Journal of Hydraulic Engineering

The adoption of baffles is relatively common in the construction of culverts, to assist with the upstream passage of migrating fish species. However, there still is a lack of systematic studies of the complicated hydraulic conditions induced by the baffles to optimize the designs. Herein, near-full-scale physical modeling was performed, focusing on the oscillation and instability of open-channel flow in a fish-friendly culvert equipped with full-height sidewall baffles. High-resolution measurements of the instantaneous flow velocity were obtained using an acoustic Doppler velocimeter. The physical results were marked by the existence of some low-frequency oscillations. A triple decomposition technique was applied to the free-surface and velocity time series. The low-pass components confirmed a unique flow structure, consisting of a high-velocity zone in the main channel and a low-velocity flow reversal within the lateral cavities. The band-pass components corresponded to the low-frequency flow oscillations, highlighting the complicated transverse interactions between the lateral cavity and the main channel. The high-pass velocity components were related to the true turbulence characteristics. This study provides a quantitative data set in support of the sustainable design of culverts to assist with upstream fish migration in artificial and natural fast waterways.

This study conducted experiments to investigate the impact of bottom obstacles, specifically broad-crest weir-type and cylindrical obstacles, on the flow within a sewer pipe. It was observed that these two types of obstacles, when possessing the same cross-sectional area and length, exhibited nearly equivalent hydraulic responses for both supercritical and subcritical flows. The obstacle height was found to be the primary factor responsible for choking and the transition between different flow regimes. The results of the one-dimensional analysis show that the threshold value of obstacle height depends on the approaching flow conditions, including the Froude number and the filling ratio that is defined as the flow depth to the pipe diameter. In the context of a partially blocked sewer, the relationships established by using the one-dimensional model between the size of the bottom obstacle and the inflow characteristics enable the identification of flow regimes, assessment of reduced hydraulic capacity, and evaluation of local energy loss. The findings of this study elucidate the hydraulic behavior of obstructed sewers, contributing to the development of more effective strategies for the identification of obstructions through monitoring data.

The Nikuradse roughness height for a river channel or, equivalently, the friction factor ($f$) or Manning’s coefficient, are parameters required to calibrate hydrodynamic models used for flood risk management. In general, modeling hydraulic roughness includes the contributions of skin friction, related to sediment grain size, and bed-form roughness, related to bed-form geometry. Frequently, bed-form resistance is larger than the resistance due to skin friction. An empirical approach has been presented in the literature to estimating bed-form roughness, which is widely used for engineering purposes. This contribution presents an alternative physically based formulation for dune roughness estimation. Flow resistance generated by the bed form is assumed to arise from the drag force exerted by the bed, based on depth-averaged flow quantities. The new formulation agrees with existing experimental laboratory results and new field data from the Tercero (Ctalamochita) River, Córdoba, Argentina.

In the aftermath of the worldwide severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, the disposal of plastic and nonwoven fabric materials originating from personal protective equipment (PPE) into freshwater bodies like lakes and rivers has significantly increased. Surface plastic transport and accumulation have become a relevant source of pollution that adversely affects the quality of water and fluvial ecosystems. Although attempts have been made in the past to study the flow characteristics of plastic at the river surface, an in-depth study of structural configurations that can act as potential plastic traps, facilitating reduction of pollution due to plastic transport, is necessary. This paper investigated the hydrodynamics of surface plastic transport in the presence of several control structures. In addition, useful empirical equations were derived to predict the kinematic and trapping efficiency of the mentioned structures, valid for both straight and curved channels under a variety of hydraulic and geometric conditions. It was found that the Froude number and position of the structure play a dominant role in influencing surface plastic transport mechanism and the overall efficiency of structures in limiting plastic transported downstream.

