Head tube: a simple device for estimating velocity in running water

Construction and operation of a head tube are described. A head tube is an inexpensive, easily-built alternative to a current meter for measuring water velocity in streams and rivers. When water flowing in a channel is obstructed by an object, its depth increases at the point of zero velocity (stagnation zone). The difference between the flowing-water depth and depth in the stagnation zone is the head (h). A head tube consists of a clear, hollow, 2.5 cm diameter acrylic tube and a sliding sleeve made of clear plastic. The tube is placed vertically on the river bottom. The difference (head) between water level inside the tube and the height of water against the tube's upstream face is measured against a scale (cm s^–1) drawn on the sleeve, which provides a direct reading of velocity. Graduations of the scale are calculated from a hydrodynamic equation relating mean velocity () to head (h): = (2gh)^0.5, whereg is acceleration due to gravity. When tested in a laboratory flume, the head tube gave very precise estimates of velocity (R ² of relationship = 0.98), although the original calibration scale overestimated current meter-measured velocity by 22 percent. The relationship between head tube and current meter estimates of mean velocity determined in a river with stony substrate was less precise than the correspondence observed in the laboratory (R ² = 0.81). However, estimates of discharge based on head tube measurements were within 8 percent of estimates based on current meter readings.