Dams and pipes2026-10-03

Dam pipe, spillway or siphon: which your pond needs, and how big

Almost every dammed pond needs two outlets: a principal spillway pipe through the dam for ordinary runoff, and a wide grassed emergency spillway in undisturbed ground at one end of the dam for the storm the pipe cannot carry. A siphon over the dam adds a principal spillway to an existing dam without cutting a pipe through it, and still needs the emergency spillway beside it. The pipe is sized for a 1 to 10 year storm on your watershed, the emergency spillway for a 10, 25 or 50 year storm, and your county NRCS office runs those numbers.

The three ways water leaves a pond

The principal spillway is a pipe. In the usual drop inlet design a vertical riser stands in the pond with its crest at normal pool, joined to a barrel laid through the base of the dam. The Alabama Extension pond design guide says "The primary spillway is usually a pipe system (standpipe, siphon system, or a pipe at the waterline through the top of the dam) that is sized to carry all but the very heaviest rainfall." NRCS Agriculture Handbook 590 says "The principal spillway normally is sized to control the runoff from a storm ranging from a 1-year to a 10-year frequency event."

The emergency spillway (NRCS says auxiliary spillway) is an open earth channel, and Handbook 590 gives the reason for it: "No matter how well a dam has been built, it will probably be destroyed during the first severe storm if the capacity of the spillway is inadequate." Conservation Practice Standard 378 waives it only for a pipe of "3 square feet or more," which is roughly a 24 inch pipe.

The siphon is the choice for an existing dam. Alabama Extension says "Consider the siphon for older ponds that are being renovated and require a new principal spillway system." Their full guide describes an inverted V over the dam with its crest at normal pool and an air vent that breaks the siphon there, and warns that "a plugged air vent could cause the pond to drain."

How the sources size them

Handbook 590 sets the storm: "The spillway for an ordinary farm pond generally is designed to pass the runoff from a 25-year frequency storm." Its Table 7 makes that specific.

Minimum spillway design storm (NRCS Agriculture Handbook 590, Table 7). Effective height is the difference between the auxiliary spillway crest and the lowest point along the dam centerline.
Drainage area (acres)	Effective height of dam (ft)	Storage (acre-ft)	Frequency (yr)	Minimum duration (hr)
20 or less	20 or less	Less than 50	10	24
20 or less	More than 20	Less than 50	25	24
More than 20	20 or less	Less than 50	25	24
All others			50	24

The NRCS method is to look up the 24 hour rainfall for that storm in your county, give the watershed a runoff curve number from its soil and cover, read the runoff depth in inches from the handbook's Table 5, and estimate how long water takes to reach the pond from the far edge of the watershed. Peak discharge is a unit peak read from a chart, multiplied by the drainage area in acres and by the runoff in inches. The pipe takes a small share of that peak, and the emergency spillway takes the rest at a speed the grass can survive.

For the pipe, Handbook 590 says "use table 12 or 13 to select an adequate pipe size for the barrel and riser," with one hard minimum: "Because pipes of small diameter are easily clogged by trash and rodents, no pipe smaller than 6 inches in diameter should be used for the barrel." Table 12 follows in full.

Discharge values for smooth pipe drop inlets, in cubic feet per second (NRCS Agriculture Handbook 590, Table 12). Columns are barrel diameter to riser diameter in inches. Total head "is the vertical distance between a point 1 foot above the riser crest and the centerline of the barrel at its outlet end." Footnote: "Length of pipe barrel used in calculations is based on a dam with a 12-foot top width and 2.5:1 side slopes. Discharge values are based on a minimum head on the riser crest of 12 inches. Pipe flow based on Manning's n = 0.012."
Total head (ft)	6:8	8:10	10:12	12:15	15:24	18:36
6	1.54	3.1	5.3	8.1	13.6	20.6
8	1.66	3.3	5.7	8.9	14.8	22.5
10	1.76	3.5	6.1	9.6	15.8	24.3
12	1.86	3.7	6.5	10.2	16.8	26.1
14	1.94	3.9	6.8	10.7	17.8	27.8
16	2.00	4.0	7.0	11.1	18.6	29.2
18	2.06	4.1	7.2	11.5	19.3	30.4
20	2.10	4.2	7.4	11.8	19.9	31.3
22	2.14	4.3	7.6	12.1	20.5	32.2
24	2.18	4.4	7.8	12.4	21.0	33.0
26	2.21	4.5	8.0	12.6	21.5	33.8

Alabama Extension puts the riser "usually one and a half to two pipe sizes larger than the barrel pipe." Standard 378 sets "a minimum inside diameter of 4 inches," and Missouri Extension G9474 describes the drain pipe as "usually about 3 to 6 inches in diameter."

No federal source publishes a width table for the emergency spillway. Alabama Extension says "Emergency spillways can be anywhere from 10 to 50 feet wide or wider." The one published rule of thumb is Virginia Tech publication 420-011: "Recommended spillway size can be calculated by adding 15 feet to one-half of the total drainage area acres. For example, a 50-acre drainage area should have 40 feet of spillway, 100 acres requires 65 feet of spillway, and 200 acres requires 115 feet of spillway."

