overflowing and weather is dry. But it can be tricky to tell the difference between ground water and a leak, at the outlet end of the pipe.
Leaks around the outside of a pipe can cause more than low water troubles. Over time the
leak can carry away soil, and the flow will increase. Eventually this can lead to dam failure.
Another cause of dam failure is allowing trees to grow on the dam. Roots penetrating the soil can compromise soil compaction, and water may find its way from the inside of the pond to the roots, and beyond. If the dam is big enough, it may be possible to plant shallow rooted species at the back of the dam without risk. A real dilemma can occur if trees have been allowed to grow by an earlier owner, or through neglect. Cutting the trees may be as much of a problem as letting them stand, because the dead roots can create waterways. In some cases, tree removal may require stump removal as well, and careful rebuilding of the dam. Willows are notorious for sending out roots that can cause leaks. Another reason for not letting trees grow on dams is that if a big tree blows over it can pull up soil and roots, creating an avenue for leaks.
Using concrete for dam construction is sometimes done, but requires engineering savvy. Especially in the north, concrete is liable to crack due to frost damage. Using concrete to line a pond spillway should be avoided.
Dams built on ledge or springs can cause trouble. On ledge it may be difficult to
create a leak resistant seam between the foundation for the dam and the earthen structure built on top. Cutting a notch in the rock and packing in clay can help create a core trench to stop leaks. If there are springs in the foundation, they may undermine the dam and cause leaks and possibly dam failure. One solution can be to pipe the springs out of the foundation area, to prevent undermining.
Good material is essential to creating a tight dam. A good percentage of quality clay, 10 to 20 percent, is required. Avoid sand, shale, and gravel. If a dam is built with poor material it may be possible to reduce leakage by lining the dam with good clay. But the installation technique is challenging, and the result often unsatisfactory.
The last resort for leaky dams may be installing a membrane liner. This can be more difficult if added after a problem occurs, rather than at the time of initial construction. Better to do your prep work thoroughly before building -- you'll be properly dammed if you do.
July 1, 2016