Journal of Waterway, Port, Coastal, and Ocean Engineering

This paper summarizes the results of an experimental study on scour around spherical bodies and self-burial in steady current and in waves. The equilibrium scour depth below a fixed sphere in steady current for live-bed conditions was found to be $S∕D=O\left(0.3\right)$ , $D$ being the sphere diameter. The effect of the Shields parameter, $\theta$ , and that of the position of the sphere with respect to the bed, $e$ , were investigated. It was found that, under live-bed conditions, the effect of $\theta$ is quite weak. It was also found that the effect of $e$ is significant. No scour occurs when $e∕D>0.8$ . It was demonstrated that, in the case of waves, the Keulegan-Carpenter number, $\mathsf{K}\mathsf{C}$ , is the main parameter that governs the equilibrium scour depth. $S∕D$ is always smaller in waves than in steady currents. Regarding the self-burial, the equilibrium self-burial depth in steady current was found to be $e∕D=O\left(0.5\right)$ . The self-burial depth in waves is governed by $\mathsf{K}\mathsf{C}$ , similar to the scour depth for a fixed sphere. $e∕D$ is always smaller in waves than in steady currents. The experiments indicated that the time scale of both the scour process and the self-burial process is a function of $\theta$ in the case of steady current, and $\theta$ and $\mathsf{K}\mathsf{C}$ in the case of waves.

The hydrodynamics around a static horizontal cylinder and five static scoured bed profiles subjected to steady flow were simulated using a CFD model, FLOW-3D. Both a two-equation $k\text{-}ϵ$ model and a Smagorinsky LES turbulence closure scheme were evaluated. The mean horizontal and vertical velocity estimates of both turbulence models show reasonable agreement with the data. Wake characteristics such as the reattachment point and vortex shedding were examined. The reattachment point for the $k\text{-}ϵ$ closure scheme was consistent with the observations. However, the vortex shedding induced by the LES closure scheme resulted in a considerably larger and unsteady reattachment point. In all profiles, the size and strength of the eddies formed by the LES closure scheme were found to be grid dependent. The $k\text{-}ϵ$ model was found to be grid dependent for the vortex shedding process.

A two-dimensional dynamical model is established to simulate current circulation and sediment transport induced by storm surges in a schematic harbor. The model accounts for both settling and resuspension processes of cohesive fine sediments in a salty water environment. The numerical results demonstrate that the current follows wind albeit with a time lag, and so does sediment concentration. The time lag between current and wind is of the order of interval needed for wind to transfer momentum to the whole water body. Whereas, the retarded dynamic balance of sediment entrainment and settling fluxes is responsible for the time lag between sediment concentration and current. When a tropical cyclone, or conventionally so-called typhoon in the Pacific area, is passing through the harbor, the higher sediment concentration inside the harbor is found to the right of the path rather than to the left of it. Bed deformation is found to be substantially influenced by typhoon tracks relative to the harbor. For a harbor facing east, a northward typhoon results in erosion and the southward or westward one causes deposition. Both erosion and deposition occur along the coast with little bedform variations near the harbor center.

In a recent paper, a verification study was presented of the time-domain mooring simulation program TERMSIM. In that paper, the verification was restricted to the behavior of loaded ships. Extension to the verification of the model for different loading conditions is made in this technical note. For the downtime analysis of a new liquefied natural gas (LNG) terminal, the influence of the loading condition is often neglected and the analysis is only carried out for the loaded ship. This assumption seems valid, because the draft difference between laden and ballast is small for LNG ships. However, prototype measurements of line forces showed quite large increase of mooring forces during the loading process of the ship. This strong dependence also follows from computer simulations presented in this technical note. Large differences in moored ship behavior in swell waves between the loaded and ballasted condition can occur because of a different roll period and ratio between the wave frequency and stiffness of the mooring system.

Predicting the retreat of a coastal soft cliff is a difficult and uncertain operation, which has both theoretical and practical significance. Recession of soft cliffs occurs through a combination of processes, including slope failures and surface erosion, that are difficult to model jointly. Deterministic models for predicting coastal retreat are hampered by the complex nature of coastal erosion, which is highly nonuniform in space and episodic in time. To overcome these limitations, stochastic approaches have been proposed. These models assume distributions for the size and the time of the recession events. In this paper we investigate the frequency-size statistics of soft-cliff erosion based on two historical datasets of coastal retreat measurements at two sites in England. We find that the two datasets exhibit a similar behavior. The frequency of the recessions decreases with increasing size of the retreat. For small retreats the decrease is slow. For medium to large retreats the decrease is rapid and follows a power law. The frequency-size statistics of soft-cliff erosion is similar to the statistics of medium to large landslide areas, which are also power-law distributed. This is a significant but not conclusive result. More data are needed to confirm this finding.