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Hydraulic scour is one of the major causes of bridge and embankment failure during typhoon-induced or monsoon-induced floods in steep intermittent rivers in Taiwan. In this study, field data for sediment load and riverbed scour, measured in gravel-bed and sand-bed reaches between 2003 and 2013 are analyzed to determine their differences under nonequilibrium sediment-transport conditions. The riverbed scouring potential was extremely high during the fast-rising stage of a large flood in the bimodal gravel-bed reach. By contrast, the unimodal sand-bed reach with a higher suspended load typically demonstrated a lower scouring rate, although it was still higher than that of a perennial river. Existing bridge-scour formulas suitable for intermittent rivers were examined by separating short-term general scour and localized scour. A maximum general scour-depth formula is developed in this study for use as a potential tool for emergency evacuations and bridge closure purposes. [ABSTRACT FROM AUTHOR]

Many riverine systems have been disconnected from their receiving basins by flood-protection levees and other engineered systems. Reconnecting these alluvial rivers with their receiving basins is a viable option to nourish and sustain existing coastal wetland systems as well as to build new land. This sediment nourishment can be accomplished through direct dredging and placement or through sediment diversions. Efficient design of sediment diversions is important to maximize the land building potential. This study's objective is to quantitatively identify key design attributes of sediment diversions, influencing their ability to capture sediment. The outfall channel alignment angle (ϕ), intake invert elevation, and diversion size are hypothesized as key parameters. The analysis is limited to sediment grain sizes larger than 63 µm and has been performed using a validated three-dimensional numerical model. A time integrated sediment-water ratio was used as an indicator to reflect efficiency of sediment capture. Analyses indicate ϕ has minor impact on total diverted sediment load and limited to the medium sand (M) fractions (250-500 µm) compared to the fine (125-250 µm) and very fine (63-125 µm) size classes. The sediment water ratio increases as intake invert elevation is deepened to a certain limit, then it plateaus around yInv=yBar ~ 0.75, implying that deeper invert beyond that limit may not be beneficial. The analysis also shows that the sediment water ratio sharply increases up to a water discharge extraction ratio of 0.1 as the size of the diversion is increased, and plateaus around 0.2. [ABSTRACT FROM AUTHOR]

Eulerian equations for the vertical flux and momentum of suspended particles in dilute sediment-laden open-channel flow in equilibrium have been derived using the two-fluid approach. Reynolds averaging has been applied in order to allow validation of individual terms with experimental data. Consideration of the various terms of the vertical momentum balance with experimental flume data indicates that the drag force has to be separated into a mean and a turbulent contribution with different timescales, respectively, the (gravitational) particle timescale and the integral turbulence timescale. The resulting formulation further provides a new theoretical closure for the turbulent Schmidt number. [ABSTRACT FROM AUTHOR]

A laboratory characterization of cohesive sediment has been carried out in which data obtained from standard sedimentation and rheological measurements were combined in a determination of the critical solid concentration for the detection of elasticity in a weakly cohesive suspension. The corresponding storage modulus and shear stress are very critical in any in situ rheometry of sediments, especially in the study of mud-water surface erosion in a flume. Sedimentation results showed that particle size distribution rather than surface treatment controlled the rheological behavior of the suspension while the critical solid concentration for the appearance of three-dimensional space-filling network, showing some measurable elasticity in the suspension, occurred in the region of 0.015. This parallel between the consolidation behavior and shear rheology development for the flocculating system has been established. This technique could be an adjunct to the laboratory characterization of cohesive sediments for the estimation of critical shear stress for surface erosion, especially in a typical flume experiment under water wave pressure. [ABSTRACT FROM AUTHOR]

The original derivation of the Einstein bed-load formula involves several oversimplified assumptions concerning step length of a particle, exchange time, and probability of being lifted into motion. A modified version of the Einstein formula that relaxes these assumptions and incorporates a nonuniform sediment model is proposed and tested with both field and laboratory data. The modified formula shows promise as being more reliable than the original version in estimating bed-load transport rates. [ABSTRACT FROM AUTHOR]