Journal of Waterway, Port, Coastal, and Ocean Engineering

This paper is concerned with the permanent displacement of composite breakwaters under wave impact. It demonstrates that permanent displacement is a more rational criterion to design composite breakwater as well as to assess its performance when compared to merely a factor of safety. The horizontal wave force and uplift force acting on the caisson are determined from the Goda formulae and expressed as a fraction of the effective weight of caisson using the wave coefficients. A yield wave coefficient is defined for the horizontal wave force when the limiting condition against direct sliding is reached. For a storm that renders a wave coefficient larger than this yield value, a rigid-body motion is induced in the caisson. The equation of motion is double-integrated to obtain the magnitude of permanent displacement. The proposed procedure is illustrated with design example and compared with 35 case histories among which sliding was observed for 24 of them.

Submarine pipelines, one of the most important subjects in coastal engineering, are widely used in coastal structures. In order to ensure these pipelines are stable and functional during their project life, attention must be paid to their design and coastal processes. In the design phase, the possibility of local scouring under submarine pipelines must be taken into consideration. If local scour occurs under the submarine pipelines, the pipelines may either vibrate due to the hydrodynamic forces or induce additional static or dynamic loads due to self-burial. Moreover, they might be destroyed partially or fully and thus be unable to perform their functions. This paper presents experimental investigations of scour depth with respect to water depth using regular waves. The investigations encompass shoaling region conditions. Results of the measurements of velocities and beach profiles are enlightening, both for understanding of the mutual interaction between the disturbed flow field around the pipe and the seabed morphology as well as in making predictions of scour depth around submarine pipelines. Based on the present experimental data, a new equation is proposed, relating the maximum equilibrium scour depth to the modified Ursell number for the live bed condition for a rigid pipe fixed initially in contact with the bed.

The entrapped wedge is a specific type of stratified flow problem that is found in estuaries when a barrage is used to control water levels. To avoid problems with water quality, it is essential that any wedge be removed. The sensible mechanism by which this should occur is via the momentum of the freshwater flow. To investigate this, a laboratory investigation of the erosion of an entrapped saline wedge caused by a buoyant overflow has been undertaken. The shear flow has been found to induce mixing at the interface between the two fluids, and a mixed layer results. The characteristics of the layer have been found to change with time with two different mechanisms being identified. The behavior of the layer depends on the test conditions, in that certain combinations of velocity, depth, and density difference yield different mechanisms of erosion. The temporal behavior of the interfacial layer with various freshwater velocities and depths and density differences are presented.

Results of laboratory model tests are used to assess the accuracy of the American Petroleum Institute (API) method of predicting wave-current loading on a shallow-water production caisson in extreme random waves. Model tests were conducted using a 1-to-20 scale model of a 36-in. (×2.54-cm) diameter caisson in a 50-ft (×3.048-m) water depth. Tests were conducted with and without in-line currents in extreme random waves, many of which were breaking. The experimental results for wave-current kinematics, wave forces, and overturning moments are compared to predictions based on the API procedure using Stream Function wave theory. Based on analysis of 132 extreme random waves from the test program, it is found that the in-line fluid velocities measured just below the wave trough level were 6% larger than measured values on average. Predicted in-line wave forces and moments were, on average, 10% and 15% larger than measured values, respectively. Measured wave loads included dynamic amplification of about 10% due to ringing of the structure following the impact of extreme breaking waves. The standard deviation between measured and predicted values were quite large, however, and measured wave loads for a few waves that plunged directly on the caisson model were between 1.5 and 2.2 times larger than the predicted loads.

The changes in the wave spectrum and significant wave height near a reflective structure are investigated. Significant wave-height measurements with a one wave gauge show a standing wave pattern near the structure, extending offshore to about two times the spectral-peak wavelength for a JONSWAP incidental wave spectrum. The measured spectra also reveal a pattern of nodal and antinodal frequencies, even to much larger distances from the structure. These findings are in agreement with results from linear wave theory. Multigauge techniques can be used to discriminate between incident and reflected waves. Despite the standing wave pattern these techniques can be used to determine the incident wave height accurately up close to the structure—the distance being limited by the extend of the evanescent wave modes attached to the structure.

Two different methods for estimating frequency-dependent reflection coefficients for waves incident on a coastline using collocated measurements of elevation (or pressure) and horizontal current are investigated by simulating time series with known true reflection coefficients and added uncorrelated noise. The methods are applicable to measurements made in the nearshore zone where waves propagate essentially shore-normally. A time domain method is shown to introduce a significant positive bias in the estimated reflection coefficient. A contour plot is calculated giving the bias as a function of the estimated reflection coefficient and the coherence between the estimated incoming and outgoing wave time series, which can be used to provide corrections for the bias. A new principal component analysis (PCA) technique is found to be essentially bias-free. For this method, 95% confidence levels on zero reflection coefficient are computed for a range of numbers of degrees of freedom. Spatial separation between the sensors equivalent to a time delay, at the wave phase velocity, of τ_x produces a spurious peak with a reflection coefficient of one at frequency 1/(4τ_x). The peak is very sensitive to small errors in estimating the time delay, which result in apparent reflection coefficients even higher than one. The conclusion is that the time delay must be made as small as possible in order to push the spurious peak to a high frequency. Application of these results is demonstrated using field data taken on a natural beach.

In 1989 Vincent and Briggs found that irregular waves with directional spread typical of natural wave systems were not as strongly focused behind a shoal as would be predicted from use of monochromatic waves. These conclusions were based on an analysis of significant wave height alone. Reexamination of the data in terms of H_max and H_1/100 indicated that the largest waves behind the shoal were close to those predicted from the Rayleigh distribution based on the observed significant height.