Journal of Hydraulic Engineering

Reaeration is a primary path of reoxygenation in streams, fundamental to environmental and ecological integrity. Previous laboratory studies of reaeration rates in subcritical flows showed large scatter in results, with differences in mass transfer coefficients of more than one order of magnitude between comparable flow conditions. Although supercritical flow is commonly observed in natural streams and engineered channels, systematic measurements of supercritical flow reaeration rates have been unavailable. Experiments in a laboratory open channel flume encompassing sub- and supercritical flows have been undertaken. The subcritical data were consistent with a large body of previous studies. Supercritical flows showed 6–10 times stronger reaeration rates than comparable subcritical conditions, while local rates at hydraulic jumps systematically exceed those in the supercritical flows upstream by a factor up to three. A close relationship between reaeration rates and turbulent dissipation rate is observed, and a systematic Froude number dependency is demonstrated for both sub- and supercritical flows. Observed mass transfer coefficients do not correlate as well with flow Reynolds number and shear Reynolds number. The higher reaeration rates associated with supercritical flows indicates that a change in open channel flow regime for the same Reynolds number may be used to improve water quality.

This study investigated multivariate relationships between critical erosion thresholds of reservoir sediments and their physicochemical and biological characteristics to unravel the effect of sedimentological parameters on fine sediment erosion. We collected 22 sediment cores from the deposits of two reservoirs located in southern Germany, Grosser Brombachsee (GBS), and Schwarzenbachtalsperre (SBT). An erosion flume and an advanced photogrammetric method were used to quantify critical erosion thresholds for a succession of vertical layers over sediment depth. The functional relationships between the critical erosion thresholds and a collection of sediment parameters including bulk density, sediment composition, percentiles, cation exchange capacity, organic content, extracellular polymeric substances (EPS proteins and carbohydrates), and chlorophyll-a were examined. The clay-dominated sediments of the GBS with comparatively low total organic carbon and sand content were, on average, 10 times more stable than the sandy sediments of the SBT. Consequently, for the clay-dominated sediments, strong positive correlations were found between the erosion thresholds and clay content. In contrast, the sandy sediment layers experienced strong positive correlations with the sand content and percentiles. The bulk density was mainly positively correlated, and the total organic carbon content was mainly negatively correlated, with the erosion thresholds. Furthermore, EPS and chlorophyll-a were not good indicators for the erosion thresholds, suggesting an ambiguous influence of biology. Generally, the strength of the relations decreased for sediment layers deeper than 10 cm. Overall, our results underline the need to investigate the influence of sediment characteristics on fine sediment erodibility from varying natural environments.

In this research, laboratory experiments and numerical models were used to develop a new formula for the hydraulic scour rate and a bedrock module for estimating soft bedrock erosion rate, and they were then applied in a study of the Zhuoshui River in Central Taiwan. From laboratory experiments, the relationship between the critical shear stress and hydraulic scour rate with the dominant bedrock property (uniaxial compressive strength) was established. For the soft bedrock module, only one parameter (i.e., the rock erodibility coefficient $k_v$) was calibrated once the bedrock properties were known. An unsteady two-dimensional depth-averaged numerical model incorporating the new formula for hydraulic scour rate and the bedrock module was developed. After calibration and validation, the developed model well reproduced soft bedrock erosion during high-flow periods in this study reach. The simulation results showed that hydraulic scour is dominant in the upstream reach, while abrasive scour accounted for 65%–80% of the total erosion in the downstream reach. In addition, more bedrock erosion was observed at the locations with greater contributions from abrasive scour.

Experiments were conducted in open channel flows under near-critical mobility conditions to quantify local evolution of the sediment bed induced by emergent and submerged rectangular vertical porous walls in yawed conditions. The goal was to design minimally invasive hydraulic structures, or vanes, redirecting sediment mass flux to be coupled with intake pipes in dam bypass systems for sustainable sediment redistribution. Theoretical scaling for the maximum scour depth derived from the phenomenological theory of turbulence was extended to our specific emergent vane geometry under varying structure porosity, angle, and size. Most of our attention was devoted to quantifying the asymmetry of the induced bathymetric effects and developing an appropriate metric to identify the most effective vane configuration to steer sedimentation along a desired direction. The orientation angle was shown to significantly affect the geometry and volume of the scour and deposit, the spanwise shift of the deposit volume, and the velocity profiles in the wake, for both emergent and submerged vane configurations. The optimal streamwise and spanwise spacings in yawed, submerged, porous vane arrays were determined based on the spatial evolution of the wake flow, ensuring minimal sheltering effects and maximal lateral directionality of the sediment deposit. We here provide supporting evidence that wind and marine hydrokinetic turbine wake models can be extended to the case of porous planar structures to capture the key physical mechanisms governing sediment deposits.

