Journal of Hydraulic Engineering

During flood events, transported large wood (LW) may accumulate at river infrastructures or is retained intentionally at retention structures. In both cases, the accumulation results in backwater rise upstream of the clogged cross section. Previous studies determined various governing parameters for the estimation of backwater rise, but the findings are still contradictory. In addition, the effect of organic fine material (e.g., branches or leaves) was so far neglected. Therefore, a series of flume experiments was conducted to clarify the influence of the governing parameters on backwater rise due to predefined LW accumulations. The hydraulic conditions, LW accumulation characteristics (compactness, accumulation length, LW diameter), and the organic fine material were varied systematically. Based on the results, backwater rise mainly depends on the approach flow Froude number, compactness of LW accumulations, and percentage of organic fine material. The findings of this study resulted in a design equation to calculate backwater rise due to LW accumulations, contributing to the estimation of the hazard potential assessment of river infrastructures for flood events. Furthermore, the present study contributes to an enhanced process understanding of LW accumulations in rivers.

Compact regression formulas were developed through one-dimensional (1D) unsteady flow numerical model simulations for the analysis of slit dam (SD) hydraulic behavior in torrents. The formulas allow an easy evaluation of the sediment-trapping efficiency and of the depth of the sediment deposit upstream from SDs at the peak time of flood. The use of the developed formulas also provides a first estimation of the dam self-cleaning effects at the end of the event associated with the receding stage of the hydrograph. Compared to the approaches available in the literature, the results of this work allow the extension of the analysis of the trapping processes behind SDs to conditions of unsteady flow. The developed formulas may represent a simple and reliable tool for SD design in mountain gravel bed rivers characterized by relatively steep slopes (1–5%).

A series of physical experiments under wave-only and combined wave-current conditions are conducted in a large-scale wave flume. It is found that waves with strong nonlinearity may lead to downstream movement of the maximum scour point under wave-only conditions, which is attributed to the unbalanced effects of the wave crest and wave trough. Based on the surveyed topographies of scour holes and measured temporal variation of scour depth in different locations in the hole, the weighting ratio $U_{cw}$ is adopted as a criterion to distinguish the wave-dominated and current-dominated combined flow. This classification criterion is found to have a characteristic value of 0.44, at which the nondimensional characteristic timescale coincidentally gains a turning point at lower wave Shields numbers. The equilibrium timescale is found to be represented linearly by the characteristic timescale. Empirical formulas established based on small-scale flume data yield reasonable results of scour depth for wave-only conditions and for a relatively larger Keulegan–Carpenter (KC) number in combined wave-current conditions but underestimate the results for the lower KC number conditions in combined flow.

Simultaneous effects of centrifugal force, floodplain interferences, and variation of the depth-averaged velocity are combined in an analytical model to derive a relationship for estimating the secondary current parameter in bends of compound open channels. This parameter is evaluated using the experimental data. To generalize the obtained relationships, numerical modeling is performed using open-source computational fluid dynamics (CFD) software. In the present study, the Reynolds-averaged Navier-Stokes equations (RANS) are solved, and a single-phase solver is applied with an appropriate boundary condition on the free surface. The shear stress transport (SST) $k\text{-}\omega$ turbulent model is applied for turbulence modeling of the flow field. To obtain general correlations for the hydrodynamic characteristics of the flow, such as the velocity components, different numerical models are developed. Accordingly, all the present numerical results, as well as the available experimental data, are used to derive correlations for the secondary current parameter in curved compound open channels.

The effects of step edge and cavity shapes on skimming flow properties were investigated in a large-size 45° stepped chute model configured with uniform triangular steps, partially blocked cavities, and chamfers. The focus of this experimental study was the air–water flow regime and the energy dissipation performances. Visually, the partial cavity blockage and chamfers were respectively associated with an increase and a decrease in flow stability, while causing no substantial change in the general flow regimes. Comparisons of characteristic air–water properties indicated better aeration performance for the sharp edges than for the chamfers. A substantial reduction in friction factor was observed with the chamfers, while partial cavity blockages appeared to slightly improve flow resistance. A strongly negative correlation between total air entrainment and flow resistance was identified, which was more observable for the sharp edges. A comparative study revealed that sparsely spaced sharp edges at slopes between 30 and 45° might be optimal in terms of aeration and energy dissipation performances.

River mouth bars are strategic morphological units primarily responsible for the development of entire deltaic systems and coastline configuration. Although many works during the last decades analyzed the effects of factors such as tides, wind waves, or sediment properties, the control that some aspects of the geometrical configuration of the river mouth exerts on the sedimentary processes remains unclear. This paper addresses the role of the receiving basin slope and the river discharge angle in the hydrodynamics of an exiting sediment-laden turbulent jet and in the resulting mouth bar morphodynamics using Delft3D. Keeping all other factors constant, we find that when varying the discharge angle and the receiving basin bottom slope, the turbulent jet changes its direction and spreading rate. We also establish a theoretical framework for the specific morphology of the mouth bar as a function of the river discharge angle and basin slope, highlighting their importance for properly designing managing strategies. Finally, predicted mouth bar morphologies are compared with good agreement to river mouths in real locations.

Reservoir sedimentation is gaining growing attention as dams are aging, due to economic and environmental consequences. Venting of turbidity currents is one of many sediment management techniques, highly recommended when water is in shortage. The venting operation is experimentally investigated using two reservoir bed slopes. The main research questions concern the opening timing of bottom outlets and the duration of venting. The timings tested are relative to the arrival of the current at the outlet. The results showed that in-time venting, synchronized with the arrival of the turbidity current at the outlet, is more efficient than early or late venting. It is recommended to start opening the gates when the turbidity current is around 300 m upstream of the outlet, so that the evacuation is synchronized with the arrival of the current at the dam. Additionally, venting should not be stopped immediately after the end of the turbidity current flow but should instead last for a certain time in order to evacuate the muddy lake depending on the outflow discharge.

Saline density currents are a growing environmental concern due to their increasing occurrence generated by brine discharges. The present paper shows an experimental global characterization of the quasi-steady-flow properties of nonconfined saline currents using nonintrusive particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques. This global characterization includes the study of the influence of the bottom slope and the initial flow conditions such as the flow rate, the thickness, and the saline concentration on the plume behavior. The results achieved reveal the self-similarity of the mean variables’ vertical profiles and the collapse of higher length scales of turbulence along the density currents. In addition, this study finds the existence of a normal flow state from which the vertical mixing becomes the main mixing mechanism in the plane of symmetry of the currents. Finally, a comparison between widespread numerical integral approximations and the measured data reveals that these approximations need to be reformulated. The high-quality database presented in this study is a key tool for future calibration and validation of different numerical approximations.

Plunging-flow dropshafts can entrain a large amount of air into sewer systems, pressurize downstream airspace, and cause potential sewer odor issues. In this study, a new design for a plunging-flow dropshaft is investigated by installing an internal divider to form a wet shaft and an air shaft. The air shaft allows the air entrained by the falling water to partially circulate back to the wet shaft, thus significantly reducing the air demand from the outside. Laboratory experiments were conducted using a 7.72-m-tall dropshaft with a divider with different sizes of opening for air circulation. External air demand and air pressure variation in the dropshaft were examined. The effects of inflow impingement and breakup of inflow into small water drops were found to be the main cause of the air pressure gradient along the dropshaft. Air flow circulation was modeled and compared well with the experimental measurements. The divider was found to effectively reduce the external air demand and the air pressurization of the dropshaft.