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.

Shallow bays along the fringe of an estuarine channel impact salinity distribution. The primary mechanism of impact has been termed “tidal trapping,” and is a result of the phase difference between the filling and emptying of the bay and the flow in the channel. This mechanism has been proposed as a major contributor to salinity intrusion in some locations. This paper creates a conceptual model to explain the increase or decrease in salinity intrusion based upon the channel tidal waveform and the character of the adjacent bay. Two idealized scenarios are used for this explanation: (1) Standing wave in the channel with a small phase lag in the bay tide; and (2) Progressive wave in the channel again with a small phase lag between the channel tide and the bay. This conceptual approach is useful for inspecting and understanding salinity intrusion processes in a complex estuary bay setting since the form of the wave can often be determined by generally available water surface gages along the estuary. A constructed numerical model of an estuary supports the conceptual model.

Piping systems are commonly designed to withstand the first transient pressure peak, which is unaffected by dissipation. However, for multiple operations of control equipment, for example, pump start-up following pump shutdown, and load acceptance following load rejection on hydraulic turbines, an accurate prediction of the dissipation of pressure oscillations is needed to select a suitable time for the second operation. For this purpose, following a simple procedure used for computing the dissipation of vibrations of bridges and other structures with time, a method is presented to compute the dissipation of pressure oscillations in piping systems. Similar to structural engineering, this method is simple to apply, does not require simulation of the entire system, is not computationally intensive, and gives reasonable results for practical applications for a complex phenomenon whose mechanics is not well understood at present. An empirical equation for the damping ratio is developed using dimensional analysis and by nonlinear regression. Comparisons of the computed and experimental results for 17 tests conducted in laboratories all over the globe show good agreement. It is found that the damping ratio increases with increases in the Reynolds number or Mach number and decreases with the diameter-to-length ratio of the pipeline. Uncertainty, quantified using a Bayesian inference approach, shows that the model predicts the value of the damping ratio successfully.

An apron downstream of an unsubmerged weir can cause the main jet to flip up and down cyclically in a local scour hole, significantly influencing the scouring process. This behavior was experimentally investigated using particle image velocimetry (PIV). The flow inside the scour hole is characterized by the presence of coherent structures (i.e., four different types of roller), and the flipping behavior is linked with the cyclical appearance and disappearance of a bed roller beneath the main jet. With the cyclical jet-flipping, the main jet has a pivotal region with significant average kinetic energy (AKE) immediately downstream of the apron, and the turbulence kinetic energy (TKE) distributions attain their peak values around this region. The jet development also exhibited self-similarity along the streamline passing through the peak AKE when scaled by the local length and velocity scales. Compared with the no-apron case, the apron reduced the maximum scour depth by dissipating the jet energy and decreasing the impingement angles of the jet to the bed profile and the water surface.

Detailed spatiotemporal analyses of near-field outfall dynamics are reported for an inclined brine multiport diffuser discharging into a dynamic open-coastal embayment. Three-dimensional variations in near-field discharge dynamics were captured using near-continuous in-situ monitoring of physicochemical properties in parallel with measurements of dissolved oxygen and ambient hydrodynamic conditions. Temporal analyses were conducted using principal component analysis and concentration–duration–frequency methods to show near-field salinity variations are generally localized to within 30 m of the diffuser and are highly sensitive to ambient crossflow dynamics. Periods of low near-bed velocities and high-velocity shear corresponded to the lowest return and boundary brine dilutions with instantaneous near-bed salinity increases of up to $1.5\mathrm{g}/\mathrm{kg}$ and dissolved oxygen (DO) reductions up to $1.9\mathrm{mg}/\mathrm{L}$ measured immediately downstream of the diffuser. Both trajectory and dilution demonstrated strong correlation with near-bed crossflow magnitude and were assessed against laboratory-based empirical models. Measured dilutions were well approximated by models that accommodate crossflow; however, trajectory properties were generally overpredicted. The quantitative characterization of temporal plume dynamics within an unsteady coastal setting provides valuable insights into the applicability of existing modeling approaches and regulatory assessment. Improvements to monitoring strategies are also proposed.