The present paper discusses an experimental study on the critical submergence for lateral dual horizontal intakes that are placed in a row and perpendicular to a uniform approach flow. The effect of clear spacing between the dual square intakes extracting the same flow discharge on the critical submergence is critically studied and found that the dual intakes show mutual influence on the flow withdrawal and critical submergence, which decreases with an increase in the clear spacing. The intakes behaved as isolated intakes in the same flow condition with separate vortices causing multiple vortices at a limiting clear spacing of twice the intake size. The downstream intake of the dual intakes is first affected by the air entrainment from surface vortex formation when both intakes operated under the same discharge condition for spacing less than twice the intake size. The effects of other significant parameters like sill height and intake size under various approach flow and intake flow conditions on the vortex formations at critical submergence of dual intakes are also studied. The intake flow Reynolds number and Weber number significantly affect the critical submergence of the lateral dual intakes, indicating the relevance of viscosity and surface tension effects on surface vortex formations. An empirical relationship for the prediction of critical submergence for dual side intakes has been developed and found to be in good agreement with the observed data with an error of less than $\pm 15\%$. The developed equation is also applicable for the computation of critical submergence for single-side horizontal square intakes under uniform approach flow. The outcomes of the study shall be helpful for the field engineers in fixing the invert level of the single and dual intake for avoiding the air-entraining vortex.

In this paper, we propose a new predictor for the time-dependent, spatially-maximum scour depth at a circular pier in clear-water flow conditions. In spite of a number of approaches used in scour research, a simple predictor based on data correlation still has merit because more insightful studies of the process dynamics lack engineering impact. Furthermore, the simple condition of a circular pier is a reference for several variabilities to be considered afterwards. The present formula is obtained using laboratory data from 30 sources, corresponding to 328 experiments over 66 years. The predictor accounts for 5 dimensionless parameters (pier slenderness, flow intensity. sediment coarseness, sediment uniformity. and time) and has a good predictive ability, largely outperforming that of 27 literature equations. The proposal of a new formula is accompanied by several considerations based on additional parameters and operational conditions, as well as by an analysis of the uncertainty of the computed scour values and recommendations for conservative predictions of the scour depth in engineering practice.

In the derivation of his bedload formula, Bagnold approximated the critical near-bed flow velocity for incipient sediment motion as the particle settling velocity. However, this approximation has not yet been validated since Bagnold’s bedload formula was proposed in 1973. By comparing the drag coefficient defined based on the critical near-bed flow velocity and that defined based on the particle settling velocity, this note shows that the difference between the two velocities is negligible, which substantiates Bagnold’s approximation.

The effect of turbulence intensity on riprap stability was studied experimentally. Riprap stones were placed on a flat streambed and turbulence intensity was changed with a turbulence generator. An acoustic Doppler velocimeter (ADV) was used to measure turbulence intensity. Results showed that in a constant flow condition, by increasing the near-bed turbulence intensity from 10% to 25%, the stable riprap diameter increased about 2.5 times. Based on the results of the present work and using all existing data, an empirical equation was developed for calculating stable riprap stone size considering the time averaged near-bed velocity and near-bed turbulence intensity.

Grass-lined spillways are flow conveyance structures with environmental benefits. The stability of such spillways has been typically assessed considering soil erosion, but the design of grass-lined spillways based on hydraulic considerations has rarely been conducted. While subcritical flows in channels with grass have been studied extensively, studies of velocities and flow resistance in supercritical flows in spillways are limited. Herein, this experimental study investigated the application of a multilayer velocity model for supercritical self-aerated flows on a spillway with submerged artificial grass. Velocities were measured with a pitot tube and dual-tip air–water flow conductivity probe, providing the most systematic assessment of velocities and flow resistance in supercritical vegetated flows to date. The velocity distributions were well described with a multilayer velocity model previously developed for subcritical flow conditions, and a constant interfacial velocity supplemented the observed supercritical aerated free-surface layer. Based on this velocity model, explicit expressions for the mean flow velocity and the friction factor were developed for grass-lined spillways, which were also applicable for subcritical flow conditions with comparable vegetation cover. The flow resistance model provides a theoretically developed design option for grass-lined spillways that is solely based on vegetation properties and hydraulic boundary conditions.