Journal of Waterway, Port, Coastal, and Ocean Engineering

The quantity of resuspended solids (RSS) created during dredging operations is both a production and environmental issue. Numerical models predict the movement of sediment plumes from dredging operations; however, these far-field models need a “source term” as input. The purpose of this work was to generate a “source term” by developing a near-field resuspension model for cutter suction dredging operations. The near-field cutter resuspension model (NFCRM) used a discretized domain to obtain a more accurate velocity structure and incorporated real laboratory data to model the fluid dynamics of the cutter suction dredging environment. A two-dimensional finite-difference method produced temporal evolution of resuspension from the dredging operation. The output of the near-field model clarified dynamics of the cutter-head environment and provided a source term for a far-field model. To develop and verify the near-field model, turbidity and velocity were measured around a 34.3 cm (13.5 in.) diameter cutter head. Turbidity measurements were equated to RSS concentrations. The NFCRM incorporated laboratory data to determine the velocity field and the turbulent diffusion. Settling velocity of sediment was the primary scaling variable. Cutter speed provided a direct relationship to scale the advective field. The Péclet number scaled the diffusive field. Both point-specific and overall source-term resuspension values predicted from the NFCRM were validated with laboratory testing and field data. Conclusions from this research demonstrate that undercutting produced a larger point-specific RSS concentration than overcutting in laboratory testing. RSS concentration was positively correlated with cutter speed and thickness of cut, and inversely correlated with suction flow rate. The NFCRM produced higher point-specific regions of RSS for undercutting but larger mean values of RSS for overcutting. The NFCRM was comparable to all laboratory testing and field data with suitable mean absolute error (MAE) values. Greater accuracy was achieved for overcutting simulations. The NFCRM followed laboratory trends with cutter speed and suction flow rate but did not show a strong correlation with cut thickness. The NFCRM was numerically sensitive to swing speed because of instabilities in the advective field. Future research should investigate different cutter-head designs and three-dimensional modeling.

Results from a numerical modeling study are presented to investigate the tidal elevations, tidal current velocity, bed deformation, and suspended sediment concentration in the Qiantang Estuary, China. The Qiantang Estuary is well-known for its macrotide, which generates a hydrodynamically complex environment. This presents challenges for numerical modelers to accurately simulate the flow field and sediment transport in the region. This paper presents a mathematical model using finite volume method with unstructured mesh to simulate the tide-induced water elevation, current velocity, bed deformation, and suspended sediment transport in the Qiantang Estuary. The parameters in the model were determined using the long-term observed field data of the Qiantang Estuary. The simulated tidal elevations and current velocities agree well with the field observations. The numerical prediction of the bed deformation in 5 months is reasonably compared with the field measurements carried out in the same period. However, relatively large deviations exist between the simulated and observed suspended sediment concentrations, which are discussed in this paper.

This paper presents new experimental data and predictive equations for the reduction of the tsunami inundation force provided by finite-length seawalls. The hydraulic model experiments were conducted in a rectangular basin equipped with a large-stroke piston-type wavemaker to produce a transient pulse on the basis of an error function to best simulate the initial phases of tsunami inundation. The bathymetry had a mild slope constant in the cross-shore direction and was followed by a flat section raised above the mean water line. The tsunami force, pressure, and run-up were measured on an instrumented specimen located on the flat section, and a seawall was placed between the specimen and the shoreline. The incident wave conditions, seawall positions, and seawall heights were varied systematically to quantify the reduction of the maximum and average force relative to the baseline conditions without a seawall. Reduction factors ranged from 1.0 (no reduction) to 0.10 (90% reduction). Two empirical formulas were derived to predict the reduction factors for the maximum and average force using as input the incident (unbroken) tsunami height, the bore height at the seawall (in the absence of the wall), the seawall height, and the position of the seawall relative to the shoreline and design structure. The equations predicted the data for which they were calibrated with $R^2$ values of 0.86 and 0.83 for the maximum and average forces, respectively. The equations were used to predict two other laboratory data sets: one conducted in the same facility under the same conditions with the specimen replaced by a column-supported structure, and a second, previous study in a two-dimensional wave flume at larger scale.

