Surface films affecting velocity profiles of slowly moving water in open channels

Summary

Rates of movement of surface films and underlying water were measured using observable solid and liquid tracers. Water flow rate, surface width, water depth, and channel length were varied in flumes in which the water was allowed to flow over a broad weir crest at the tail end. In some runs the surface was blocked 5 cm in front of the weir. When this was done, velocity of the surface film immediately upstream from the block decreased to a small fraction of its previous value and this area of slow surface velocity built upstream with time, extending to as far as several meters under steady state conditions. In this reach, the film at the air-water interface of the water causes drag on the moving water similar to that at a solid-water interface.

Water surface in channels one cm wide and 100 cm long stopped moving when the shear force caused by water flowing beneath it dropped to less than 0.0013 dynes/cm². This indicates structure in the surface which does not deform or shear at rates proportional to the force applied. Average water flow velocities from 0.3 to 1 cm per second provided shear stresses in excess of 0.002 dynes/cm² and moved these surface films. However, the velocity distribution across the surface of the channel was not parabolic, and indicated that most of the shear in the film was taking place near the edges of the channel. Extrapolation of these observations to water film dimensions present in unsaturated soils indicates that air-water interfaces in unsaturated soils are usually static.