Journal of Hydraulic Engineering

The processes of ripple evolution are studied through numerical simulation using a coupled computational fluid dynamics (CFD)–discrete particle method (DPM) model with focus on discussing the effect of the size of the computational domain on ripple evolution. Ripple-induced form resistance and bed load transport rate are also discussed. Fluid movement is simulated using the CFD computation with the introduction of large eddy simulation for turbulent closure. The movement of sediment particles is simulated using the DPM. It is found from the results of simulation that for a two-dimensional case the ripple evolution involves three stages from wavelet, merging of wavelets to equilibrium. The ripple sizes increase during the merging process and reach a stable state at the end of the merging process. The ripple sizes obtained in the final equilibrium stage are closely related to the streamwise size of the computational domain and have an upper bound for given sediment and flow conditions. If the streamwise size of the computational domain is set to approximately 6 times the ripple length or beyond, the discrepancies among the equilibrium ripple lengths obtained from using different streamwise size of computational domain can be below 9.2%. The ripple lengths modeled in the wavelet stage agree well with the experimental results. During the process of ripple merging, an abrupt reduction in the form resistance and an increase in the bed load transport rate are observed.

Gravity currents descending a slope in a linearly stratified environment can be frequently encountered in nature. However, few studies have quantitatively investigated the evolution process of lock-exchange gravity currents in such environments. A new set of analytical formulae is proposed by integrating both mass conservation and linear momentum equations to determine the front velocity and the front location of a downslope current. Based on the thermal theory, the formula considers the influence of ambient stratification by introducing a newly defined stratification coefficient in the acceleration stage. As for the deceleration stage, the formula is derived by adding a parameter that takes into account the density distribution of the ambient water. The transition point between the acceleration and deceleration stages and the maximum front velocity are also determined by the proposed formulae. Lock-exchange gravity current experiments are conducted in the flume with linear stratifications to provide data for the validation of the formulae. The comparisons between the calculated and measured data in terms of front location and front velocity show satisfactory agreements, which reveal that front velocity presents a rapid acceleration stage and then a deceleration stage in a linearly stratified ambient.

A field experiment was conducted at an inclined multiport brine diffuser in an open-coastal embayment for plant operating capacities ranging 33–100%. A three-dimensional array of conductivity and temperature sensors captured high-resolution brine plume dynamics. Ambient velocity and wave characteristics were also measured. Measured localized outfall salinity concentrations did not exceed background levels by more than $1.09\mathrm{g}/\mathrm{kg}$, whereas deviations from time-averaged ambient salinity ranged from $-4.30$ to $2.83\mathrm{g}/\mathrm{kg}$ over the 14-day sampling period. Plume trajectory and dilution were compared with commonly applied laboratory-based empirical methods. Although there was general agreement in time-averaged distributions for the 100% plant operation case, the effects of ambient hydrodynamic processes on plume dynamics were dominant. The results have implications for the application of empirical models in the design, regulation, and management of these outfalls.

Vortex drop shafts serve to overcome important elevation differences in drainage systems. If well designed, they are performant in terms of energy dissipation and safety. However, the standard design requires well-defined approach flow conditions to ensure the reliability of the structure. In practice, these conditions are frequently ignored due to space restrictions and the fact that several inflow branches may arrive at various elevations with supercritical and/or subcritical flows. The literature provides preliminary concepts for such situations that were not adaptable to the case discussed herein. Nevertheless, in order to apply a standard vortex drop shaft, a novel concept was developed that comprises a junction chamber to merge the inflow branches followed by a very short and steep inlet channel. Extended physical model tests have proven its feasibility. The hydraulic aspects discussed herein allow, at least partially, the adoption of the presented concept in similar situations.