Freeboard and elevation steps, quoted from each source
Source	Requirement
Handbook 590	"If your pond is less than 660 feet long, provide a freeboard of no less than l foot. The minimum freeboard is 1.5 feet for ponds between 660 and 1,320 feet long, and is 2 feet for ponds up to a half mile long." Measured from the water surface "when the spillway is discharging at designed depth."
Standard 378	"a minimum of 1 foot of freeboard between design high-water flow elevation in the auxiliary spillway and the top of the settled embankment," and "a minimum 2 feet of elevation difference between the crest of the auxiliary spillway and the top of the settled embankment when the dam has more than 20 acre drainage area." Pipe crest "a minimum of 6-inches" below the spillway crest at 20 acres or less, "a minimum of 1 foot" above 20 acres.
Penn State	"the top of the dam for a one-acre or smaller pond should be at least 1.5 feet above the high water level or the elevation of water designed to flow through the emergency spillway." Pipe "about 12 inches below the bottom of the emergency spillway."
Missouri Pond Handbook	Freeboard "is the distance between the planned water level, or spillway level of the pond, and the top of the dam. It should be at least 3 feet."
Alabama Extension	Freeboard "is the distance between the designed flow of the emergency spillway and the top of the dam, usually about 1 foot."

On seepage along the pipe, Handbook 590 accepts a filter diaphragm or collars: "If the dam is less than 15 feet high, one antiseep collar at the centerline of the fill is enough." Alabama Extension says the fill around the barrel "is often the weakest portion of the earthfill and where the majority of dams fail."

A 5 acre pond with a 50 acre watershed

Handbook 590's own Example 1 is a 50 acre watershed in Cole County, Missouri, with curve number 66 and a "25-year, 24-hour rainfall = 6 inches." Runoff comes out at 2.44 inches and the design peak at "qp = 0.7 x 50 x 2.44 = 85 ft3/s," which is 85 cubic feet per second. A 5 acre pond averaging 8 feet deep holds about 40 acre-feet (the PondTools volume calculator, also standalone, takes length, width, average depth and shape and returns gallons; divide by 325,851 for acre-feet). Over 20 acres of drainage and under 50 acre-feet of storage puts this pond on the 25 year row of Table 7. In Table 12 the largest combination, an 18 inch smooth barrel with a 36 inch riser, passes 24.3 cubic feet per second at 10 feet of head. No pipe in the table carries 85 cubic feet per second. The emergency spillway, with credit for what the pond stores above normal pool, carries the rest, and the Virginia Tech rule gives it 40 feet of width. Standard 378 then puts the pipe crest at least 1 foot below the spillway crest and the dam top at least 2 feet above it. The real numbers come from your own rainfall, soils and slope, and working them out is an NRCS or engineer's job.

What pond owners on Pond Boss do

esshup (Moderator, Ambassador, Field Correspondent) says your county NRCS office will come out and size the pipe and spillway for free, and that there is no fixed rule of thumb for emergency spillway design. ewest (Moderator, Hall of Fame 2014) lays out a redundant four system design for a large lake: a small pipe at full pool, a 30 to 50 inch main pipe 18 inches higher, a siphon 2 feet above full pool, and a 30 foot emergency spillway 4 feet above full pool. His rule is don't undersize the drainage pipe; the spillway should be reserved for emergencies only.

The reported failures are undersized or plugged pipes. snrub, a Kansas member without a forum title, put a 6 inch overflow pipe on a sediment pond and wrote that "During large rain events this is not near enough," so his 12 foot wide emergency overflow has become the main outlet in storms. esshup thought a 10 inch pipe had his drainage covered, then added a 12 inch pipe after water arrived from places that had never flooded. Cecil Baird1 (Hall of Fame) abandoned a buried 6 inch pipe that could no longer pass about 40 gallons a minute once algae clogged it. With 18 inches of floodwater over his drain tube, iowanagofishin (Lunker) tried to extend the 8 inch pipe and was sucked into it to the groin.

Trash guards split the forum. Theo Gallus (Moderator) argues a guard should be built so that if it clogs, water can still reach the pipe; Brettski (Ambassador, Field Correspondent) would not put one on an 18 inch horizontal pipe, because it would be the first thing to clog. esshup says a siphon can handle more water than a same size regular pipe and that a bad old pipe can be grouted shut on the pond side and replaced with a siphon.

Where the tiers differ

The sources disagree on a minimum pipe: Handbook 590 says nothing under 6 inches for the barrel, Standard 378 allows 4, Missouri Extension describes 3 to 6, and the forum's clogging reports are all 6 inch pipes. Freeboard is counted from different lines: NRCS and Alabama measure from the water surface while the spillway is flowing, Missouri from normal pool, so Missouri's "at least 3 feet" and the NRCS 1 foot do not contradict each other. Neither Handbook 590 nor Standard 378 uses the Virginia Tech width rule. The 100 year storm that circulates on the forum for emergency spillways is not in the federal tables; Table 7 tops out at 50 years.

What to do

Measure the watershed first. Estimate storage with the volume calculator, then take watershed acres, storage and dam height to your county NRCS office and ask for the design storm, pipe size and spillway width in writing. For a new dam, use smooth pipe no smaller than 6 inches, a riser one and a half to two sizes larger, a trash guard that still passes water when fouled, and collars or a filter diaphragm. Put the emergency spillway in undisturbed ground at the end of the dam, grassed and treeless, at least 1 foot below the dam top and 2 feet if the watershed exceeds 20 acres. For an existing dam with a bad pipe, a siphon avoids opening the fill, but guard the vent and keep the earth spillway. Do not reach into a flowing pipe, and do not treat a pipe of any size as a substitute for the grass channel beside it.