Hydrodynamics and Sediment Dynamics in a Receiving Basin for Sediment Diversion: a Case Study in Barataria Bay, Louisiana, USA

Barataria Bay is a receiving basin of Mid-Barataria Sediment Diversion in Louisiana, USA. In this region the data of sediment transport and hydrodynamics are scarce but important for the design and planning of sediment diversion to be implemented in near future. Four-months bottom boundary layer observation was conducted to study winter and spring hydrodynamics and sediment dynamics in the bay. Hourly waves, tides, currents and bottom suspended sediment concentration were measured using multiple optical and acoustic sensors attached to two tripod platforms. High-temporal resolution data indicated that during winter, salinity at northern bay was mainly controlled by northerly wind during cold fronts, and tidal currents kept southern bay salinity high during the same period. In spring, frequent pervasive southerly winds and the westward shelf transport of flooding freshwater from Southwest Pass of Mississippi River Delta lowered the salinity in southern bay. Density spectral analysis showed that wind-driven currents played the most important role in generating wave-current combined shear stress that triggered bottom sediment resuspension. Style-Glenn 1-D boundary layer model was also applied for sediment flux calculation. During the cold front passages sediment transport directions generally rotated and its magnitudes changed greatly when southeasterly wind shifted to intensified northwesterly wind. Southerly pre-front winds facilitated wetland sedimentation by transporting sediment to the northern bay during high water level conditions such as flooding spring tides. Conversely, northerly winds during cold fronts could dominate over bidirectional tidal currents and led to southward net sediment transport and eventual sediment loss in the bay. Timing of diversion openings, orientation of receiving basins, dominant wind directions and water levels should be considered in the planning and management of future sediment diversions in coastal areas

Wave Attenuation by Constructed Oyster Reef Breakwaters

Biloxi Marsh, located along the shoreline of Eloi Bay in St. Bernard Parish, Louisiana has experienced significant shoreline erosion in recent years. The Living Shoreline Demonstration Project, completed in November 2016, constructed three miles of living shoreline structures to attenuate waves and thus combat marsh edge erosion along the shoreline of Eloi Bay. Several types of constructed oyster reef breakwaters were installed for this demonstration project. Due to the experimental nature of these products, available performance characteristics are limited. This research measures wave attenuation across the constructed oyster reef breakwaters using bottom-mounted pressure gauges. Seven pressure gauges were deployed to obtain wave characteristics on the unprotected and protected sides of four types of breakwater structures. The raw pressure data were processed to determine water surface elevations, significant wave heights, and peak wave periods. In addition to the wave gauges, two water level sondes were deployed to record water surface elevations at the site. Topographic and bathymetric surveys were also conducted along cross-shore transects at the wave gauge locations to provide a profile of the shoreline and structures. The wave attenuation and transmission characteristics of the oyster reef breakwaters from the field measurements are presented. A range of transmission coefficients were calculated for each breakwater structure type

Numerical Modeling and Field Investigation of Nearshore Nonlinear Wave Propagation

First, a phase-resolving frequency-domain wave model that solves nonlinear wave-wave interactions is improved to account for wave dissipation and modulations over viscoelastic mud layer. Model results show satisfactory agreement with laboratory measurements. The model is then used to investigate the combined effect of mud viscoelasticity and nonlinear wave-wave interactions on surface wave evolution using cnoidal and random wave simulations. In general, qualitative measures such as shape of cnoidal waves or pattern of variation in Hrms of random waves are dictated by direct mud-induced damping which, due to resonance effects, has a substantially different structure over viscoelastic mud compared to viscous mud. Nonlinear interactions affect spectral shape and distribution of energy loss across the spectrum. Subharmonic interactions cause indirect damping of high frequencies but ameliorate damping of harmonics around mud’s resonance frequency. Therefore, neglecting mud elasticity can result in significant errors in estimation of bulk wave characteristics and spectral shape. Next, a phase-resolving frequency-domain model for wave-current interaction is improved to account for wave modulations due to viscoelastic mud. Results indicates that copropagating currents decrease frequency-dependent damping at low frequencies while they increase it at higher frequencies. The opposite is true for counterpropagating currents. The impact of currents at high frequency increases with increase in mud shear modulus and it is observed in both monochromatic and random wave simulations. The Performance of two mud-induced wave evolution models are compared. One model assumes that the mud layer is thin and the other is applicable to mud of arbitrary depth. It is found that a model based on thin-mud assumption overestimates damping over viscous mud in both monochromatic and random wave scenarios. However, for viscoelastic muds, this model slightly underestimates and overestimates damping for monochromatic and random wave scenarios, respectively. Finally, a preliminary field measurement and data analysis of wave and flow over a seagrass meadow is conducted. In addition, a computational model for hydrodynamics of wave-vegetation interaction is linked to a computational biophysical model for seagrass growth. As a result of this integration, the wave-vegetation model provides improved information on leaf orientation to the seagrass growth model

Understanding the Influence of Nonlinear Seas on Wind Generated Waves

An understanding of the influence of wind of the surface on irregular waves is important for improving ocean forecasting models. While many studies have investigated the phenomenon of wind wave suppression on the surface of mechanically generated waves in the laboratory, few studies have investigated the occurrence of this phenomenon for irregular waves. Chen and Belcher (2000) developed the first model to predict the suppression of wind waves as a function of the steepness of the long wave on which they travel. The Chen and Belcher (2000) model however, was only validated using monochromatic waves, not irregular waves, which are more representative of real ocean sea states. Additionally, few studies have investigated turbulence under irregular waves in the presence of wind in a controlled environment. This thesis aims to satisfy two research objectives. The first is to determine the applicability of the Chen and Belcher (2000) wind wave suppression model to irregular sea states. The second objective is to provide a procedure for selecting the appropriate method for indirect measurement of turbulence beneath waves in a laboratory. To meet these objectives, a comprehensive data set consisting of wind velocity, surface elevation, and water velocity data were collected in the Alfond W2 Ocean Engineering Lab at the University of Maine. The data set consisted of a variety of irregular and monochromatic wave environments and wind speeds. Through the use of multiple data analysis techniques, this study reveals that in order for the Chen and Belcher (2000) model to be directly applicable to irregular seas, a modification must be made to the long wave-induced stress term. This modification accounts for the wave energy associated with each frequency in wave spectrum for irregular waves, whereas the original model only accounts for a single wave frequency. The modified model is able to accurately predict the trend in the suppression of wind waves on the surface of irregular, long waves as a function of the long wave steepness. Additionally, in this work a case study is presented that reveals several limitations associated with the existing methods for indirect measurement of turbulence in a laboratory. The results of this work expand the implications of the Chen and Belcher (2000) model to be more applicable to ocean waves. This can aid in better prediction of model parameters, such as the drag coefficient and the sea surface roughness length, which are controlled by the high frequency waves on the ocean surface. This work also provides a guide for planning an experiment to measure TKE dissipation, ε, under waves in the presence of wind in a controlled, laboratory setting, which will aid in the planning of future experiments