Felix Jose

The Effects of Bed Friction on Wave Simulation: Implementation of an Unstructured Third-Generation Wave Model, SWAN

Abstract Parallel implementation of an unstructured Simulating Waves Nearshore (SWAN) model with the Wave Model (WAM) cycle 4 formulation was used to evaluate the performance of a third-generation wave model over large spatial scales. Data from a network of National Data Buoy Center (NDBC) buoys and the Wave Current Information System (WAVCIS) stations were used to assess the skill of the input and output of the wave model. The simulation results reveal that the underestimation of energy in the low-frequency band (0.12–0.17 Hz) can be ameliorated if the model is calibrated using site specific in situ measurements instead of the Pierson-Moskowitz spectra. This process led to more than a 25% decrease in the root mean square error between simulated significant wave height and in situ observations. Use of the verified model for the Gulf of Mexico, with bed friction computed from grain-size distribution, as opposed to a default constant bed-friction formulation, showed that the wave height difference can exceed 1.5 m or 40% of local wave height for a large spatial extent during extreme events, such as Hurricane Dennis. In addition, with the use of eddy viscosity bed-friction formulation with usSEABED (U.S. Geological Survey), the sediment data results were in better agreement with the in situ observations during Hurricane Dennis, with less than a 4% increase in computational cost. The mean wave-height distribution over several cold fronts also demonstrates the influence of bed grain-size parameterization in wave transformation, especially in water depths shallower than 15 m, thereby demonstrating the significance of this study in advancing our understanding of sediment-transport modeling.

Seasonal Hydrodynamics along the Louisiana Coast: Implications for Hypoxia Spreading

ABSTRACT Allahdadi, M.N.; Jose, F., and Patin, C., 2013. Seasonal hydrodynamics along the Louisiana coast: implications for hypoxia spreading. Summer and fall inner-shelf flow characteristics, including the vertical current structure, obtained from three WAVCIS (WAVE-Current-surge Information System) stations off the Louisiana coast were analyzed to delineate the hydrodynamic conditions that contribute to the formation of seasonal hypoxia at each station location. Two of the WAVCIS data stations used for analysis are located west of the Mississippi bird-foot delta, whereas the third is located east of the delta. Relatively small vertical gradients in the horizontal velocity current (i.e., u and v velocity components) were observed during summer (2009) for both CSI-6 and CSI-9 stations, which are located well inside the hypoxia-prone zone west of the delta. In contrast, the summertime vertical gradient of horizontal current at CSI-16 (located east of the delta) was significantly higher than that of western stations. Significant differences in the vertical gradient of flow velocities along with contrast in water column density gradient during the summer culminated in a strong stratification, which is considered the main physical requirement for the formation of hypoxia. A computed criteria based on the Richardson number also pointed to higher potential for stratification at both stations west of the delta, whereas the water column east of the delta still remained subject to vertical mixing. Furthermore, summertime current fields at CSI-6 and CSI-9 were significantly less compared with CSI-16, suggesting less reoxygenation driven by advection from surrounding waters. These conditions may exacerbate hypoxia.

THE FATE OF SEDIMENT PLUMES DISCHARGED FROM THE MISSISSIPPI AND ATCHAFALAYA RIVERS: AN INTEGRATED OBSERVATION AND MODELING STUDY FOR THE LOUISIANA SHELF, USA

The dispersal behavior of river sediment plumes from the Mississippi and Atchafalaya rivers were investigated using a 3-D hydrodynamic and sediment transport model implemented for the Louisiana inner shelf to demonstrate the application of numerical models, field data and satellite images to study river sediment plumes for the area. The key tasks of calibration and skill assessment of the model were performed using vertical current profile data from ADCP’s deployed at WAVCIS coastal observing stations. Seasonal hydrodynamic features, including inertial oscillations, were also considered for fine tuning of the calibration parameters. Plume dispersion patterns, computed from the sediment transport model, were verified using satellite-derived suspended sediment concentration data. Sediment transport characteristics on the innershelf were simulated for a typical spring season river plume discharge scenario and also when significant sediment resuspension occurred along the shelf, resulting from waves generated during the passage of cold fronts. The results show that the river plumes were significantly influenced by the prevailing spring hydrodynamics along the Louisiana shelf and that the waves also facilitated the resuspension and the transport of fine grained sediments.

