Journal of Hydraulic Engineering

Transported driftwood and woody debris during floods may lead to accumulations and blockages at river bridges or weirs, and can result in excessive scour or an increase in backwater that may lead to flooding of the nearby areas. Although driftwood-related problems occur predominantly in forested mountainous regions, driftwood may eventually reach densely populated lowland areas. Prevalent retention structures, such as nets or racks, across the river often fail for larger alpine rivers because of the excessive structural loading or the resulting backwater rise that may overtop the flood embankments. Therefore, this paper presents an overview of driftwood risk analysis and retention measures in large alpine rivers. Several methods on how to determine the potential driftwood volume for a given catchment area are summarized. The most reliable data are thereby obtained with a detailed investigation of the catchment characteristics. Observations during past flood events may also add valuable information for future driftwood management. Furthermore, methods on how to determine the likelihood of driftwood blockage at river crossings are presented in order to identify endangered structures prior to a flood. A driftwood transport diagram is introduced that indicates how much driftwood is expected at a certain location along the river and provides the basis for an optimum location for a driftwood retention structure. Finally, a novel driftwood retention structure is presented, where the driftwood is retained in a bypass channel located at the outer river bend. Because of the secondary currents that are induced by the river bend, the bed load remains in the primary channel, whereas the floating driftwood is directed into a bypass section. The bypass retention was tested in flume experiments and successfully implemented in a small-scale case study.

This paper is motivated by the need to develop low cost, wireless velocity sensors for hydraulic research and application in streams. A velocity bend sensor (VBS) is a flexible plastic polyimide substrate sheet with an electronic resistor connected to a voltage divider. Drag of a moving fluid bends the sensor, changes the electronic resistance, and produces a voltage drop that can be related to the time-averaged freestream velocity of the fluid. VBSs were tested in a recirculating hydraulic flume with a gravel bed. The VBSs show transition from rigid to elastic bending with increasing freestream velocity, which can be described using dimensionless fluid and beam-bending properties. The relationship between stream velocity and voltage drop across the circuit is nonlinear. A semitheoretical approach to estimate time-averaged streamwise velocity from the voltage drop based on fluid drag, elastic member bending, and circuit principles is applied and shows good agreement with experimentally derived calibration curves. The triple decomposition theorem and spectral analysis are performed on VBS and acoustic Doppler velocimeter (ADV) time series. Results show that the VBS captures low-frequency characteristics of macroturblence present within the turbulent open channel flow but is unable to measure smaller-scale characteristics of eddy shedding for these hydraulic conditions. Turbulent intensity calculated using VBS data is 12% of that from the ADV attributed to the lack of detection of shedding-sized eddies. But, the linear fit between turbulent intensity from the VBS and ADV suggests that the VBS can be used as a proxy for more detailed turbulent measurements when applied in streams.

Bioretention cells have become an accepted technology for storm-water management due to their potential benefits in runoff volume reduction and water quality improvement. Over the years, a considerable amount of research has been done to improve performance, whether by improved functionality or by introducing engineered filter media to target specific contaminants. The design for a full-scale demonstration project carried out in Oklahoma incorporated a sand and fly ash filter for enhanced phosphorus adsorption and surface sand plugs for improved infiltration. Samples taken from the filter media during construction exhibited heterogeneity in fly ash content, and therefore, hydraulic conductivity due to the pozzolanic property of fly ash. Maximizing contaminant attenuation will require achieving uniform flow through the cell to ensure use of the entire filter volume. Thus, it is important to understand how the small-scale filter heterogeneity and the large-scale sand plugs impact performance of this design. To do this, a three-dimensional finite element model was developed in a software tool to simulate saturated flow through a bioretention cell. Three general configurations were modeled for three different scenarios. A filter-only configuration was evaluated to assess the effect of filter media hydraulic conductivity heterogeneity on flow through the cell. The second configuration added a top soil and sand plug layer with six sand plugs measuring 1.5 m by 1.5 m, which was similar to the constructed cells. The final configuration evaluated a top soil and sand plug layer with 14 smaller sand plugs only 1 m by 1 m. Three different scenarios were evaluated for each configuration that varied by size and distribution of the filter media heterogeneity. The first scenario used the measured scale and range variability, while the second used the same scale with double the variation, and the third used the same variability, but increased the scale volume by a factor of 27. Model results indicate that variability in fly ash content creates complex flow through the filter medium, but does not result in significant preferential flow. Sand plugs create some flow concentration but do not dominate flow within the cell, and the number of sand plugs is not significant provided that their total area is appropriate to maintain the desired drainage rate.

