M.S. - Ocean and Resources Engineering

Permanent URI for this collectionhttps://hdl.handle.net/10125/2120

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A morphology study of Makapuʻu Beach with a lidar topographic survey buggy
(University of Hawai'i at Manoa, 2025) Stegman, Dylan D.; Cheung, Kwok F.; Ocean & Resources Engineering
This thesis investigates short-term beach morphology at Makapuʻu Beach, Oʻahu, in response totwo distinct wave events using a combination of high-resolution mobile LiDAR surveys and process-based wave modeling. A custom-built survey buggy equipped with RTK-GPS and LiDAR was developed to enable efficient topographic data collection across the beach face. Surveys were conducted before, during, and after each event to capture dynamic changes in beach elevation and sediment distribution. The first event, a relatively long-period north swell, caused substantial erosion and offshore sediment transport, particularly in the northern embayment. The second event, driven by moderate trade wind energy from the northeast, resulted in more uniform retreat and greater sediment retention. V olume change calculations and elevation differencing revealed spatial patterns of erosion, scarp formation, and partial recovery. These patterns are closely mirrored by XBeach Non-Hydrostatic (XBNH) model outputs, which show wave energy focusing, offshore-directed currents, and circulation features that help explain the spatial distribution of sediment loss and accumulation across the beach in each case. This study highlights the effectiveness of integrating mobile LiDAR and numerical modeling for high-resolution, event-scale beach monitoring. The results underscore the influence of incident wave characteristics on morphological response and provide valuable insights for shoreline management, sediment transport studies, and coastal engineering applications.
Feasibility of a wave-powered lithium extraction system
(University of Hawai'i at Manoa, 2025) Bourjeaurd, Griffin; Gedikli, Ersegun D.; Ocean & Resources Engineering
Limited supply and rising demand in lithium-ion batteries due to growth in EVs, consumer electronics, and grid energy storage requires alternative sources of lithium (Li) to achieve our sustainability goals. The ocean has 5,000 times more Li than the world’s total land reserves, with elevated concentration levels found in several Li “hotspots”. A novel, coupled Li extraction and cathode material manufacturing process is a potential solution for our Li production needs that reduces time, environmental impact, resource demand, and costs associated with the Li-ion battery supply chain. The process still requires a substantial amount of energy to meet forecasted demand, and most Li hotspots are in remote locations without access to a grid-scale energy source. Integrating wave energy conversion (WEC) with the coupled Li production system has potential to be an emission-free, self-powered system that is deployable in virtually any favorable wave resource environment. This thesis investigated the feasibility of deploying a wave-powered Li extraction system by evaluating the integration of two reference model WEC devices with the coupled process at four selected Li hotspots. Preliminary results for the selected locations have shown that a single WEC can generate enough energy to extract 33,500 to 250,000 kgLi, producing 800,000 to 6,500,000 kg of LiMn2O4 (LMO) cathode active material at a cost of 4.01-9.00 $/kgLMO, substantially lower than the market price of 15 $/kgLMO.
