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Overfall BEST

You can practice noticing how the depth and speed of water affect the overfalls by watching a river or stream flowing over gravel. Where the water is deep and slow, the surface is calm. But where the water is quick and shallow, the surface becomes agitated.



This paper presents a dataset obtained from fifty-two laboratory experiments of nappe oscillations on free overfall structures. Data were collected on two complementary experimental setups, each consisting of a linear weir model. The dataset covers test configurations involving varied geometric parameters (i.e. weir crest shape, weir width, fall height and nappe confinement) and inflow discharges. The following experimental data were produced: assessment of nappe oscillation occurrence and associated frequencies. The later measurements were performed using characterization techniques (image and sound analysis) developed for this research. Reuse of the collected data will support efforts to improve the understanding of the physical processes underpinning nappe oscillation and to validate numerical modelling of the phenomenon.

Free-overfall structures (such as weirs and crest gates) are commonly used as flow control structures for a variety of open channel/free surface flow applications including irrigation, water treatment, and dam safety. The gravity-driven free falling jet on the downstream side of these structures, called the nappe, may display a variety of behaviors and instabilities1. In particular, nappe oscillations, also known as nappe vibrations, can occur under relatively low-head discharges and have been observed to occur with a variety of weirs (linear, labyrinth), crest gates and fountains2,3,4,5,6,7.

A review of the scientific literature10 shows that nappe oscillations have been subject to various theoretical2,11,12,13,14,15,16,17 and experimental3,6,18,19,20,21,22 modelling attempts over the last 80 years. It emerges that nappe oscillation is a complex hydraulic process. A number of theories regarding the cause or the origin of the oscillations appearance and development have been put forward. However, no definitive understanding of the phenomenon has been proposed to date. While most experimental research so far focused on vertically falling water sheets generated by a thin slot in a pressurized tube, only few of them explored the nappe oscillation problem associated with free overfall weirs. Such systems, widely used as dam safety structures for flood release, may however experience strong nappe oscillations6. Nappe oscillation is a timely topic since many non-linear structures were built in recent years3. Such structures due to their improved hydraulic efficiency, operate more frequently under low-head conditions than linear ones and therefore are more often subject to nappe oscillation. An improved understanding of this phenomenon is thus required to better manage its potential occurrences and address the practical consequences.

The present dataset23 is the outcome of an experimental research aimed at providing a thorough characterization of nappe oscillation occurring on free overfall structures. A comprehensive test program was conducted (52 tests) on two laboratory setups of varied size scales, shedding light on nappe oscillation occurrence and its characteristics, for varying nappe confinement, weir crest shape, weir width and fall height. Based on distinct acoustic and optical observations of the phenomenon, respectively measured for each test with a microphone and a high speed camera, two original characterization methods have been developed to quantify the nappe oscillations properties. The application of these methods allows determining the occurrence of the phenomenon and identifying of its dominant frequency.

This dataset is intended to bring insights into nappe oscillation characteristics in particular those affecting free overfall structures. A large amount of nappe characteristics, in term of discharge range of occurrence, noise production and oscillations frequency, as well as associated geometric and hydraulic parameters, were collected. This dataset is expected to prove particularly valuable since only scarce data were available in literature and only few of them provide sufficient quantitative information in terms of geometry and hydraulic features as well as discharge range of occurrence and associated frequencies.

Abstract:River slopes can be changed due to an extreme event, e.g., a large-scale earthquake. This can uplift a riverbed greatly and thereby change the behavior of the river flow into a free or submerged overfall. Corresponding damage, including extreme erosion, on bridge piers located in the river can take place due to the aforementioned flow conditions. A reconstructed bridge pier in the same location would also experience a similar impact if the flow condition is not changed. It is important to identify these phenomena and research the mechanism in the interaction between overfall types and scour at bridge piers. Therefore, this paper is aimed at studying a mechanism of free and submerged overfall flow impacts on bridge piers with different distances by a series of moving-bed experiments. The experiment results showed clearly that bridge pier protection requires attention particularly when the pier is located in the maximum scour hole induced by the submerged overfall due to the z directional flow eddies. In many other cases, such as when the location of the bridge pier was at the upstream slope of a scour hole induced by a flow drop, a deposition mound could be observed at the back of the pier. This indicates that, while a pier is at this location, an additional protection takes place on the bridge pier.Keywords: bridge pier; overfall; scour; landform change impact on pier 041b061a72

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