Journal of Hydraulic Engineering

Abstract

Hydraulic parameters, including bed shear stress, are challenging to calculate for flash floods. Applying theoretical equations to strongly unsteady flows requires comprehensive and accurate flow data that are difficult to collect for natural events. To empirically evaluate the extent to which simpler calculations can reasonably predict shear stresses in rapidly changing hydrographs, and to determine how bed shear stresses differ for hydraulic bores propagating over dry beds compared to shallow water, we conducted laboratory flume experiments on idealized short-duration flash floods with bores. We compared the Saint-Venant shallow water equation, which theoretically captures the depth-averaged momentum balance of gradually varying unsteady flow to eight simpler methods, with assumptions that are not met in these flows. The Saint-Venant method predicts higher shear stresses immediately after bore arrival, but all of the simpler methods are nonlinearly correlated with the Saint-Venant method even when flow is rapidly changing. While none of the methods we evaluated should strictly apply to rapid changes in depth and velocity of bores, the correlations we found between methods just after bore arrival suggest that, for applications where shear stresses must be calculated but data are insufficient to apply the full Saint-Venant equations, simpler methods may provide meaningful shear stress constraints. Compared to flood bores propagating over a dry bed, bores propagating over shallow flowing water had steeper water surfaces but resulted in significantly lower bed shear stresses immediately after bore arrival.

Practical Applications

The common characteristic of tsunamis and waves moving onshore, and of dam breaks and flash floods occurring in rivers, is the presence of a sudden large increase in water depth and velocity termed flood bores. These result in an abrupt and large increase in shear stress (the force of the water on the bed), causing fatalities and extensive damage to infrastructures. We generated flood bores in a flume, with instrumentation that automatically monitored water depth, velocity, turbulence and water surface slope of bores arriving on a dry flume bed, with additional experiments of bores arriving on an existing shallow flow. We then calculated and compared the temporal change in shear stress by different methods, and found that, with the exception of the immediate arrival of a bore, shear stress calculated using simple assumptions is similar to the most demanding method. Another conclusion is that the shear stress is very large upon arrival of a bore on a dry bed—larger even than over a body of water. Hence, flood bores unexpectedly arriving on a dry bed are particularly dangerous to people, and their potential damage quantified by shear stress can be calculated using simplified formulas.

Our data show large scatter in the critical Shields stress values for initial sediment motion. The main sources of dispersion are related to the methodological procedures defining the inception of movement (i.e., visual observations or extrapolation of sediment transport rate) and to the estimation of the bed shear stress. The threshold for sediment motion varies with many factors related not only to grain size but also to bed composition (e.g., presence of fine sediments in a coarse matrix), arrangement (e.g., bed roughness, grain orientation, and characteristic lengths of bed structures) and slope. New models to estimate the critical Shields number are proposed using grain size or bed slope or a combination of both. Model parameters and uncertainty are estimated through Bayesian inference using prior knowledge of these parameters and measured data. Apart from the uncertainty in observations, two types of uncertainty can be evaluated: one related to the parameter estimation (i.e., parametric) and one related to the choice of the model (i.e., structural). Finally, a four-parameter model based on the grain size and bed slope yields the best results and demonstrates potential interaction between these two parameters. Model uncertainty, however, remains large, which indicates that other input parameters may be needed to improve the proposed model.

Accurate estimation of local scour depths at piers is important since underestimation can result in structural failure and possible loss of lives and overestimation in needless structure cost, which can be significant for large structures. An improved mathematical model for estimating equilibrium structure-induced (local) sediment scour depths at bridge piers with complex geometries founded in noncohesive sediment is proposed. It is the result of an increased understanding of local scour processes, the contributions of individual pier components to the effective pier width, and the quantity and quality of recently published laboratory data. A large quantity of published laboratory data was compiled and screened for test duration, method of extrapolation to equilibrium, flume dimensions, and instrumentation used. This resulted in a data set of 454 complex pier local scour tests with which to test the model presented here and other commonly used models. The methods compared are those designed for use with a wide range of complex pier shapes founded in noncohesive sediments and subjected to either clearwater and/or live-bed flow conditions.

Stormwater management model (SWMM) software has recently become a modeling tool for the simulation of intermittent water supply systems. However, SWMM is not capable of accurately simulating the air behavior in the pipe-filling phase, missing therefore a relevant factor during pipe pressurization. This work proposes the integration of a conventional accumulator model in the existing SWMM hydraulic model to overcome this gap. SWMM source code was modified to calculate the air piezometric head inside the pipe based on the system boundary conditions, and the air piezometric head was incorporated in the SWMM flow rate pressure component. Experimental data were collected during the rapid filling of a pipe system for three possible configurations that are likely to occur in intermittent water supply pipe systems: no air release, small air release, and large air release. Results show that the improved SWMM better describes the effect of the air behavior using the extended transport (EXTRAN) surcharge method when compared to the original SWMM. Results also show that the SLOT method with predefined slot width is not suitable for this purpose; thus, further research is needed to assess if an adjusted slot width could provide better results.