Abstract

Cyclic lateral loads on monopile foundations of offshore wind turbines induce vibrations that affect the clear-water scour process around the foundations. Previous experimental studies showed that the current-induced equilibrium scour depths at vibrating monopiles may be less than those at static monopile foundations. However, this observation may not be comprehensive when applied to the cases of clear-water scour around vibrating monopile foundations subjected to different flow intensities. This paper presents an experimental study that investigated the clear-water scour mechanism at vibrating monopile foundations and obtained new findings. The experimental results indicate that the scour process is influenced by three major factors: vibration-induced sediment subsidence, vibration-induced sediment refill, and current-induced erosion. Based on the three major influencing factors and experimental data, a dimensionless equation for equilibrium clear-water scour depth at vibrating monopile foundations was derived. Furthermore, a regime transition flow intensity is defined, at which the equilibrium scour depth remains constant regardless of any changes in the vibration amplitudes. Two distinct scour trends under low and high flow intensities were identified. The two different trends were found to be induced by different dominant factors. The vibration-induced subsidence effect dominates in the low flow intensity regime, whereas the vibration-induced sediment refill effect dominates in the high flow intensity regime. The new findings of this paper help to understand the characterization of clear-water scour process around vibrating monopile foundations.

Practical Applications

Monopile foundations of offshore wind turbines experience cyclic lateral vibration caused by winds and waves. The cyclic lateral vibration of monopile foundations potentially can affect the current-induced scour process around the foundations. The experimental results of this study revealed that the scour depth around vibrating monopile foundations under clear-water scour conditions is influenced by a coupling effect of flow intensities and vibration amplitudes and frequencies. The findings emphasize the influence of vibration on the scour process around monopile foundations of offshore wind turbines. These results provide valuable insights to optimize the design and manufacturing processes of monopile foundations. Moreover, the findings contribute to the development of more-effective scour protection measures in offshore wind engineering.

This study focused on the effect of the transversal bed slope on scouring at bridge piers. Twenty-four experiments were conducted, half of them being on a horizontal bed and the other half on inclined beds. Three inclination angles, three flow intensities, and three pier diameters were combined within the degrees of freedom of the available experimental setup. Velocity measurements were performed to characterize the velocity field at triangular cross sections. Contrary to intuition, it was concluded that the equilibrium scour depth around cylindrical piers inserted in an erodible inclined bed decreases with the inclination angle for angles between approximately 11° and 25°, which suggests that practitioners can safely use scour depth predictors derived for horizontal beds to calculate the scour depth at inclined beds, provided that they properly assess the approach flow velocity and the critical velocity for the beginning of sediment motion on inclines. Within the range of nondimensional values covered by this study, practitioners can also use a new scour depth predictor.

Abstract

Hydraulic jacking is a serious threat to concrete spillway chutes, demonstrated by the catastrophic spillway chute failure at California’s Oroville Dam in 2017. To improve the understanding of uplift pressure and joint flow developed at open, offset joints and enable design and evaluation of anchors, drain systems, and joint remediations that could help prevent such failures, laboratory tests were performed in a supercritical flume furnished with a model joint where the gap width to offset height ratio was varied over a $725:1$ range. The tests included measurement of boundary layer velocity profiles approaching the joint. Uplift pressures were normalized to the velocity head near the boundary, which is related to the depth-wise velocity profile exponents determined in the experiments and can be estimated for field applications from the chute friction factor. The normalized uplift varies with the joint aspect ratio and the flow depth to offset height ratio. The new relations reduce the uncertainty of modeled uplift pressures by a factor of 2.87 over previous methods. Example applications demonstrate practical upper limits for potential uplift pressure. Subsequent articles will address discharge into offset joints, the dissipation of uplift via drainage, and the effect of different methods for remediating existing offsets to reduce uplift.

Practical Applications

Concrete spillway chutes develop cracks and must necessarily be constructed with joints, both of which are prone to displacement over time that may create offsets into the flow. Flow striking such offsets is brought suddenly to rest, similar to a pedestrian tripping on a sidewalk crack. The local stoppage of flow at the offset creates dangerously high pressures that can be injected into the foundation, leading to erosion beneath the slab and potential uplift failure, often called ‘hydraulic jacking’. This mechanism has caused several notable failures including the Oroville Dam spillway in 2017. Protection against such failures is usually provided by a combination of the weight (thickness) of the slab itself, anchors that hold the slab down, and subsurface drains that reduce the buildup of pressure. This paper provides experimentally based equations for predicting uplift pressure to enable effective design of new spillways and evaluation of existing spillways. The new equations are significantly more accurate than previous methods because they account for the roughness of the chute surface and the reduced flow velocity near the boundary. Subsequent papers will address flow rate through joints, pressure dissipation by drainage, and methods for treating existing offsets to reduce potential uplift.

