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Bridges crossing rivers wider than 50 m are typically supported by piers. In a mobile riverbed, scour occurs around bridge piers, and it is the main cause of bridge collapses worldwide, especially during floods. While bridge pier scour has been extensively studied, there is still a lack of measuring systems for scour monitoring in the field. In this paper, we present existing devices for scour measurement and analyze their comparative advantages and disadvantages. A study case with a scoured bridge pier in supercritical flow is presented. Results show that supercritical flow patterns previously reported at the laboratory scale also occur in the field. The measured scour supports the hypothesis that supercritical flows, even when having high flow speeds, do not produce higher scour than subcritical flows. A possible explanation linked with the sediment sizes of rivers with supercritical flows is discussed. Further, field measurements of scour around bridge piers are needed to enhance our understanding of this complex and nearly unexplored situation. [ABSTRACT FROM AUTHOR]

Hydropower is experiencing a development boom worldwide. However, at the same time, challenge sustainability, mainly due to synergistic effects of several projects in the species distribution area. What and how hydroelectric projects will affect species, depends on a number of attributes. We analysed Chilean freshwater species distributed over 10 major basins of Central Chile threaten by 1124 plants (12,338 GW) planned in a hotspot of biodiversity. The exposure of the species to the planned hydropower projects was analyzed at the scale of species and as an assemblage. The hazardousness was evaluated by type, according to characteristics of planned hydropower plant: size, dam height and turbine type. Finally, considering species and assemblage exposition the most threaten genera were defined. Project analysed compile 45% of the exploitable potential, with a total of 198 (75.5%) project with capacities < 20MW, and 280 (88.1%) with low head dams ( 60). The latter species pertain to genera Trichomycterus, Percilia, and Diplomystes, defining a critical fish assemblage that facilitates the management of an important part of the cases (40.4%) through mitigation technologies.

Scales involved in bridge pier scour include the Reynolds number, the Froude number, and the ratio of the pier to the sediment diameter, among others. Consequently, it is not possible to achieve perfect similitude, and thus scale effects between model and prototype arise. In fact, these scale effects distort the scour estimations typically used in hydraulic design of bridges, as scour formulas are based mainly on results from scour experiments at the laboratory scale. In this paper, proper scales of the flow, sediment, and scour are derived theoretically. The suitability of these scales to achieve perfect similitude of scour is tested through carefully designed laboratory experiments conducted in three different flumes having different sizes and piers of different diameters, with five bed materials of different sizes and densities. Results show that the dimensionless effective flow work, W* is a suitable criteria for scaling the effects of river flows on the local scour process around a pier in a bed composed by grains represented by the dimensionless grain diameter D*. In this context, scale effects of the ratio of the pier to the sediment diameter is quantified and shown to be negligible.

Experimental results on scour at circular and square cylinders in uniform gravel under the clear-water scour condition are presented. Nonintrusive, high-resolution topographic measurements of developing and equilibrium scour holes were performed during running experiments with an experimental installation using a laser distance sensor (LDS) and precision step motors. Measurements were taken by the LDS in different azimuthal half-planes with θ = 0°, 15°, 30°, 45°, 60°, 75°, 90°, 105°, 135°, 150°, 165°, and 180° to study the spatio-temporal variation of geometric properties of scour holes. Experiments were conducted over 40 h until equilibrium, with bed shear stress equal to 95% of the critical bed shear stress for the initiation of sediment motion at the undisturbed plane gravel bed having a [d.sub.50] of 3.25 mm. Results show that scour in gravel progresses more slowly around the square cylinder, where steeper slopes than at the circular cylinder are Observed. During experiments the scour slopes do not exceed the observed repose angle of the sediment particles. Maximum scour depth in different azimuthal half-planes and scoured volume are well correlated with the maximum scour depth at azimuthal half-plane with θ = 0°. Key words: scour, bridges, piers, sediment transport, loose sediment hydraulics. Cet article presente les resultats experimentaux sur raffouillement autour de cylindres circulaires et carres dans un lit de gravier uniforme sous des conditions d'affouillement en eau claire. Des mesures topographiques non intrusives et a haute resolution des trous d'affouillement, en developpement et a I'equilibre, ont ete prises lors d'essais utilisant un capteur de distance au laser (LDS) et des moteurs pas-a-pas de precision. Les mesures ont ete prises par le LDS sur differents demi-plans azimutaux dont θ = 0[grados], 15[grados], 30[grados], 45[grados], 60[grados], 75[grados], 90[grados], 105[grados], 135[grados]. 150[grados], 165[grados] et 180[grados] afin d'etudier la variation spatiotemporelle des proprietes geometriques des trous d'affouillement. Des essais ont ete realises pendant 40 heures jusqu'a I'equilibre, avec une contrainte de cisaillement du lit de 95 % de la contrainte de cisaillement du lit critique pour I'initiation du mouvement des sediments dans le lit de gravier non perturbe ayant un [d.sub.50] de 3,25 mm. Les resultats montrent que I'raffouillement dans le gravier progresse plus lentement autour des cylindres carres, oil des pentes plus raides qu'autour des cylindres circulaires ont ete observees. Durant les essais, les penles d'affouillement n'excedaient pas I'angle de repos observe des particules de sediment. La profondeur maximale d'affouillement dans differents demi-plans azimutaux et le volume affouille concordent bien avec la profondeur maximale d'affouillement a un demi-plan azimutal de θ = 0. Mots-cles : affouillement, ponts, piliers, transport de sediments, hydraulique de sediments meubles. [Traduit par la Redaction], Introduction Scour at cylinders has been extensively studied in the past. Nevertheless, most of the past studies focused on scour in alluvial sand beds. Work on estimation of maximum scour [...]

An experimental analysis of the dynamics of avalanches for different stages of development and turbulent flow conditions is introduced. Experiments were conducted in a laboratory flume using clear-water conditions and a video camera placed within an acrylic cylinder. The results of a proper orthogonal decomposition of the pre-processed image sequences highlighted the existence of two types of sand avalanches: Type I corresponding to a narrow and vertically long-advancing slide, and Type II characterized by an elongated and short-advancing pattern. Temporally speaking, sand avalanches were triggered aperiodically, resembling the behaviour of the horseshoe vortex system. The frequency of occurrence of avalanches decreased with time, while the estimated volume of mobilized sand increased in the same period. The sand slide magnitudes and associated inter-avalanche time were Log-Pearson 3 distributed. Sand avalanches are an important scour mechanism, which depending on the flow intensity contribute to about 40% to 60% of the scoured volume. [ABSTRACT FROM AUTHOR]