Evaluation of coastal imaging georectification for measuring runup with UAVs on Hawai‘i beaches
(University of Hawai'i at Manoa, 2025) Nelson, Gabriel Sespe; Cheung, Kwok Fai; Ocean & Resources Engineering
Coastal inundation presents a significant and escalating threat to island communities such as Hawaii, driven by rising sea levels and increasing wave activity. Under these circumstances, precise measurement of wave runup is crucial for accurate coastal hazard assessments, effective mitigation strategies, and sound engineering practices. UAV-based photogrammetry paired with the Coastal Imaging Research Network (CIRN) Toolbox offers a powerful tool for coastal image processing. However, CIRN’s standard georectification method assumes a flat reference plane, which can introduce significant elevation errors in complex topographic environments. This study evaluates the accuracy of the flat-plane approach versus a topographically-informed method that incorporates digital elevation models (DEMs) for image georectification, with a focus on its applicability to Hawai‘i beaches.Deterministic analysis using surveyed ground truth data shows that the topographically- informed method produces three times lower elevation error than the flat plane method (mean absolute error of 0.063 ft vs. 0.229 ft), with improved consistency across varied sites and viewpoints. Time series comparisons of runup exceedance metrics (R1%, R2%, R10%) show that average differences between georectification methods ranged between 2.25 to 8.45% with beach slope, beach elevation, and UAV orientation influencing results. Differences between runup statistics combined with the insights from deterministic analysis indicate that the topographic correction improves georectification accuracy for runup metrics—particularly under the steep and irregular beach conditions common in Hawai‘i. To support survey planning, a geometric error estimation formula is developed to predict Flat Plane georectification error. With an average predictive error of less than 0.05 ft, the formula provides a validated tool for assessing and minimizing error tolerance before field deployment and deciding when topographic correction is necessary. The tools developed here enable practitioners in Hawai‘i and beyond to optimize UAV-based coastal monitoring for both efficiency and accuracy.
Path optimization for acoustical oceanography applications
(University of Hawai'i at Manoa, 2025) Jandial, Prajna; Nosal, Eva-Marie; Zhu, Frances; Ocean & Resources Engineering
This Master’s thesis contributes to underwater acoustic sampling techniques through machine learning. It has two objectives: (1) Optimizing the sampling process for underwater sound fields and(2) Optimizing data assimilation for ocean acoustic tomography. To address the first objective, we developed an approach that leverages autonomous underwater vehicles (AUVs) to sample unknown sound fields. Unlike fixed sensor networks with spatialconstraints, AUVs can make real-time decisions and adaptively survey a region. The proposed algorithm, sound exploration with active learning (SEAL), adaptively samples a survey region based on the sound field characteristics. SEAL uses an active learning strategy based on Gaussian Process (GP) regression to characterize a static sound field in a survey region. With each location sampled, the algorithm employs a GP to estimate the field and quantify the uncertainty in the predicted sound field. The uncertainty metric is used to choose the next sampling location. This dynamic approach maximizes the information gained by the AUV at the locations that it samples. SEAL also ensures efficient convergence toward the true distribution of underwater static sources in the sample region. Our algorithms were developed via simulation and were validated with a controlled experiment in a swimming pool [work funded by the NSF AI Institute in Dynamic systems and Catalyst t Awards for Science Advancement]. For the second objective, we aimed to optimize the process of integrating acoustic data intoocean models via ocean acoustic tomography. Ocean acoustic tomography (OAT) derives water column properties from acoustic observations. OAT traditionally uses ray tracing and requires making frozen ray approximations that can be limiting in some cases. Another approach involves iteratively updating sound speed profiles and re-running sound propagation models until the modeled travel times agree with measured travel times. However, this approach is computationally expensive. To optimize the iterative approach, we developed a machine-learning pipeline to map perturbations in sound speed profiles to corresponding changes in acoustic ray paths. The 2010–11 North Pacific Acoustic Laboratory (NPAL) Philippine Sea experiment dataset was used to develop and validate a neural network. To constrain the model’s learning to small perturbations in ocean states and observed acoustic travel times, sound speed profiles were decomposed using empirical orthogonal functions, with principal components used for training, while ray paths were represented as Fourier functions. The proposed neural network focuses on the variability in sound speed profiles and ray paths so that a predicted decomposed ray can be obtained for small changes in the ocean state [work funded by the Office of Naval Research].