Trash screens are commonly installed at culvert entrances to prevent the ingress of debris that might otherwise become lodged within the structure. However, these can be a hazard in themselves if not cleared at an appropriate interval. There are currently no tools available to support making decisions regarding screen inspection requirements based upon potential site-by-site blockage risks. The analysis reported here was performed to address this issue. The parameter considered as key in the decision-making process was the probability of screen blockage. To determine this for any screen under consideration, a stochastic predictive model was developed using inspection records, obtained from 140 screens in Belfast, Northern Ireland, to relate blockage probabilities to seven potential drivers extracted from channel, land-use, meteorological, temporal, and social deprivation factors, employing a logistic regression approach. To allow for randomness in the data set, a Bayesian framework was adopted through which the uncertainty associated with any prediction could be reported using appropriate credible intervals. The predictive accuracy of the model was also assessed using appropriate measures and, despite documented uncertainties, was shown to be well within acceptable limits.

Computational fluid dynamics (CFD) software was used to generate water depth and discharge data for 51 Cutthroat flume sizes, including all 24 standard sizes published by the flume’s developer and 27 intermediate sizes. The hydraulic model was validated by using data from several sources, including physical model testing for 15 standard sizes and three intermediate sizes. Grid-size independence was also confirmed for a range of computational cell sizes and the calibration results for 15 standard flume sizes were compared with previously published calibration parameters. Through the validated CFD simulations, a full set of calibration and validation data was generated for each of 51 flume sizes, filling the gaps in the experimental data and permitting a full analysis of the performance of the flume. The study resulted in a generic calibration for free-flow conditions for any Cutthroat flume in the dimensional range of the 24 standard sizes and specified dimensional ratios according to the original design criteria. The error in the predicted discharge for all flume sizes is less than 3% of the full-scale discharge.

This work presents an experimental investigation on turbulent flow characteristics in a typical gully with reverse flow. Four different flow rates (the inlet Reynolds number ranged between $2.8\mathrm{e}+04$ and $7.4\mathrm{e}+04$) were characterized in a laboratory facility using an acoustic Doppler velocimeter. A complete signal analysis was carried out to identify and remove the influence of errors in acoustic Doppler measurements on the computation of the mean velocity, turbulent kinetic energy, and Reynolds stresses. The flow showed a similar pattern between the different analyzed experimental conditions in terms of mean velocity and turbulent kinetic energy. A macroanticlockwise spanwise axis vortex was identified based on the mean velocity field in the region encompassed between the upstream wall and the inlet centerline. Maximum mean velocity magnitudes were approximately 25% of the inlet velocity and characterized the outer flow of the vortex. Higher turbulent kinetic energies were observed at the inlet centerline region with values approximately 5% of the inlet velocity square. The Reynolds stresses exhibited some anisotropy, which was revealed by both the shear stresses values and by the differences in the normal stresses. Normal stresses were higher than shear stresses, with maximum values at the inlet centerline region (about 3%, 5% and 2% of the inlet velocity square in the streamwise, spanwise and vertical direction, respectively).

Knowledge of flow pattern and water age distributions in service reservoirs is of the utmost importance for ensuring water quality supplied to the customers. In this article, a balancing reservoir was first investigated as a benchmark case. Using the computational fluid dynamics (CFD) method coupled with dynamic meshes, the flow pattern and water age distributions in the reservoir were simulated under actual service conditions. The calculated flow pattern indicated the migration and evolution of flow recirculation regions in the reservoir, which would lead to nonuniform lateral and vertical distributions of the water age, and therefore residual chlorine concentration. A passive flow control method, by adjusting the original circular nozzles to elliptical nozzles, was explored to enhance the water quality. It was found that using elliptical nozzles could assist fluid mixing in the reservoir. The water in the modified reservoir with elliptical nozzles had 5% lower age values than those of the original. Next, bubble plumes were used as an active method to enhance the water quality in the balancing reservoir. It was found that bubble plumes could enhance the fluid mixing significantly in both the vertical and lateral directions. Consequently, the modified reservoir with bubble plumes had 10% lower age values than those of the original reservoir. Therefore, the use of bubble plumes is suggested to be a viable approach for enhancing the water quality in a balancing reservoir, which is deemed to be novel in this field.

