Mitigation Techniques for Nappe Oscillations on Free-Overfall Structures

Nappe oscillation is a phenomenon that can have an undesirable impact to the local environmental (e.g., nearby residences, parks) as the phenomenon has been observed to produce a significant level of noise caused by acoustic pressure waves. As a result, a detailed investigation was undertaken to identify practical and effective mitigation solutions to address these effects for free-overfall structures. Nappe oscillations were studied using a prototype-scale linear weir (weir length of 3.5 m and fall height of 3 m), a high-speed camera, and audio equipment. The effectiveness of various crest modifications to reduce nappe oscillation and any corresponding impacts to hydraulic efficiency were evaluated. The test matrix included the optimization (position and spacing of elements) of three mitigation solutions: 12 configurations with projecting elements, 5 configurations with deflectors and one configuration with a step. These three countermeasures proved to be effective after the optimization of their characteristics. In particular, deflectors or a step produced the maximum noise reduction while decreasing the discharge coefficient by less than 3 % over the range of experimental conditions.