Journal of Waterway, Port, Coastal, and Ocean Engineering

A suspension bridge over a river is generally supported on a structure mounted on large concrete piers placed on the river bottom near the two banks. Because of its massive size the piers are built by pouring concrete at site in a floating position. During this construction period, the floating caissons are moored in place to withstand any environmental currents and waves, which may be prevalent at the site. Since the draft of the caisson changes during construction, the mooring system must be capable of holding the caisson at all these drafts. Once the construction sequence reaches the required height, the caisson is ballasted down to rest on the river bottom and subsequently during further ballasting penetrates the soil to its desired depth. The design and construction of the piers in place and their mooring system provides several technical challenges. This paper describes these challenges experienced in the design of a caisson system for a new bridge at Tacoma Narrows, close to Seattle. This design process included towing the caissons at a shallow draft from the dock to the site, design and installation of the mooring system, construction of the caissons in place, and final touchdown and penetration to soil. The current at the Narrows was extremely high, which gave particular challenges in the construction sequence. Some of the critical areas of the associated design and their solution techniques are highlighted. It should be noted that the dimensions of the bridge caissons and their mooring system are of similar order of magnitude as typical offshore structures exposed to severe environment.

Short-crested waves are three-dimensional waves that may be generated through a reflection of a two-dimensional Stokes wave on a seawall. Thus, they are more likely to be observed near ports or any consequent marine structure. Two numerical methods are used to compute three-dimensional surface gravity short-crested waves on deepwater. The first method is an asymptotic procedure and the second computes a direct numerical solution. One of the main properties is the four-wave resonance. Such resonance introduces nonuniqueness with several solution branches connected through a turning point. We show that both computational methods are reliable for nonresonant waves, but that the direct numerical method converges faster. For resonant waves, the direct method is more appropriate because all solution branches can be obtained. The asymptotic method computes only one branch of solutions for any given parameter values, and is uncertain around and past any turning point. Stability analysis of the branches shows that, although sporadic, an instability associated with harmonic resonance is more likely to appear for one branch in the vicinity of the turning point. Consequently this could amplify the unstable resonant mode. The nonuniqueness of the solution requires careful attention in every study on the impact of surface waves on marine structures. It is shown here that the wave force exerted on a seawall may change drastically from one branch of solution to another.

An approximate analytical assessment of long wave development due to nonlinear shoaling is obtained and verified against wave-flume experiments. The phase averaged equation for the nonlinear evolution of shoaling waves by Agnon and Sheremet serves as a starting point. Their shoaling interaction function $J$ is significantly simplified through the mild slope assumption. Results, which may be used to assess the long wave input for engineering applications such as harbor resonance studies, are given.

The complex dynamics of the Venice tidal barrier subjected to broadbanded wave spectra are investigated by applying empirical orthogonal function (EOF) analysis to experimental data sets. Space and frequency patterns of the natural modes of the barrier are reconstructed and satisfactorily compared with the recent theory of Li and Mei. The results indicate that when broadbanded wave spectra are considered, natural modes with the longest spatial wavelengths seem more likely to be excited than those with shorter wavelengths.

Offshore-detached structures are used to limit shoreline erosion, and to provide a safe haven for marine craft or bathers. Their function is to reduce the amount of wave energy in their lee, and to enhance sediment deposition at the shoreline. Planform shoreline changes in the vicinity of an offshore structure represent the integrated effect of natural shoreline variability, and engineering factors including sand nourishment, the formation of a shoreline salient, and a secondary “groyne effect” induced by a gradient in the alongshore transport. To quantitatively assess a structure’s performance, it is necessary to first distinguish the structure-induced effects from the pre-existing shoreline variability, and second to discriminate and quantify these differing modes of shoreline response. The easily applied technique “odd–even function analysis” can be used to discriminate the principal modes of shoreline behavior in the vicinity of offshore-detached structures, and to distinguish this induced response from naturally occurring shoreline variability. The successful application of the technique is illustrated to identify early trends in coastline adjustment in the lee of a new submerged reef structure at the Gold Coast, Australia.

We investigate the drift of small bodies (bodies which do not significantly disturb the fluid) in waves. We begin with the slope sliding models of Rumer and Marchenko, and we establish that the models are different and that the correct model is due to Marchenko. We then investigate the slope sliding model when we have linear wave forcing of constant frequency (for which the differences between the two models vanish). We establish that the drift velocity, for this case, is independent of initial position, which contradicts published results in which drift velocity was shown to depend on initial position. The reason for the error in previous results was the slow decay of the transient motion. For this reason we present a numerical method to calculate the drift velocity, which avoids the problems associated with the slow decay of the transient motion. Some results for the drift velocity are given.

A video technique has been developed to study the motion of individual sediment particles under waves. The technique was developed for moveable bed conditions under waves in the Davidson Laboratory’s large wave tank. Experiments were conducted with grain sizes of 1, 2, and $3\mathrm{mm}$ . All runs were applied to a modified Shields diagram and accurately predicted the region of motion and no motion. This proves that shear stress is an accurate predictor of grain movement initiation for a flat bed. The individual particle velocities were also measured. An ensemble average of particle velocity with respect to wave phase compared well with Sleath’s expression for instantaneous bed load transport.

In this paper, an accurate Chebyshev finite-spectral method for one-dimensional (1D) Boussinesq-type equations is proposed. The method combines the advantages of both the finite-difference and spectral methods. The spatial derivatives in the governing equations can be calculated accurately in an efficient way, while some flexibility is allowed for treating irregular grids. The efficiency and accuracy of the proposed method are verified by successfully solving the problem of solitary wave propagation over a flat bottom where analytical solutions are available for comparison. A simple formula to calculate the wave celerity of the solitary wave propagation has also been derived. Finally, the applicability of the numerical method to periodic and random waves was validated by the simulation of nonlinear wave propagation over a bar where laboratory data are available for comparison. The method can be easily extended to treat 2D problems.