Separating and reattaching flows beneath semisubmerged ice blocks with smooth and rough undersurfaces were systematically studied using a planar particle image velocimetry system. The examined Froude and Reynolds numbers based on the incoming free-stream velocity and water depth were 0.06–0.24 and 55,900–204,000, respectively. The ratios between the submerged thickness of the ice block and water depth varied between 0.02 and 0.18. The results indicate that there exists a mean separation bubble beneath the head of an ice block. The mean reattachment length when scaled by the incoming water depth correlates very well with the Reynolds number based on the submergence thickness. The undersurface roughness tends to reduce the mean reattachment length, and this reduction diminishes as the submergence thickness increases. The streamwise mean velocity scales well with the mean reattachment length within the mean shear layer. The results also show that turbulence transition initiates at the leading edge of the ice block. Meanwhile, the magnitudes of Reynolds stresses are significantly elevated within the mean shear layer and peak in the rear part of the mean separation bubble. When properly scaled, the Reynolds stresses in the lower portion of the shear layer exhibit self-similar profiles. These self-similar profiles are close to a Gaussian function and are not sensitive to either the undersurface roughness or Reynolds number.

Abstract

Artificial open-channel junctions have sharp corners and a smaller width-to-depth ratio. Despite numerous studies on channel junctions, knowledge on turbulent structures and their role in mixing at artificial junctions is lacking. To fill this research gap, the present study uses a large-eddy simulation (LES) model to investigate the three-dimensional (3D) turbulent structures and their role in pollutant transport at a right-angled laboratory open-channel junction with a deformed bed. The deformed-bed junction represents a quasi-equilibrium condition and consists of a scour zone and deposition bar. The numerical model is validated against experimental data of the velocity field and turbulent kinetic energy. Comparisons of the flow field between the flat-bed condition and quasi-equilibrium deformed-bed condition showed a reduced flow separation zone and less developed recirculating gyre in the latter case because of the strong secondary currents. The coherence of the turbulent structures was drastically disrupted at the deformed-bed junction because the Kelvin-Helmholtz (KH) instability results in more randomly oriented residuals. In contrast, the breakdown of the shear layer at a flat-bed junction displayed the trail of the arch-shaped vortices. The role of turbulent structures in pollutant transport is elucidated by using a neutrally buoyant conservative tracer. Large turbulent structures associated with the KH instability help the tracer evolve faster at the deformed-bed junction.

Practical Applications

The open-channel junctions of artificial systems (e.g., drainage networks) have sharp corners and a smaller with-to-depth ratio (compared with natural river junctions). This paper describes the detailed three-dimensional (3D) flow field and pollutant transport at a laboratory-scale right-angled channel junction with a quasi-equilibrium deformed bed to elucidate the hydraulics of the artificial channel junction. Right-angled channel junctions are a common feature of artificial channel junctions. Given the limitations of the experimental studies, a 3D numerical model, namely large-eddy simulation, is used in the present study. The quasi-equilibrium deformed bed of the junction was obtained from the initial flat-bed condition. The flow field, including the 3D turbulent structures, was modified remarkably at the deformed-bed junction compared with the field under the initial flat-bed condition. The study shows that bed topography plays a major role in controlling turbulent structures. The role of the 3D turbulent structures in pollutant transport is investigated. The present study improves the understanding of the flow field and mixing patterns at artificial open-channel junctions.

Two- and three-dimensional analyses of transient flows considering the air–water interaction have been a challenge for researchers due to the complexity in the numerical resolution of the multiphase during emptying in pressurized water pipelines. The air–water dynamic interaction of emptying processes can be analyzed using thermodynamic and hydraulic laws. There is a lack in the current literature regarding the analysis of those phenomena using 3D models. In this research, several simulations were performed to study the complex details of two-phase flows. A 3D model was proposed to represent the emptying process in a single pipeline, considering a PVoF model and two-equation turbulence model. The model was numerically validated through 12 experimental tests and mesh sensitivity analysis. The pressure pulses of the air pockets were evaluated and compared with the experimental results and existing mathematical models, showing how the 3D models are useful for capturing more detailed information, such as pressure and velocity patterns of discrete air pockets, distribution of air and water velocity contours, and the exploration of temperature changes for an air pocket expansion.

Water quality degradation, in the form of undesirable algae, occurs near the Nottawasaga River mouth and along Wasaga Beach, southeastern Georgian Bay. The ability to manage this water resource is compromised due to lack of monitoring and science. In the present study, a 3D hydrodynamic model was applied to gain an understanding of how advection and dilution of the river plume can trap nutrients along Wasaga Beach. Simulated water temperatures and currents had a root-mean-square error between 1.5°C and 2.5°C and $\sim 0.06\mathrm{m}{\mathrm{s}}^{-1}$, respectively. Maximum nearshore river concentrations occurred near the beach during high river discharge $>20{\mathrm{m}}^3{\mathrm{s}}^{-1}$ and westerly winds. Generally, the river plume was advected by winds $>\sim 4\mathrm{m}{\mathrm{s}}^{-1}$, traveling along the northern shoreline during southerly winds and along the southwest shoreline during northeasterly winds. This process was analytically modeled when the wind strength indicator or Froude number (ratio of the characteristic wind-velocity scale to the buoyancy–velocity scale) was greater than one.