Sensitivity of a third generation wave model to wind and boundary condition sources and model physics

Three different packages describing the white capping dissipation process, and the corresponding energy input from wind to wave were used to study the surface wave dynamics in South Atlantic Ocean, close to the Brazilian coast. A host of statistical parameters were computed to evaluate the performance of wave model in terms of simulated bulk wave parameters. Wave measurements from a buoy deployed off Santa Catarina Island, Southern Brazil and data along the tracks of Synthetic Aperture Radars were compared with simulated bulk wave parameters; especially significant wave height, for skill assessment of different packages. It has been shown that using a single parameter representing the performance of source and sink terms in the wave model, or relying on data from only one period of simulations for model validation and skill assessment would be misleading. The model sensitivity to input parameters such as time step and grid size were addressed using multiple datasets. The wind data used for the simulation were obtained from two different sources, and provided the opportunity to evaluate the importance of input data quality. The wind speed extracted from remote sensing satellites was compared to wind datasets used for wave modeling. The simulation results showed that the wind quality and its spatial resolution is highly correlated to the quality of model output. Two different sources of wave information along the open boundaries of the model domain were used for skill assessment of a high resolution wave model for the study area. It has been shown, based on the sensitivity analysis, that the effect of using different boundary conditions would decrease as the distance from the open boundary increases; however, the difference were still noticeable at the buoy location which was located 200-300km away from the model boundaries; but restricted to the narrow band of the low frequency wave spectrum. Uncertainty in wind data and wave boundary conditions were evaluated.Model sensitivity to different formulations of white capping and wind input were examined.Buoy and remote sensing data were used for model skill-assessment.Importance of having several measured dataset in evaluation of model performance is shown.

PARAMETERIZATION OF WAVE ATTENUATION IN MUDDY BEDS AND IMPLICATION ON COASTAL STRUCTURES

The complex interaction between surface waves and muddy sea bottom is pivotal for wave evolution studies in coastal regions. Considerable absorption of wave energy resulting in decreased wave height had been observed in numerous geographical locations which include countries like China, India, South Korea and Louisiana (Gulf of Mexico), USA. Strong dissipative effects of cohesive sedimentary environments on waves are well known, but little understood. The spectral action balance equation used in numerical wave modeling expresses evolution of wave energy as a function of frequency and direction, balanced by various source and sink mechanisms. In this research paper, we propose an improved parameterization of bottom induced wave attenuation which is an inherent limitation in the existing state-of-the-art third generation wave models. We explain through theoretical formulations wave propagation and attenuation in heterogeneous environments, where significant wave damping over entire wavelength is observed in bottom strata having mixture of sand with mud (hereafter referred to as slurry). In the context of engineering relevance, such a mixture is termed “slurry” commonly referred in dredging technology. Hence, in this work we propose and incorporate a parameterization form of wave attenuation in the action balance equation which accounts for dissipative mechanism for slurry dominated bottoms. The attenuation coefficient is thereby determined and a comparison is made with the theoretical formulation due to slurry and sand for a one-dimensional case. The study also addresses implication of such wave attenuation on coastal structures.

WINTER STORM AND TROPICAL CYCLONE IMPACTS ON THE SHORT- TERM EVOLUTION OF BEACHES AND BARRIERS ALONG THE NORTH- EASTERN GULF OF MEXICO

Here we present data indicating the complexity and highly variable response of barrier islands and beaches to the impacts of tropical cyclones and winter storms along the northern Gulf of Mexico. Data indicate that (1) barrier islands can conserve mass during catastrophic hurricanes; (2) less severe hurricanes and tropical storms can promote rapid dune aggradation and contribute sediment to the entire barrier system; (3) cold fronts play a critical role in the post-storm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe along the bayside beach through locally generated, high frequency, steep waves; and (4) barrier systems along the northern Gulf do not necessarily enter an immediate post-storm recovery phase, although nested in sediment-rich nearshore environments. The fluid mud environment off west Louisiana coast plays a significant role in damping wave energy associated with tropical cyclones. However, no significant surge attenuation appears apparent. It is anticipated that these findings will have important implications for the longer-term evolution of coastal systems in the northern Gulf of Mexico.

HETEROGENEITY AND DYNAMICS ON A SHOAL DURING SPRING -WINTER STORM SEASON, SOUTH-CENTRAL LOUISIANA, USA

Ship Shoal, a shore-parallel elongate sand shoal and a remnant of a late Holocene active delta has a unique heterogeneous sedimentary feature strongly affected by winter storms and fluvial sediment input from the Atchafalaya River. The interaction between fluvially derived sediment and subsequent deposition on the shoal has not been quantified; implications for hydrodynamic modeling are profound given that the shoal surface vacillates between sand and fluid mud. Thus, attenuation effects on waves and currents vary greatly. The results of a field survey undertaken during spring flood and winter storm periods showed that during fair weather, river-borne sediments transported to the shoal, forms a distinct fluid mud layer. Bottom sediments were re-suspended and transported by storm-induced waves and prevailing northerly/southerly