Journal of Hydraulic Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Landscape-scale changes in hydrology, sediment dynamics, and riverine ecosystems are raising questions about how resilience to changing conditions should be integrated in the design of river restoration projects. This study synthesized current strategies and challenges for river restoration in systems governed by change, emphasizing (1) an international survey of practitioners to document the state-of-the-practice, including challenges facing practitioners implementing river restoration and (2) a literature review of the state of the art in designing for resiliency, including advantages and limitations of established and more-experimental design approaches and tools. Survey results highlighted fundamental challenges associated with identifying appropriate project objectives within a design practice largely relying on historical data, the significance of social and regulatory barriers, and the need to focus on underlying processes. Important discrepancies between the expectations for resiliency of projects and the practices being implemented emerged. The state-of-the-art literature review summarized how design strategies may be modified to address those discrepancies, and discussed the divergent approaches for increasing resilience through (1) increasing resistance to change (i.e., stabilization) and (2) restoration of a channel’s dynamism to reduce the risk of, and recovery time following, disturbances. Both the state of the practice and the state of the art highlight the lack of resiliency within the practice of river restoration itself, and illustrate the importance of designers, funders, regulators, and stakeholders adapting and innovating as the natural and human landscape they are managing changes.

The present study examines the effect of upstream channel bank angle on the turbulent flow characteristics and the underlying coherent structures associated with the development of local scour in the vicinity of the leading edge of a longitudinal structure. Three laboratory experiments with channel bank angles of 28°, 45°, and 65° were carried out. Flow measurements were obtained employing a stereoscopic particle image velocimetry (SPIV) system. The characteristics of underlying energetic flow features in each of the experiments were investigated by using the proper orthogonal decomposition (POD) and optimized dynamic mode decomposition (optDMD) techniques. The SPIV results indicated that for a moderate bank slope $\theta =28°$, the horseshoe vortex (HV) amplifies turbulent kinetic energy and, consistently, bed shear stresses that consequently contribute to the local scour development. As the channel bank angle increases, and thus the degree of flow obstruction decreases, the intensity of HV declines and eventually diminishes for $\theta \ge 65°$. Thus, the erosional capacity of the flow is reduced. The modal analyses specified that with the increase of channel bank angle, the flow energy is distributed among a larger number of POD modes and the similarity between the POD and DMD modes decreases. The relatively low energy content of the POD and DMD modes suggests the aperiodic behavior of the main necklace vortex and verifies the significant reduction in its coherency with the channel bank slope.

Large wood debris can cause critical damage to bridges and other riverine structures, and increase flood risk. Although their effects on hydrodynamic actions and flood levels have been investigated in recent research, little effort has been devoted to reducing the amount of debris that can accumulate at structures. This paper proposes and experimentally tested a new type of large wood debris retention system in which a series of alternating porous and rack-type modules is placed in-line with the current. Laboratory tests showed that the proposed retention system can offer high levels of efficiency in trapping large wood in rivers. The geometrical features of the structure play a major role and can be chosen carefully to optimize trapping efficiency. Results showed that large wood debris trapped by these structures have limited effects on the increase of the upstream water levels. Further development of the solution proposed in this work can pave the way for use of low-cost, highly effective debris retention systems for effective river management and large wood debris removal in practice.

Three laboratory-scale Archimedes screw generators were tested for their power production. They were tested at different inclination angles corresponding to their lengths in order to maintain a constant head for each screw. Experimentation was used to develop initial insight into the effect of inclination angle on Archimedes screw power production. Computational fluid dynamic simulations were then compared to the experimental data and evaluated for their accuracy. The model was found to be acceptable and was then used to extend the data set to a total of seven inclination angles across the range of $\beta =10°$–40°. It was found that Archimedes screw generators with shallower inclination angles produced more power. It was also noted that shallower inclination angles correspond to longer screws and, thus, more civil infrastructure costs. The authors suggest that the ideal inclination angle for an Archimedes screw generator installation is between $\beta =20°$ and 25° based on this laboratory-scale testing.

