news from the waterfront by Tim Matson
DO'S AND DON'TS OF POND SPILLWAYS
FOR THE SPRING.
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 is the spillway?”
“Well, now that you mention it, the pipe is an old steel thingamagig . . . and I think it’s leaking.”
Spillways are a key element in pond construction, but often overlooked.
That is, until the pond starts leaking.
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 (spillway) system—use pipe or create a natural earthen channel resembling a stream.
Pipe is especially helpful when there are large volumes of discharge and where the pond dam doubles as a 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 water level. Overflow water drops several feet down the standpipe, takes a 90-degree turn and flows out the discharge pipe buried at the base of the pond embankment. Sometimes a straight run spillway pipe is 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 drainpipes are installed below frost level to prevent winter damage.
On flat terrain where there is no pond embankment, culvert pipes at the pond water level may be used, but care must be taken to avoid frost damage and leakage around the pipe. Flat terrain ponds often lend themselves better to natural earthen spillways.
Common Problems with Pipe
Often the steel standpipe intake is the first area to decay. The lip and upper barrel start to rust and leak. Pretty soon the pond water level is not holding up. 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. It can be knocked around by moving water and ice if not reinforced.
Using a natural spillway gave the owner a chance to enhance the waterscape with an attractive custom maple bridge.
Switching to a Native Earthen Spillway
Some contractors may offer to insert plastic pipe inside a rusting pipe 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 is 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 the old pipe completely and refill the ditch with good material packed in compressed layers. Ideally the pond has been dewatered and it is a dry time of year. A constructed stream may be prepared before or after pipe removal. Large ponds will need a spillway soon after the overflow pipe system is shut off.
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. The route that takes overflow back quickest to the original watershed drainage stream is best.
The spillway should be ample enough to handle peak run-off floodwaters, or there should be a secondary emergency spillway at a slightly higher exit elevation.
4 Elements of a Natural Spillway
There are four main elements of the natural spillway: approach channel, width, depth and discharge. Discuss these design criteria with your contractor.
New natural spillways are most vulnerable to erosion, so they are often lined with small stones or riprap. Bricks laid like paving can be used. Avoid concrete (potential for frost damage) and construction fabric (prone to undermining).
The bigger the spillway, the more need for large stones to reinforce the channel. Some people prefer the look of native or round river stones, to industrial riprap. Occasionally spillways are paved like walkways with large flat stones or 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.
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, which like nothing better than to plug them up. Beaver control options range from baffles and other devices, 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.
Ponds: Planning, Design, Construction (USDA Handbook 590) www.nrcs.usda.gov
Article appears in Living the Country Life, Spring 2014,
reprinted here March 16, 2014