With conventional drainage, the pipe needs to be installed with enough grade that the water flowing through the pipe will have enough velocity that contaminants won’t clog the system over time, thus the term “self cleaning velocity.” Self Cleaning Velocity in drainage manuals is stated as a minimum flow of 3 feet per second. At this flow rate, a pipe will be flushed of contaminants and remain open.
The slope needed for a pipe to maintain this velocity is based on the Manning number of the pipe. This number calculates the friction as the result of wall construction and diameter. It is generally accepted that smoothwall 4″ pipe placed at a 1% slope will provide adequate velocity in a system to maintain flow. Larger pipes can be installed at less of a slope and still maintain adequate velocity.
Let’s look at how this affects installation costs on flat properties. Smoothwall 4″ corrugated pipe needs to be installed at a 1% slope to achieve the proper velocity. If seepage water needs to be collected three feet deep at the source, and the relief point is 500 feet away, this means that a pipe would need to be installed 8 feet deep at the relief point on a flat property. Most flat properties don’t have eight feet deep relief points, and even if they do, the cost to install pipe 8 feet deep becomes tremendously expensive.
The options may be as follows. Either install 12″ pipe at an adequate slope of .25% or 4.25 feet deep (3 feet at start, plus 1.25 feet at end of the 500 feet), or install a siphon. The cost of the 12″ pipe and installation time can be upwards of $25.00 per foot from most contractors, and may be a total overkill in terms of capacity requirements.
…siphons are designed from the beginning to have the capability of being flushed with the pressure from the irrigation system.
A siphon, on the other hand, may be installed with a trencher at any depth; yes, even higher than the three foot depth needed initially to collect the seepage water, at a much lower cost per foot. Siphons are not designed to have self-cleaning velocity. In fact, siphons reach a resting level between rainfalls when there is no velocity at all in the system. Obviously, when the water stops flowing and is at rest, contaminants will settle out and accumulate in the pipe. However, siphons are designed from the beginning to have the capability of being flushed with the pressure from the irrigation system. They are constructed from HDPE pipe, the same pipe that is used in many new irrigation systems, and can be completely pressurized. This ability to pressurize the system can flush out any contaminant, even systems that have been completely inundated with silt in a floodplain.