Jordan Journal of Civil Engineering

Influence of Crest Roughness on Weir Flow Conditions for Improving Nappe Stability


Mohanad Khodier; Blake Tullis;


Low head nappe flow over hydraulic structures (such as weirs and fountains) can generate significant noise and pressure waves, through nappe oscillation creating the potential for negative impact on the hydraulic structure safety and surrounding environment.. One potentially effective mitigation technique involves increasing the physical roughness of the weir crest surface. The exact phenomena (e.g., increased local turbulence, disrupted boundary layer growth, roughened air-water interface, etc.) generated by the roughened crest surface responsible for mitigating the development of nappe oscillation isn’t clear. In an effort to better understand the influence of weir crest surface roughness on nappe instabilities, an experimental investigation was conducted using a laboratory-scale linear weir with three different crest shapes (rectangular, quarter-round, and half-round) to investigate changes in nappe velocity profiles relative to smooth and artificially roughened weir crests. Artificially roughening a weir crest affected velocity profiles, discharge coefficients, and nappe trajectory profiles, relative to the smooth crest conditions. Maximum horizontal velocities decreases with increasing crest roughness, particularly for smaller unit discharges. Roughening the weir crest did improve nappe stability, however, adding crest roughness also reduced the hydraulic efficiency of the weir as noted by a reduction in discharge coefficient.


Nappe; Oscillation; Particle Image Velocimetry (PIV); Unaerated; Roughness.