The current literature for labyrinth weirs suggests that little attention has been paid to the complex free-surface flows generated downstream of these structures. In particular, there is no available guidance on the most appropriate model implementations to reproduce these flows numerically. This study presents new insights into the three-dimensional (3D) computational fluid dynamics (CFD) modeling of the free-surface flow over a labyrinth weir and spillway. The volume of fluid (VOF) model is implemented in both the OpenFOAM and ANSYS Fluent version 17.2 solvers to simulate four flow rates over a $1:25$ scale Froude number physical model. The results reveal the VOF method with the $k\text{-}\epsilon$ standard turbulence model and the piecewise linear interface construction algorithm is capable of well characterizing the complex flow behavior and features and provides appropriate predictions of velocities and depths. The model is also able to adequately estimate the labyrinth weir rating curve and the flow situation for various levels of tailwater in the spillway channel. The numerical predictions from the two solvers present greater consistency for the low flow rates. The increased discrepancies occurring for the largest flow rates are attributed to the different sensitivity to the mesh cell size as well as to the interface capturing schemes utilized.

Scour at bridge abutments usually consists of local and contraction scour that correlate because of the flow field that develops both scours. Flume experiments were conducted using simulated compound channels to investigate the effect of a setback vertical-wall abutment’s streamwise contraction length on scour depth and pattern. The adequacy of the current design method for riprap aprons was also examined. The prescour flow measurements showed that an abutment with small streamwise length may divert the highly turbulent zone toward the main channel and downstream that beyond the protection of the apron. By contrast, the turbulent zone of a long abutment tends to occur near the abutment face and within the apron area. The erodible bed experiments showed that the abutment length may significantly affect scour depth, morphology, and temporal development for apron-protected abutments but only has a minor influence on unprotected abutments. For apron-protected abutments, an increased abutment length reduces the magnitude of the deepest scour and moves the scour hole closer to the contracted section and the abutment face, which is consistent with the prescour flow field. Depending on the extent of setback, scour may also extend into the main channel. It was found that the existing abutment scour predictors should consider streamwise abutment length in order to reduce underestimation for short contractions and overestimation for long contractions. The design of riprap aprons should also be improved accordingly and integrated with the predictors.

The dispersion of oil droplets in rivers was numerically investigated for uniform flow in a hypothetical wide river with a depth of $3.0\mathrm{m}$. The river hydrodynamics profile was used in conjunction with the VDROP model to produce the oil droplet size distribution (DSD), whereas the NEMO3D model was used to track the movement of the oil droplets. Results suggest that the gradient of eddy diffusivity significantly affected the upward-normal (i.e., quasi-vertical) transport of the droplets, and caused them to mix rapidly through the depth. We also found that an increase in buoyancy resulted in a decrease in the streamwise variance and spreading coefficient. Oil droplets broke up due to the relatively large energy dissipation rates in the river at approximately $1.0\mathrm{m}$ below the surface and deeper. The droplet breakup varies DSD in the river water column, which may subsequently affect other chemo-physical processes (e.g., oil-particle aggregation). The breakup efficiency is affected by a system-dependent parameter $K_b$, which reflects the uncertainty of a system. The steady-state DSD was bimodal for the case $K_b=0.05$, whereas it was unimodal for larger $K_b$ values (i.e., $K_b=1.0$ and 0.25), respectively. More small-sized droplets were generated and persisted in the deep-water column with larger $K_b$ values. The droplet breakup also enhanced the streamwise spreading of the plume. The effect of droplet entrainment on oil dispersion was studied by assuming constant entrainment probabilities of surface oil droplets. The oil DSD varied with different droplet entrainment probabilities, and the number of oil droplets generated in the water column decreased significantly with a decrease in the entrainment probability of the oil droplets.

This study investigates the flood flow characteristics in a compound channel subject to both lateral contraction (caused by bridge abutment/embankment) and vertical contraction (caused by bridge deck submergence) at bridge sites. Three abutment setback distances and three pressure flow types [free surface (FS), submerged orifice (SO) and overtopping (OT)] are tested. The results show that turbulence structures in the approach channel remained the same irrespective of downstream obstruction. However, lesser abutment setback from the main channel led to greater flow acceleration as the flow approached the bridge section, where SO flow had the highest flow intensity and FS flow had the lowest. At a bridge contraction, the distributions of flow velocity, turbulence intensity, and Reynolds shear stress are significantly affected by the type of pressure flow and the extent of contraction. Enclosed counterclockwise secondary circulating flows in the main channel may occur for a combination of long or medium setback abutments and a FS flow, while other conditions usually feature open-ended upward flows due to flow relief. The strong downslope flow component detected at the main channel bank has a noticeable bank erosion capability. The bed shear stress, which is an indicator of sediment scour, is highlighted by apparent peak zones at the main channel bank and abutment toe. Finally, clear relationships between turbulence intensity, unit discharge ratio, and lateral contraction length are found. In general, normalized turbulence intensities on the floodplain and in the main channel can be used to assess the contribution of macroturbulence to the final bed topography after scour.