This study numerically investigates and compares the secondary currents, velocity dips, turbulence properties, and boundary shear stresses in supercritical narrow open channel flows (OCFs) and in narrow duct flows (DFs) using an updated Launder–Reece–Rodi Reynolds stress model in OpenFOAM, which was validated previously for supercritical flows using experimental data. Six steady state simulations were performed at a bulk velocity of $2.31\mathrm{m}/\mathrm{s}$ covering Reynolds numbers from $3.08\times {10}^5$ to $6.16\times {10}^5$ and aspect ratios (width to flow depth) of 1.0. 1.25, and 2.0, which in combination with the observed Froude numbers from 1.65 to 2.33 for OCFs are comparable to sediment bypass tunnel flows. Although free surface produces greater maximum secondary flows, the top wall in DFs creates stronger bulging of the longitudinal velocity above the velocity dips, which generates marginally higher maximum longitudinal velocity and significantly higher velocity fluctuations compared to OCFs. Two pairs of corner vortices are observed in each half width for DFs. However, such vortices differ in OCFs, in which the reduction of aspect ratio develops intermediate vortices. Such differences in the secondary currents are interrelated to the observed variations in the distributions of longitudinal velocity and Reynolds stresses. Higher average bed and sidewall shear stresses are obtained for DFs than for OCFs. The bottom vortices undulate the bed shear stress distributions. Similarly, the sidewall corner vortices (for DFs) or intermediate vortices and inner secondary vortices (for OCFs) undulate the wall shear stress distributions. These undulations are further influenced by the aspect ratio. Moreover, the flow characteristics below the mid depth observed for OCFs are comparable to those obtained for DFs, especially for the square cross sections with aspect ratio of 1.0.

Complex interactions between flowing water and bedforms in rivers with sand beds during changing flow rates may result in bedforms, flow resistance, and sediment transport rates that differ on the rising and falling limbs of flow hydrographs. To study this phenomenon, laboratory experiments based on six realistically shaped hydrographs with durations of 1–6 h in 1-h increments were performed in a recirculating flume at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) National Sedimentation Laboratory. Bed topography, water-surface slope, flow rate, and sediment transport rate were continuously monitored. Counterclockwise hysteresis between discharge and sediment transport rate as well as bedform dimensions were observed for all hydrograph lengths, and the magnitude of hysteresis was quantified. The amount of hysteresis in transport rate, bedform amplitude, and bedform length varied according to the length of the hydrograph, although the relationship was not monotonic. The Engelund–Hansen method was able to approximately predict sediment transport rates based on measured hydraulic parameters. Maximum bedform amplitudes increased with hydrograph length, while bedform lengths increased only minimally beyond the 4-h hydrograph period.

Abstract

In this paper, we leverage the direct numerical simulation (DNS) data for closed-channel flow for a range of friction Reynolds number ($R{e}_{\tau }\sim 180–\mathrm{5,000}$) to develop a new one-point friction velocity method (OPFVM) to calculate friction velocity $U_{*}$ in terms of free-surface velocity $U_m$, flow depth $h$, and kinematic viscosity $\nu$. In contrast to prevalent methods that require several cumbersome near boundary measurements to obtain friction velocity, the OPFVM relies on a single easy-to-measure free-surface velocity measurement. The formulation is used to obtain friction velocity for a closed-channel flow (CCF) DNS regime with $R{e}_{\tau }=\mathrm{10,049}$ and on four open-channel flow (OCF) DNS regimes with $R{e}_{\tau }\sim 180–\mathrm{2,000}$. The same formulation was then experimentally verified in our laboratory. To avoid being prescriptive, a sensitivity analysis was performed to determine the permissible variation in $U_m$ to restrict the error in estimated $U_{*}$ to 2%. The relationship between the depth-averaged velocity $U_b$ and the maximum free-stream velocity $U_m$ is also explored using the DNS data sets and an approximate relationship between $U_b$ and $U_m$ is proposed. With advances in remote-sensing technology that enables free-stream velocity measurements, this method extends the potential to measure even the friction velocity remotely.