The flow through and downstream of a row of seven open draft tubes in a barrage has been investigated through laboratory experiments in a wide flume, a three-dimensional (3D) computational fluid dynamics simulation, and a two-dimensional depth-averaged computation. Agreement between the experiments and the 3D modeling is shown to be good, including the prediction of an asymmetric Coandă effect. One aim is to determine the distance downstream at which depth-averaged modeling provides a reasonable prediction; this is shown to be approximately 20 tube diameters downstream of the barrage. Upstream of this, the depth-averaged modeling inaccurately predicts water level, bed shear, and the 3D flow field. The 3D model shows that bed shear stress can be markedly magnified near the barrage, particularly where the jets become attached.

Extended blockages in a pipeline system are expected to impose changes onto the system resonant frequencies where the size and nature of the frequency shifts can be used to determine the blockage characteristics. Although a theoretical method for detecting and locating extended blockages in pipeline systems using these changes in the system frequency response (SFR) was developed by the authors in a previous paper, the impact of an extended blockage on SFR has yet been verified experimentally and is the topic of this paper. The impact of six different extended blockages under a range of different Reynolds numbers on the frequency response is used to confirm the theoretical behavior of an extended blockage. These experimental tests are conducted in the pipeline hydraulic laboratory at the University of Canterbury, New Zealand. An analytical simplification of the original SFR-based method is used to identify the key blockage parameters governing the frequency shifts and shows that the magnitude of the frequency shift increases with severity of blockages and is related to the changes in characteristic impedance and wave propagation coefficient of pipeline (pipe diameter, thickness, and/or wavespeed) imposed by the blockage. The experiments show that the length and location of potential extended blockages in the pipeline can be accurately predicted by the proposed method. Significant error exists, however, in the prediction of the pipe constriction diameter and is a result of the nonlinear operations in the experiments such as full valve closure in this study and the inability of existing models for perfectly replicating transient events in pipes with severe constrictions.

Inverted siphons are particular structures in sewerage and drainage systems that are frequently associated with difficulties in hydraulic design and operating conditions. Air injection in the base of the rising branch of the sanitary siphons may solve problems often found in conventional design, such as the available hydraulic head and the flow velocity required, or the frequent deposition of suspended solids and organic matter. A model was developed for numerical integration of the energy differential equation for gradually varied steady flow of the two-phase flow of the rising leg considering isothermal conditions. The results of the model application were compared with published data measured on air-lift pumps and with data obtained in a siphon experimental setup with two barrels of different diameters, which was specifically constructed and tested for the research. The numerical results for the set of parameters used and corresponding calibration data show a good agreement with the experimental measurements, primarily for the barrel with the smaller diameter.

Acoustic doppler velocimeter (ADV) data can be contaminated by spikes from various sources. Available despiking methods were found to encounter difficulties in despiking ADV data from a turbulent jet flow. An iteration-free despiking algorithm was developed for highly contaminated ADV data by applying a bivariate kernel density function and its gradient to separate the data cluster from the spike clusters. It is shown that the new method overcomes some of the deficiencies of the existing despiking methods.

European statistical hydraulic models developed in France and Germany were tested on how they predicted the frequency distributions of water depth and point velocity measured in 14 reaches in Ecuador during 25 surveys. First, the observed frequency distributions were fitted to parametric functions defined in Europe and the parameters were predicted from the average characteristics of reaches (e.g., discharge rate and mean depth and width) using European regressions. When explaining the frequency of three classes of velocity and three classes of depth among reach surveys, the fitted and predicted distributions had a low absolute bias ($<3\%$). The residual variance of fits relative to the mean class variance was $<18\%$. The residual variance of predicted frequencies was 30–61% for velocity classes and 20–36% for depth classes. Overall, the European models appeared appropriate for Ecuadorian stream reaches but could be improved. This study demonstrates the transferability of statistical hydraulic models between widely separated geographic regions.