Journal of Waterway, Port, Coastal, and Ocean Engineering

This paper presents the results of a combined laboratory and numerical investigation into the role of infragravity motions in the wave runup process over fringing coral reefs. Laboratory experiments were performed with a reef profile typical of fringing reef systems along the southeast coast of Guam. Spectral analysis of the measured time histories of surface elevation over the reef face and flats show significant changes to the wave energy spectrum shoreward of the break point. Most of the wave energy in the incident wave frequency band is dissipated within a few wavelengths of the reef face with the wave motions over the reef flat and shoreline dominated by oscillations at infragravity periods [O(100s) prototype]. The infragravity wave energy is minimum at the reef crest and increases as waves propagate shoreward over the reef flat and also with increasing water level on the reef. The dominant infragravity mode is the first reef oscillation mode with a wavelength approximately equal to four times the width of the reef flat. This component is resonantly amplified at the shoreline relative to the incident infragravity energy at the reef crest. A numerical model based on the Boussinesq equations is applied to the laboratory data and is able to describe complex changes to the wave spectrum over the reef flat due to nonlinear wave-wave interactions and wave breaking as well as runup at the shoreline.

Cohesive suspended particles in a river aggregate into microflocs with average diameters of $10–20\mu \text{m}$ , which on their arrival in the estuary aggregate to macroflocs with average diameters of $100–200\mu \text{m}$ . We performed a laboratory study on samples from the Seine estuary. The experiments show a more or less systematic behavior of floc size with drivers. Macrofloc size relates to turbulence, $\eta$ , suspended particulate matter concentration ( $C$ ), salinity ( $S$ ), time ( $t$ ), and mineral and biological composition of primary particles $M$ . Generally, floc size increases with $C$ , $S$ , $t$ , and $\eta$ . We performed Eulerian field surveys of floc size and the same drivers on various sites in the Gironde estuary. Our field samples show an ambiguous behavior of floc size against salinity and suspended sediment concentration. Floc size does not seem to simply increase with $C$ , $S$ , $t$ , and $\eta$ , but flocs grow and decay with time.

An adjustable shear stress straight flume commonly used to measure cohesive sediment erosion rates has been modified to include downstream bed load traps. The new flume can be used not only to measure erosion rates, but also to analyze and quantify the modes of transport for this complex problem. The new device was used to study transport modes of quartz particles ranging in size from 19 to $1,250\mu \text{m}$ . As expected, the traps captured the coarse material (bed load) and the fine material bypassed the traps (suspended load). Transport properties of natural sediments from three locations were also studied. Fine sediments with little or no sand eroded as aggregates which maintained their integrity in the flume channel while moving as bed load into the traps. Natural sediments that included high percentage of sand also eroded as aggregates. However, these aggregates quickly disaggregated. Sand moved as bed load and fell into the traps while fine particles moved predominately in suspension.

Observations of the cross-shore variations of pressure gradients measured on a fixed barred beach in a large-scale laboratory wave flume are presented in this paper. The wave-induced components (0.1–1.0 Hz) of the free surface elevation, near-bottom vertical and cross-shore velocities, and pressure gradient are used in the analysis. The cross-shore variation of the pressure gradient showed the maximum value in the area of wave breaking over the bar, and the pressure gradients were considered to be influenced by the fluctuation of the water surface elevation, which are shown to be a rough proxy for the water surface slope. The cross correlation related to the vertical velocity showed no correlation in the wave breaking area. Surprisingly, although several measuring points were located in the wave breaking area or in the inner surf zone, the spatial variation of the pressure gradient and water surface elevation had a correlation coefficient greater than 0.8 with time lags approximately 0.1 of the wave period from offshore to onshore in both regular and irregular wave cases. This indicates that the near-bottom pressure gradient can be estimated using the water surface elevation over a complex barred bathymetry and through the inner surf zone.

To ensure the safety of a variety of marine operations, ocean weather monitoring and prediction in many coastal regions have gained prominence over the last couple of decades. To support ongoing forecasting efforts in Prince William Sound, the site of the worst oil spill disaster in U.S. history, a three-way coupled wave forecasting system is developed. The issue of wave forecast reliability is addressed in the context of the associated uncertainty that confronts the manager of engineering operations or other planners. This issue has generally not been systematically addressed in literature. High-resolution 36-h daily forecasts of significant wave heights (SWHs) are compared with measurements from buoys and satellites for about 1 year. The results show that 70% of the peak SWHs (in the range 5–8 m) were predicted with an accuracy of 15% or less for a forecast lead time of 9 h. The errors are larger for smaller SWHs. Correlation coefficients $\left(R^2\right)$ and indices of agreement $\left(D\right)$ between predicted and measured SWHs were reasonable and generally more than 0.78 for all lead times. On average, our analysis showed 70% or greater likelihood of the prediction falling within a tolerance of $\pm \left(1\times \text{RMSE}\right)$ for all lead times. Barring changes in model physics, resolution, etc., similar acceptance levels could be expected from comparable systems.

Analysis of long-term coastal tide records at Key West, Pensacola, and Mayport, Fla. examines the variability of extreme high-water events. Linear trends of event water level at Key West and Pensacola support findings elsewhere that increases in extreme water level are consistent with increases in global mean sea level. A link between event variables and the Atlantic multidecadal oscillation (AMO) is found at the Key West and Pensacola stations. Mayport does not exhibit such a link, likely the data are confounded by estuarine flows and oceanographic interactions with the St. Johns River. Quantile regressions and extreme value distributions suggest dynamic trends in the event variables at Key West and Pensacola as a function of AMO, indicating an increased variability in relation to linear models that should be of interest to coastal planners and forecasters.