THE VENTURI EFFECT AND HYPORHEIC FLOW

In the context that pressure variations over bedforms drive hyporheic flow, we proposed and provided initial evidence for a measurable Venturi Effect influencing net downward pressure imposed by the water column on the bed of a stream. This would be a natural and obvious consequence of Bernoulli's equation describing the conservation of energy in flow along stream lines. Instrumental calibration errors created the appearance of a significant Venturi Effect in our preliminary work. Upon correction of instrumental errors, the effect disappears.

Thorough analysis of data collected in a small stream over a range of velocities shows that no pattern of correlation exists between differences in velocity and changes in piezometer water levels. Further, changes of shallow piezometer water levels as a function of stream velocity are statistically indistinguishable from changes in stream stage as a function of velocity. Finally, inspection of logged pressure data shows no pattern of difference in piezometer water levels. In sum, water velocities up to 0.7 m/s do not affect net downward pressure of the water column on the stream bed, contrary to predictions based on Bernoulli's Equation.

Several explanations are possible for the absence of the effect. Water flow in the studied stream is viscous and occasionally rotational, violating Bernoulli's assumptions. Bernoulli developed the equation for the conservation of energy along a single stream line, where this work compares two parallel stream lines, assuming that the two lines have equal energy entering the meander bend. Finally, pressure variations and resulting hyporheic flow under bedforms may result not from the phenomena that Bernoulli described, but rather, for example, from conservation of momentum.

This seemingly trivial result bears on our fundamental understanding of the fluid dynamics driving small-scale hyporheic flow. Sinusoidal pressure variations over bedforms implicitly rely on Bernoulli – Venturi and/or Euler and/or Navier-Stokes to account for the distribution of pressure. Although conceptual and mathematical models rely on Bernoulli's conservation of energy equation, a necessary consequence of the theory has not been observed in a situation where it should be obvious. Other theoretical bases for observed variations may have to be explored.