This paper presents a method to incorporate the effect of climate change on the performance-based design of a caisson breakwater and its application to the East Breakwater of the Port of Hitachinaka in Honshu, Japan. The up-to-date projection estimated a future sea-level rise of 0.25–0.6 m and a wave-height increase of 1.7 m (from 8.3 to 10 m) during the 21st century in this area. The influence of a sea-level rise on caisson sliding was minimal; the sea-level rise was much smaller than the water depth at the breakwater site. When the effects of climate change were not included, the performance-based design method calculated the same caisson width as that of the constructed breakwater, partly validating the consistency between conventional deterministic methods and performance-based design methods. The effects of climate change dictate an increase in caisson width of approximately 1.5  and 0.5 m for linear and parabolic increases of wave height, respectively, which are approximately 6.8 and 2.3% of the present caisson width of 22 m. Finally, it is recommended that the caisson breakwater be designed using the projected wave height and water level in 30 years from construction, if the deterministic design method is used and the effects of climate change are to be taken into account.

As a new alternative countermeasure to protect the coastal zone and to increase the surfing possibilities in the Leirosa area of Portugal, multifunctional artificial reefs were investigated numerically in this paper. The primary surfing parameters used in the design (i.e., breaker type, peel angle, wave height at breaking, and currents around the artificial reef) were analyzed. The reef functionality was also analyzed for coastal protection. Two reef geometries with different reef angles of 45 and 66° were tested, considering two design wave conditions (storm and common) and two tide levels (medium and low). Simulations show that both reef geometries are adequate for surfing, although the reef angle of 66° is more suitable for standard surfers, and the 45° angle is more adequate for advanced/professional surfers. A morphodynamic study should be carried out to analyze the efficiency of the artificial surf reef for coastal protection.

Two-dimensional tidal flows within the Lake Pontchartrain–Lake Borgne area (Louisiana) are simulated to assess the effects of the surge protection structure on the Gulf Intracoastal Waterway (GIWW) on navigation conditions. The region of interest is modeled with a shallow-water, depth-averaged, finite-element model. The water levels and discharge are analyzed at a location in the GIWW to ascertain model behavior. It is shown that the presence of the structure produces infrequent increases in velocities in the GIWW, which can be mitigated by a proposed structure near Lake Pontchartrain.

In this study, the Bureau of Meteorology’s recently expanded sea-level observing network is assessed for its ability to contribute to tsunami warnings. This is done by using modelled tsunami travel times and examining the network’s ability to observe any potential tsunami in a timely enough manner so that sufficient warning is provided to the Australian coastline. Although the overall coverage of the network is very high, the study does highlight a number of network gaps. The value of individual observation sites and their contribution to the warning characteristics of the network is also assessed. The results provide valuable guidance for network managers and also set a benchmark for evaluating possible future changes and enhancements to the network.

The effects of external currents on the longshore current and sediment transport in the surf zone are examined using the cross-shore numerical model CSHORE, which is extended to include the alongshore pressure gradient term in the longshore momentum equation and to allow oblique waves in the swash zone on a beach. The extended CSHORE is compared with five tests conducted in a wave basin with a recirculation system. The cross-shore variations of the longshore current and sediment-transport rate are predicted fairly well for the cases of no and favorable pressure gradients. The cases of adverse and time-varying pressure gradients are computed to extrapolate the experimental results for wider applications.

A stability formula has been developed for Tetrapods armoring rubble mound breakwaters based on hydraulic model test results of the present and previous studies. The formula is applicable to breakwaters with various slope angles. It is also applicable to low-crested breakwaters and different packing densities if the corresponding terms are incorporated in the formula. The uncertainty of the formula is also described.

A total of 25 small-scale laboratory experiments simulating combined wave overtopping and storm-surge overflow on a trapezoidal levee were conducted at a nominal prototype-to-model scale of $N_L=25$. Time series measurements of irregular and unsteady flow thickness and velocity were acquired at two locations on the landward-side, $1V:3H$ slope. These measurements were used to calculate the time series of instantaneous shear stress representing the average over a 4.8-m-long (prototype scale) levee slope between the two measurement locations. Empirical relationships are presented for estimating the mean shear stresses for steady overflow and for combined wave and surge overtopping. For the latter case, additional formulas are given for estimating representative parameters of the irregular shear stress peaks associated with individual overtopping waves. The collected data were intended primarily for the design of rapidly deployable levee armoring systems; however, the data could also be used to evaluate the erosion of soil or vegetated levees and dikes.