Practical Applications

Measuring friction velocity $U_{*}$ is difficult in both laboratory and field settings for engineers and scientists. The proposed new method overcomes this challenge to estimate the friction velocity $U_{*}$ by measuring the velocity $U_m$ close to the free surface, flow depth h, and temperature (for viscosity). Because near-surface measurement of velocity is not difficult, this method greatly simplifies the measurement of $U_{*}$ with better accuracy than other prevalent methods in practice. In addition, direct numerical simulation (DNS) data has been used to estimate the average velocity $U_b$ using the measured free-stream velocity $U_m$, which further enables measurement of discharge using a single-point measurement of velocity near the free surface in smooth channels.

Air in the headspace of sewer laterals can be driven by the flowing sewage as well as air pressure. In this study, the openings on manholes for air flow are approximated as a thin continuous slot at the laterals. In this way, the governing equations can be solved analytically for infinitely long laterals without water flow. For cases with water flow, a numerical solution is developed. It is found that the air flow pattern in a long lateral can be categorized into three zones. The zone closest to the trunk is defined as the “zone of influence,” where the air flow in the lateral is dominated by air pressure, and the air pressure and air velocity decay exponentially in the lateral. The second zone is named as “zone of equilibrium” where the air pressure gradient is zero and the air flow is only driven by the drag of water and can be calculated analytically. The air flow zone farthest from the trunk is defined as the “zone of induction” where the air is inducted into the system for mass balance in the system. An explicit fitting expression on the air velocity and water velocity in a long lateral is provided. A graphic solution for air velocity and zone of influence is also developed for a quick estimation on the air movement in sewer laterals.

Recent studies show that near-bed turbulence may be better scaled using a mixed velocity scale $\sqrt{u_{*}U}$, where $u_{*}$ is the shear velocity and $U$ denotes the characteristic bulk flow velocity. This implies that the classical scaling consideration for sediment transport rates could be improved by redefining the Shields number based on the mixed velocity scale. To investigate this potential improvement, a series of analyses was performed with laboratory data from 25 sources of sediment discharge studies. The data were carefully selected by minimizing effects of some factors including sidewall, particle gradation, particle density, and fluid viscosity. The findings indicate that the relationship between sediment discharges and flow conditions can be enhanced significantly when the analysis is carried out based on the redefined Shields number. The new relationship does not depend on the type of bedforms, and therefore facilitates the prediction of sediment discharge. Finally, by comparing with previous studies, it shows that the proposed formula markedly improves the prediction of bed-material sediment discharges.

Natural accumulations of wood, as well as engineered logjams, are relevant for river ecosystems. These structures, by interacting with the flow, can lead to significant backwater effects and morphological changes. Research on engineered logjams has so far mainly relied on physical modeling, which may impede the study of different flow conditions or logjam designs and configurations. Here, a simple approach for the simulation of logjams in two-dimensional depth-averaged hydrodynamic solvers is explored, building on widely available modeling options. The effect of logjams is included by combining a local contraction and an increase in local frictional losses based on the logjam characteristics, thereby reflecting the main physical flow processes in the region near a logjam. More than 350 modeling tests were used to calibrate and validate the proposed approach. For all tests, backwater rise due to channel- and partial-spanning logjams can be estimated with $\pm 30\%$ prediction range. Given its simple design, the calibrated model is not universal and requires careful evaluation when applying it to different setups. Nevertheless, a clear advantage of the proposed approach is that it uses tools that can be adopted in most of the standard two-dimensional hydrodynamic solvers.