CHAPTER 14 - MEASUREMENTS IN PRESSURE CONDUITS

14. Small Tubes or Siphons

Plastic or aluminum siphons and tubes are commonly used to deliver water from a canal or ditch to the furrows (figure 14-15) (Scott and Houston, 1959). These conduits may also be used to measure the rate of flow. The head acting on siphons and straight pipes through banks is measured in the manner shown on figure 14-15. The rate of flow from figure 14-16 is used only for siphons and pipes where the flow exits the tube into free air. Figure 14­17 is used only for the submerged exit case. The uncertainty for discharge is about +/-15 percent. Therefore, this method of determining discharge is approximate but could be useful for managing water allotments and apportionment over acreage.

The pipe ends should be cut cleanly with no burrs. The tubes and ends should not be dented or deformed. Tight bends should be avoided. The flow past either end of the tube must be slow compared to tube velocity. Tubes can become partially or fully air locked at lower discharges of curves shown on figures 14-16 and 14­17. The siphons should be reprimed periodically when operating at the low discharge region. Both submerged ends should be located a distance of 1.5 diameters from channel flow boundaries and water surface. If a vortex forms over the siphon entrance, the entrance should be lowered if possible; otherwise, the vortex should be suppressed. The vortex can be suppressed by rafting a wide board over the intake and hanging additional cross vanes from the board if needed. Bos (1989) gives more details concerning these requirements and provides rating curves in metric units.

Figure 14-18 (NRCS, 1962a) gives discharge in gallons per minute for heads up to 20 in for siphon lengths common to furrow irrigation with pipe diameters from 2 to 6 in. As mentioned previously, accuracies better than 10 percent should not be expected.