Creating and fixing pond spillways
The call for help usually goes something 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, it’s time to get down to the nitty gritty. 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 they may be overlooked in favor of a new riding mower or vacation trip. That is, until the pond starts leaking and the waterlevel drops.
Recently there’s been a rash of spillway problems, mostly due to age. There was a pond building boom in the 70s and 80s, and the overflow pipes installed back then are well past their lifespan of 20 or so years. Eventually they corrode and leak, the pond waterlevel drops, and in extreme cases erosion around the failing pipe can tear a hole in the pond dam and destroy the embankment and flood the neighbors downstream.
What is a Spillway?
Here’s a look at the general concept of pond spillways, and how and when repairs or replacement may be needed. Also a look at how to build and maintain natural pond spillways, with their potential advantages over piping.
Good pond water quality often involves a continuous exchange of fresh water. That means water flows in, and water flows out. Just like that sparkling lake down the road. How does the water flow out? 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 for spillways 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, useful where there are cars or farm vehicles crossing. In fact many pond dams double as the household driveway. A pipe can also allow you to set an exact waterlevel, with options to raise or lower the waterlevel 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 the north it’s important that spillway and drain pipe 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 waterlevel 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 waterlevel 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 waterlevel 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. They 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 - applied by a pond owning orthopedic surgeon! But once a pipe starts to go, the rust is often present in areas buried in the dam and inaccesible. 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 which can create voids that lead water out of the pond. Replacement of the entire system is often the best “repair.” 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. Sometimes contractors suggest that 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, although water has a habit of finding its way through the earth and eventually back into the remaining pipe.
A more reliable alternative is to remove the old pipe system completely, and refill the ditch with good material packed in well 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 waterlevel. 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 flood waters, 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. A good source for spillway construction specifications is the USDA booklet, Ponds: Planning, Design, Construction (Handbook 590). It is available from www.nrcs.usda.gov
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 the potential need for large stones to reinforce the channel. Occasionally spillways will be 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 thefirst years. It’s important not to let wetlandplants get so thick they block overflow.
Some pond owners keep an eye on their spillways and do maintenance when needed; some don’t even know where the spillway is. Guess whose ponds perform better and last longer?
Good spillway maintenance means making sure debris like leaves, branches, and algae don’t clog the discharge pipe or channel. It means keeping an eye out for erosion and leaks. Trash racks are sometimes used on pipes to keep out debris, but must be monitored and kept clean. Natural spillways choked with cattails and other moist soil plants often back up and cause flooding. Keep them flowing. Spillways, piped or natural, 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 or more drastic measures.
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.
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