In situ measurements and mitigations of nappe oscillations - The papignies and nisramont dams in Belgium

Although good practices in flap gate design recommend adding splitters on the crest to provide sufficient nappe aeration and, thus, prevent nappe oscillations, oscillations problems have been detected on the flap gates of the recently commissioned Papignies weir on the Dendre River in Belgium. These oscillations were causing vibrations of the actuators, which could lead to malfunctioning. In addition, they were generating noise, which is a nuisance for people living nearby. Similar problems were reported for the down lift gates of the Nisramont weir on the Ourthe River, also in Belgium. In this context, the paper presents field measurements performed first to define the range of upstream heads prone to cause downstream nappe oscillations and, second, to quantify the effectiveness of additional splitters to mitigate the problem. Measurements with monoaxial accelerometers, microphone, cameras (including a high speed one), and an ultrasonic water level sensor were performed and compared. Data analysis shows a clear correlation between sound, image, and accelerations dominant frequencies. Results demonstrate the effectiveness of adding an extra splitter between each existing splitter in order to avoid the occurrence of nappe oscillations. These in situ measurements validate results from experimental tests performed at the Liege University on a large scaled model of a free surface weir aiming at determining the maximum spacing of splitters to avoid nappe oscillations. © 2018 7th IAHR International Symposium on Hydraulic Structures, ISHS. All rights reserved.