Journal of Hydraulic Engineering

In chute spillways, self-aeration occurs downstream of the inception point, where the turbulent boundary layer edge approaches the free surface, if they are long enough. Downstream of the inception point, a layer containing an air–water mixture extends gradually through the flow with the bulking effect. Flow bulking is essential in terms of sidewall freeboard design. In addition, the introduction of enough air quantity near the solid boundaries prevents cavitation damage. In the present work, a 2D numerical model was developed for the prediction of self-aeration and air concentration profiles across the depth and the free-surface location, together with flow bulking along the smooth chutes. The developed model deals with the solution of the one-way direction parabolic equations of mixture continuity, air mass, and air–water mixture momentum conservation. These equations are solved accompanied by the dynamic equation for the free surface, utilizing the marching technique and Prandtl’s mixing length turbulent model. The experimental data obtained by prototype measurements and laboratory tests were used to assess the accuracy of the numerical model. The relevant results were compared in terms of the induced inception point of the boundary layer development, air concentration profiles within self-entrained flows, and the consequent bulking of the flow. The capability of the numerical model for practical purposes is signified in accordance with the fairly accurate obtained results, shedding light on new horizons for further research.

Flexible barriers can be used to trap woody debris or debris flows. However, their small scale modelling is challenging because of their possible deformation. This article addresses how to meet the partial mechanical similitude of manufactured flexible barriers. Relevant dimensionless parameters are defined from flow velocity, barrier geometry, and component mechanical properties. These similitude criteria are validated using numerical simulations of barriers exposed to a hydrodynamic loading at various scales. The simulations also confirm the importance of accounting for the mechanical characteristics of the barrier components when designing model barriers in view of achieving realistic deformations. Next, a real barrier with complex features is scaled to conduct flume experiments. This scaled barrier is 3D-printed with material selected to achieve the mechanical similitude criterion. Another validation of this approach is performed considering hydrostatic loading and checking that simulated and measured deformations are similar. As an application case, the deformations measured during the experiments performed with woody debris are also compared to the hydrostatic loading.

During strong floods, rivers often carry significant amounts of sediment and pieces of large wood (LW). When bridges and hydraulic structures are unable to allow LW to pass through, it becomes necessary to trap LW through specific wood retention structures (e.g., flexible barriers). This paper presents a comprehensive analysis of the interactions between LW and flexible barriers using small scale models. A dimensionless criterion is first proposed to compute blockage probability of single logs. It is based on experiments varying log size and shape, channel slope (2%, 4%, and 6%), water discharge, and barrier bottom clearance. Based on runs using six mixtures of hundreds of logs, an equation is secondly provided to compute flow depth at a barrier accounting for the head losses related to large numbers of logs. Conditions leading to the release of LW when the barrier is severely overwhelmed are also studied. The deformation measured on the barrier proves to be lower with LW-laden flows than under full hydrostatic loading of a barrier obstructed by a plastic sheet. Overall, we demonstrate that flexible barriers are very relevant structures to trap LW. A companion paper shows how to design and manufacture a small scale flexible barrier in mechanical similitude with the prototype scale.

Channel degradation can expose river-crossing cylindrical structures (e.g., pipelines, canals, and tunnels) that are completely buried in the riverbed, altering their surrounding flow patterns to cause scour that threatens their structural safety. This paper reports a case study of scour at a river-crossing canal in a degrading river. Based on the historical topography and hydrological data, the cause, development process, and performance of countermeasures for scour at the river-crossing canal were analyzed, and recommendations for scour protection are proposed. The findings from 53 experiments of the scour at a river-crossing cylindrical structure in a degrading channel are presented. The experimental results indicate that there are three scour development modes at a river-crossing cylindrical structure in a degrading channel: (1) the exposure of the buried cylindrical structures induces downstream scour, with no tunnel erosion (i.e., scour below the bottom of the cylindrical structure) occurring at the cylindrical structure, (2) the riverbed degradation downstream of the cylindrical structure increases the water head over the cylindrical structure and seepage below the cylindrical structure, inducing tunnel erosion at the cylindrical structure, and (3) tunnel erosion is induced when a large-scale bedform trough approaches the cylindrical structure. Based on the experimental data, the critical conditions of each scour development mode were determined. The effects of the flow intensity, dimensionless water depth, and bed material roughness on the equilibrium scour depth were analyzed, and predictors for estimating the equilibrium scour depth at a river-crossing cylindrical structure in a degrading channel are proposed.

The surge tank plays a crucial role in moderating oscillations in hydropower stations, and its critical stable cross-sectional area (CSCA) is key to its stable operation. Normally, the Thoma stable area is widely used to calculate the CSCA, and a safety factor $n_f$ is multiplied for a safety margin. However, the safety factor $n_f$ is empirical and lacks theoretical basis. In this study, we present an analytical formula to calculate the safety factor $n_f$ considering the effect of the governor-turbine-penstock system (GTPS). Mathematical models of the surge tank-headrace tunnel system (SHTS) and GTPS are established, and amplitude–frequency characteristics are analyzed. The formula for $n_f$ is derived using the oscillation superposition method, and its accuracy is verified. Additionally, the effect of the governor on $n_f$ is investigated. The results indicate that the formula provides sufficient accuracy for the design of surge tanks. The effect of the governor can reduce the value of $n_f$ such that a sub-Thoma stable area can be used.

Numerous theories have been presented over the years for quantifying bedload transport in streams and rivers. These theories have been derived using physical processes, statistical methods, and semiempirical and empirical methods and rely on a variety of hydraulic, boundary, and bed material properties. Most of these equations represent instantaneous bedload transport related to temporal hydraulic and bed conditions and are not well-suited to estimate long-term average bedload over a broad range of channels. Using a pure empirical analysis, the author has developed a dimensionally homogeneous regime channel bedload transport equation not requiring bedload measurements for calibration or verification. The new equation was developed using 45 data sets and verified using 29 additional data sets encompassing a very broad range of channel conditions and was found to rely solely on stream power and bank-full stream power or, for practical applications, flow and bank-full flow. Statistical metrics indicated better performance by the new equation than by other bedload transport equations evaluated using the same source data and statistical methods.