Tim Matson is a pond designer and consultant, and author of the Earth Ponds series. His best selling book Earth Ponds: The Country Pond Maker’s Guide to Building, Maintaining, and Restoration is being published in a 30th Anniversary Edition in 2012.
The call for help goes like this: “We’ve got a pond that used to hold water, but this summer it’s way down.” After eliminating drought or water supply as the problem, the question becomes, How’s the spillway? Pause. “Well, now that you mention it, the pipe is an old steel thingamagig… and I think it’s leaking.”
What goes up must come down, and in the case of ponds, what goes in must come out – without leaking. Spillways are a key element in pond construction, but often overlooked. That is, until the pond starts leaking and the water level drops.
Age causes many spillway problems. There was a pond building boom in the 1970s and 80s, and the overflow pipes installed then are well past their lifespan of 20 years. Eventually they corrode and leak, the pond water level drops, and, in extreme cases, erosion around the failing pipe can tear a hole in the pond dam and destroy the embankment, flooding the neighbors downstream.
Types of spillways
There are basically two ways to create a pond discharge, or spillway, system -- use pipe or create a natural earthen channel resembling a stream. Piping is a popular choice because it gives the owner options not available with a natural overflow. Pipe can help avoid erosion, especially helpful where there may be large volumes of discharge, either continuously or during peak runoff periods.
A pipe spillway can eliminate the need for a bridge across the spillway, which is useful because many pond dams double as the household driveway. A pipe can also allow you to set an exact water level, with options to raise or lower the water level by adding or removing pipe extensions.
Sometimes a drain pipe at the pond bottom is connected to the spillway so the pond owner can lower or empty the pond for repairs, fish harvests, weed cleanups, etc.
Spillway pipe can be installed several ways. Often a vertical standpipe is used, cut off at the designed waterlevel. Overflow water drops several feet down the standpipe, takes a 90 degree turn and flows out the discharge pipe which is often buried at the base of the pond embankment. Sometimes a straight run spillway pipe can be installed at a downward angle, discharging at the outside of the embankment.
There are several variations on the standard pipe spillway, including a standpipe that discharges water from the pond bottom, using a siphon system. In cold climates, spillways and drain pipes are installed below frost level to prevent winter damage.
On flat terrain, where there is no pond embankment, a spillway flowing from the pond bottom may be impossible to install. There is nowhere for the pipe to discharge. Here, culvert type pipes at the pond water level may be used, but care must be taken to avoid frost damage, and leakage around the pipe exterior. Flat terrain ponds often lend themselves better to natural earthen spillways.
How do pipe systems cause problems? Often the steel standpipe intake is the first area to decay. The lip and upper barrel start to rust and eventually leak. Pretty soon the pond water level is not holding up. As corrosion continues the problem worsens and more leaks occur, some unseen, except that there may be visible flow from the discharge end, even when the water level is below the inlet. In worse case situations, water leaking through the pipe or flowing around the exterior of the buried pipe undermines the entire system. One day you may find the dam has blown out and the pond is empty.
Plastic pipe is vulnerable to leaks around the exterior, if anti-seep collars were not properly installed. Pipe can also get knocked around by moving water and ice, especially if not adequately reinforced.
Spillway repairs, replacement, and going ”native”
When pipe spillways get old their lifespan may be extended with assorted repairs. Perhaps some new pipe can be welded to the top of the standpipe barrel. Various patching materials can be used to patch holes, including roofing tar, plastic sheeting, even casting wrap. But once a pipe starts to go, the rust is often present in areas buried in the dam. Some contractors may offer to insert a new plastic pipe inside the rusting barrel, and pack it with pressurized grout. But getting the grout into every nook and cranny is difficult and expensive, and you still have the problem of the decaying exterior pipe. Replacement of the entire system is often needed. However, this can be pricey, especially in tall dams, and dams with a long pipe run.
A more economical solution may involve switching to a native earthen spillway. Instead of overflow discharging through a pipe, it flows out above ground in a constructed stream.
The first step in this transition is to remove or disable the pipe system. The cheapest and quickest method is to dig out a few feet of material around the standpipe, crush down the old pipe, and then bury it in well-packed clay. This sometimes works. A more reliable alternative is to remove the old pipe completely, and refill the ditch with good material packed in compressed layers.
But where does the overflow go now? A constructed stream may have been prepared before or after pipe removal, depending on the need for a ready-to-go new overflow. Usually removal of the old pipe will mean the pond has been dewatered (ideally in a dry time of year), so there won’t be a rush to use the replacement. However some ponds have large volume inflows that fill quickly, and will need a spillway soon after the overflow pipe system is shut off. If you’re lucky there may be an emergency spillway installed at a slightly higher level than the pipe discharge. This stream-like channel can sometimes be modified into the main spillway.
Building a natural spillway is like creating a stream. You want to dig a shallow channel with an inlet level at the desired pond water level. The channel is rarely routed over the middle of a constructed dam, to avoid erosion on disturbed soil. Often the best place for a natural spillway is near either end of the dam, so it runs over an area with at least 50% undisturbed soil. It is also important that the spillway not flow down the back slope of the embankment, which may be susceptible to erosion. The route that takes overflow back quickest to the original watershed drainage stream is best. The spillway should be ample enough to handle peak runoff floodwaters, or there should be a secondary emergency spillway at a slightly higher exit elevation.
There are four main elements of the natural spillway: approach channel, width, depth and discharge. These design criteria should be discussed with your contractor. When natural spillways are new they are most vulnerable to erosion, so they are often lined with small stones or riprap. Even bricks laid like paving have been used for reinforcement. Avoid concrete because of the potential for frost damage and undermining, or construction fabric, which is also prone to undermining. Some people prefer the look of native stones, or round river stones, to industrial riprap. The bigger the spillway, the more need for large stones to reinforce the channel. Occasionally spillways are paved like walkways with large flat stones. Spillway discharge slopes are sometimes stepped to create a waterfall effect.
Often spillways will support growth of native aquatic plants, like cattails. The roots of these plants can help hold the spillway together against erosion. Such moist soil plants will naturalize in the spillway on their own during the first years. It’s important not to let wetland plants get so thick they block overflow.
Keep an eye on your spillways and do maintenance when needed. Make sure debris like leaves, branches, and algae don’t clog the discharge pipe or channel. Look for erosion and leaks. Trash racks are sometimes used on pipes to keep out debris, but must be monitored. Natural spillways choked with cattails and other moist soil plants often back up and cause flooding. Keep them flowing. Spillways may attract beavers who like nothing better than to plug them up. Beaver control options range from baffles and other devices to protect the spillway, to live trapping.
Well-designed and maintained spillways can improve water quality, prevent erosion and flooding, and enhance a pond’s appearance. Take care of the spillway and it will take care of your pond.
Visit the NRCS website for more information on pond planning, design, and construction.