4. Turbulence

Turbulence results from relatively small parcels of water spinning in a random pattern within the bulk flow while moving downstream. Turbulence may be recognized as water­ surface boils or three­dimensional eddies which appear and disappear haphazardly. Because of this local motion within the general motion of the bulk flow, any particle of water may, at any given instant, move forward, sideways, vertically, or even backward. In effect, the water is passing a given point with accelerating and decelerating motion superimposed upon the main flow rather than with a uniform, ideal velocity. Thus, more or less water may pass a given point over a short length for short time periods, depending on the observation point chosen (figure 5-1).

Excessive turbulence will adversely affect the accuracy of any measuring device but is particularly objectionable when using current meters or propeller meters of any kind. Turbulence can be objectionable even without air entrainment or the "white water" often associated with turbulence. Turbulence is commonly caused by stilling basins or other energy dissipators, by a sudden drop in water surface, or by obstructions in the flow area such as turnouts-- operating or not--that have projections or indentations from the supply canal. Shallow flow passing over a rough or steep bottom can also cause turbulence. Weeds or riprap slumped into the flow area or along the banks, or sediment deposits upstream from the measuring device, also can cause excessive turbulence.

Excessive turbulence can cause measuring errors of 10 percent or more. Therefore, the flow approaching a measuring structure or device should be modified to resemble tranquil canal flow.