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Siphon spillway - automatically starting siphons

Smith, W.B., 2005:

Siphon spillway - automatically starting siphons. - Based on IRRIGATION-L posting from 29th July 2005.

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© W. Bryan Smith, 2005
(Click to enlarge)

The Natural Resources Conservation Service (NRCS, formerly SCS) is using siphons in small pond construction as we speak here in the US. It's usually a 6 inch or 8 inch (150 or 200 mm) pipe that is placed in the dam, but instead of going straight through the dam near the bottom, elbows are installed to bring the pipe up the front side of the dam, across the top, and then down the back side of the dam (all of this is one or two feet (30 to 60 cm) under the dam soil surface). A tee is placed in the pipe at the top of the dam facing straight up, and a 2 inch (50 mm) pipe is glued in place into that tee. The 2 inch pipe goes up about 2 feet (60 cm) or so, a 90 degree elbow is installed pointing toward the pond water, and then a 45 degree elbow is installed pointing down. More 2 inch pipe is then glued to the 45 degree elbow, with the bottom end left open at the approximate maximum water level desired for the pond.


In operation the level of the 6 or 8 inch pipe installation through the top of the dam determines the "normal" level of the pond. Any excess water in the pond drains through that pipe in a gravity flow situation. An added benefit is that any excess water is removed from the pond bottom since the inlet is near the bottom on the inner dam face, which can help prevent pond "turn-over" in our area. The deep water is typically of a lower oxygen content than the shallow water, so removing it has less potential impact on fish in the pond. We do have some stratification of pond water in our area in the summer months, and occasionally the layers of water will "turn over" or swap places due to various reasons, placing lower oxygen-content water on the surface and playing havoc with the fish population. The bottom placement of the drain pipe can help reduce that problem.


During rainfall events the pond fills quickly, and the water level rises until the large pipe through the dam is filled completely. At some point the water level rises a little more until the end of the 2 inch pipe is covered with water, which prevents air from being drawn into the pipe system and starts the siphon process, greatly increasing the flow rate. One system I did some work with had a flow of some 1300 gallons per minute (4940 liters per minute) under siphon flow, and only about 350 gallons per minute (1330 liters per minute) under "normal" flow with the large pipe completely filled. When the water level drops below the end of the 2 inch pipe as the inflow to the pond subsides, air is drawn into the system, breaking the siphon.


I've included a few simple drawings of my own that may be useful. Each shows various stages of flow.


Note: If the small 2 inch pipe is extended down further, the "normal" water level of the pond can be lowered below the level of the pipe through the dam.

© W. Bryan Smith, 2005
(Click to enlarge)

There's a nice engineering drawing of the design at the following website (along with many others). It's called a siphon pipe spillway in this list:



NRCS: Siphon Exposed on Downstream Slope.- USDA Natural Resources Conservation Service (NRCS). (pdf)
NRCS: Siphon Buried in Downstream Slope.- USDA Natural Resources Conservation Service (NRCS). (pdf)


W. Bryan Smith

Area Extension Agent - Agricultural Engineer

Clemson Extension Service

219 West Laurens Street,

Laurens, South Carolina 29360



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