Journal of Hydraulic Engineering

Density currents represent a broad class of flows driven by gravity acting on a density difference with the ambient environment. The understanding of the underlying mechanisms of density currents and their implications on fluid, species, and sediment transport have been studied extensively by others. Although confluences occur naturally in terrestrial and submarine settings, little attention has been given to understanding the confluence of two density currents. Here, we systematically studied the unsteady flow in confluences and developed a methodology for describing the flows based on bulk properties in pre- and post-confluence density currents. Numerical simulations were conducted with experimental validation in which the effects of the initial density difference and channel depth were studied in a junction. In the junction, the currents accelerated and thickened. However, the postconfluence front velocity and thickness values were lower than the preconfluence values. The front velocity, front thickness, maximum near-bed horizontal velocity, and maximum bed shear stress are presented as functions of a Froude number. For the range of cases tested, the peak and postconfluence front velocities were not dependent on the initial conditions. Unlike the front characteristics, the bed shear stress continued to rise as the current’s combined front continued downstream. Therefore, two reestablishment lengths are presented describing the length needed for the front and, separately, the body to return to a constant-value phase.

Laboratory experiments investigated the effects of bed slope, initial impoundment depth, and still water level on the hydrodynamic characteristics of tsunami-like hydraulic bores generated by a rapid-release swing gate. Different reservoir impoundment levels and still water depths downstream of the gate were selected to generate both subcritical and supercritical bores. The bores propagated over an initial fixed bed section followed by a sand bed test section that was either horizontal or inclined with a 5% upslope. A square rigid structure was placed in the middle of the sand bed test section. Time histories of specific discharge and momentum flux were calculated from measured bore velocity and depth. The bore velocities increased with impoundment depth, and supercritical bores had greater velocity than subcritical bores. Regardless of flow regime, absolute bore heights reached their maximum value of approximately 45% of the impoundment depth at the beginning of the erodible sand section. For comparable supercritical bores, propagation over the inverse slope instead of a horizontal bed increased the wave height by approximately 40% and its peak velocity by 25%.

Bed-load transport is an important process impacting riverine morphology, but until recently it has proven difficult to measure in large sand-bed rivers. Integrated Section, Surface Difference Over Time, version 2 (ISSDOTv2) method provides a means of quantifying the bed load in large sand-bed rivers. A series of flume experiments, which included frequent measurements of bed topography and an independent measurement of bed load across a range of flows, was used to provide validation for the ISSDOTv2 method. The mean difference between the ISSDOTv2 calculations and the estimated bed load was 5.6% (high flow), 12.9% (medium flow), and 56% (low flow), indicating improved accuracy for higher flow and transport conditions. It was also found that the ISSDOTv2 method responded appropriately to rapid step-downs in flow, as indicated by agreement between ISSDOTv2 load results and accompanying independent measurements of bed load.

This study introduced a new riprap placement design for the purpose of reducing bridge pier scour. The design consisted of embedding a layer of the riprap around the pier, flush with the surrounding bed, with a depression to help guide the horseshoe vortex. The new countermeasure design was tested among other conventional riprap placement designs in clear-water flow conditions and transition-flow conditions. The transition-flow condition is the evolution period from clear water to live bed where the bed has yet to reach an equilibrium state. The new countermeasure design was found to reduce greatly the depth and volume of scour in both flow conditions, up to 81.0% and 92.3%, respectively, which was more than any other riprap countermeasure tested. In addition, the new design used 18% less riprap material while also experiencing less shape deterioration when compared with a traditional flat riprap cover. Therefore, installing riprap in this specified shape yields an improved scour countermeasure that could help preserve infrastructure and increase public safety.