Occurrence and Characteristic Frequencies of Nappe Oscillations at Free-Overfall Structures

The present study investigated the occurrence and characteristic frequency of nappe oscillations on weirs by reanalyzing data from 52 experimental configurations. The considered configurations include various shapes of the weir crest and a range of weir geometric and hydraulic characteristics with two different scales. Nappes were either confined with lateral and back walls forming a nonvented air pocket between the nappe and the weir or unconfined with atmospheric pressure everywhere around the nappe. Oscillations occurred at nappes with uniformly distributed approaching flow of low velocity and unit discharge between 0.01 and 0.06  m2/s, regardless of the model scale. Weir crests that favored nappe oscillations had an upstream profile without geometric discontinuities. A dimensionless frequency of nappe oscillations was expressed as a power function of the ratio of the fall height to the water depth at the point of detachment at the crest, separately for quarter-round and truncated half-round weir crests. The exponents of the two obtained power functions were very close to each other, which allows the usage of their average so that different weir crests exhibit solely offsetting curves.