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Bendway weirs are low-level transverse structures constructed on the outer bank of a channel bend to improve navigation, create aquatic habitat and control bend migration. This experimental study was aimed at investigating the erosion and sedimentation patterns in a 90° bend by varying the relative curvature radius (i.e. normalized to the channel width), in the absence and in the presence of bendway weirs, under different flow conditions. A series of flat weirs with relative heights of 30% and 70% were considered. The results indicate that as the relative curvature increases, the erosion near the weirs decreases while the point-bar at the inner bend wall extends. Regardless of the weir height, the increase in the relative curvature leads to a deeper scour depth at the tip of the weirs. Bendway weirs with submergence ratio of 70% are found to be the most efficient in protecting the outer bank from erosion. [ABSTRACT FROM AUTHOR]

We present the results of a systematic series of 168 laboratory experiments that examine the critical conditions for the failure of landslide dams, which obstruct the course of mountain rivers. The experiments were carried out by using three different sediment materials with a quasiuniform grain size distribution and with a flume bed slope angles that ranged between 0[degrees] and 5.71[degrees] (0-10%). Three main typologies of dam failure were observed for increasing values of the dam's downstream-face angle: (1) overtopping; (2) headcutting, which led to the formation of an erosion channel on the dam's downstream face that progressively migrated up to the dam crest; and (3) initial slide of large part of superficial layer of the dam's downstream face, which was followed by headcutting. The experiments focused on the second type of failure to provide a safety criterion based on the upstream reservoir level. The quantities that govern this phenomenon have been identified and a functional relationship is proposed based on the dimensional analysis and curve fitting of the minimum level of the upstream reservoir leading to dam failure. A comparison of both the experimental findings and field data that are available in literature shows that the proposed relationship generally provides a conservative estimate for landslide dams in which comminution effects (due to fragmentation of material in the landslide process) are negligible (dam volume lower than [10.sup.6] [m.sup.3]). Finally, the applicability to larger volume landslide dams or to geometric configurations other than those investigated is discussed. DOI: 10.1061/(ASCE)HY.1943-7900.0000259 CE Database subject headings: Landslides; Slopes; Dam failures; Reservoirs; Water levels; Laboratory tests. Author keywords: Landslide dams; Dam failure; Reservoir level; Laboratory tests.

Bank retreat plays a fundamental role in fluvial and estuarine dynamics. It affects the cross-sectional evolution of channels, provides a source of sediment, and modulates the diversity of habitats. Understanding and predicting the geomorphological response of fluvial/tidal channels to external driving forces underpins the robust management of water courses and the protection of wetlands. Here, we review bank retreat with respect to mechanisms, observations, and modeling, covering both rivers and (previously neglected) tidal channels. Our review encompasses both experimental and in situ observations of failure mechanisms and bank retreat rates, modeling approaches and numerical methods to simulate bank erosion. We identify that external forces, despite their distinct characteristics, may have similar effects on bank stability in both river and tidal channels, leading to the same failure mode. We review existing data and empirical functions for bank retreat rate across a range of spatial and temporal scales, and highlight the necessity to account for both hydraulic and geotechnical controls. Based on time scale considerations, we propose a new hierarchy of modeling styles that accounts for bank retreat, leading to clear recommendations for enhancing existing modeling approaches. Finally, we discuss systematically the feedbacks between bank retreat and morphodynamics, and suggest that to move this agenda forward will require a better understanding of multifactor-driven bank retreat across a range of temporal scales, with particular attention to the differences (and similarities) between riverine and estuarine environments, and the role of feedbacks exerted by the collapsed bank soil. [ABSTRACT FROM AUTHOR]

Bottom racks made by longitudinal bars are hydraulic structures widely adopted for engineering purposes. In the present paper we revisit the problem of their hydraulic design, analyzing the data obtained from a systematic series of experiments carried out in a laboratory flume. For each run we measured the diverted discharge, the water surface longitudinal profile, and using a two-dimensional backscatter laser Doppler anemometer, we measured the velocity field over the rack and in the slit between two adjacent bars. The latter measurements, in particular, allow us to obtain the along-rack distributions of the discharge coefficient to be used to determine the rate of change of the diverted discharge. We use such distributions to derive a physically based relationship relating the overall diverted discharge to the length of the rack, the void ratio, the discharge coefficient measured under static conditions, the specific head of the stream approaching the rack, and a modified Froude number. The robustness of the proposed relationship is confirmed by the comparison between the discharges calculated through the proposed relationship and those measured in an extensive series of experiments available in the literature, characterized by ranges of the relevant flow parameters much larger than those investigated in the present contribution. DOI: 10.1061/(ASCE)0733-9429(2008) 134:1(15) CE Database subject headings: Intakes; Hydraulics; Open channel flow; Experimentation.