Investigating acoustic signal propagation between the Kaua'i beacon source and the ALOHA cabled observatory
(University of Hawai'i at Manoa, 2025) Taylor, Elizabeth; Nosal, Eva-Marie; Ocean & Resources Engineering
Acoustic tomography is a powerful technique for remote ocean sensing that measures ocean properties over integrated acoustic paths. The information gained can be used to refine ocean models and improve the understanding of oceanographic processes. Tomography has diverse applications, including the study of internal waves, temperature variability, gyre dynamics, tides, and other ocean phenomena. Acoustic tomography leverages the fact that sound speed is a function of temperature, salinity, and pressure. By observing changes in the travel time of a signal along a transect, one can invert perturbations in arrival time to measure sound speed variations, ultimately revealing temperature changes. The Kaua`i Beacon (KB) source was reactivated in March 2023 after a 15-year hiatus for long-range acoustic applications in the Pacific Basin, including tomography and navigation. The KB source is located on the seafloor off the coast of Kaua`i at a depth of 810 m and transmits a 44-period m-sequence pseudo-random noise signal with a 75 Hz carrier frequency on a nominal 2% duty cycle schedule. Recent studies demonstrated successful reception of the signal at Wake Island, Monterey Bay, the Ocean Observatories Initiative (OOI) sensor network, and Ocean Network Canada, achieving 30 dB of processing gain. However, since KB is installed on the seafloor, local effects (e.g., from bathymetry and seabed properties) potentially affect the far-field signal; several studies from the early 2000s demonstrated a relatively complex acoustic near-field at KB. Fortuitously, the ALOHA Cabled Observatory (ACO) hydrophone, leveraged here as a “KB near-field receiver of opportunity”, receives the KB signal at 166 km from the KB source with precise timing. ACO is seafloor-mounted, 100 km north of O`ahu at a depth of 4,728 m. This thesis aims to advance the understanding of underwater acoustic propagation in the acoustic near-field of KB by processing and investigating the KB signal as received at ACO. We analyze over 1,000 KB transmissions received at the ACO over 24 months and examine the temporal variability in the receptions. This variability is compared to modeled results created using a range-dependent BELLHOP model. The model simulates acoustic propagation using ray theory and known ocean properties from the Hawaii Ocean Time-Series and bathymetry from the Hawaii Mapping Research Group. Modeled acoustic arrivals are compared to ACO data, revealing similarities in the magnitude and annual phase cycle of arrival time perturbations when tracking a single ray path over a yearly cycle. The thesis will detail the methods and preliminary findings of our KB-ACO investigations, including the effects of seasonal variability on the received signal. This work forms the foundation for future research into understanding the complete acoustic arrival pattern of the KB signal received at ACO, explicitly considering the bathymetric interactions along the 166 km transect.
Resident AUV design and preliminary testing for autonomous docking and charging at Kilo Nalu observatory
(University of Hawai'i at Manoa, 2025) Chung, Norman; Krieg, Michael; Ocean & Resources Engineering
Near-shore environments are important to oceanographers because of their relationship to the biogeochemical and anthropogenic processes which occur on land and at sea. Two approaches to researching these environments are undersea infrastructure connected to shore (i.e., cabled observatories) and autonomous underwater vehicles (AUVs). Resident, autonomous underwater vehicles (RAUVs) are a novel way to combine the strengths of undersea infrastructure and AUVs to study the near-shore environment. RAUVs are AUVs which permanently reside at underwater locations and perform multiple missions at a given site of interest without needing physical human intervention. As AUVs, they can capture information at the finer spatial and temporal scales not captured as well by undersea infrastructure, but they can also use said infrastructure to recharge and exchange data over multiple missions. RAUVs used in this manner need to connect to an underwater docking station to charge and exchange data. New, wireless, inductive charging systems show promise for RAUVs over their wired counterparts because wireless systems are less susceptible to damage via seawater corrosion and permit successful RAUV docking after a mission under less stringent positioning tolerances. This research aims to assess the feasibility of RAUV operation in the rough, near-shore environment at Kilo Nalu Observatory (KNO), a near-shore observatory off the south coast of Honolulu, Hawai`i. To that end, I present the design of a low-cost RAUV meant to autonomously dock, charge, and exchange data using KNO’s existing undersea infrastructure. The basis of the vehicle is the BlueROV2 Heavy, an affordable, underwater robot sold by Blue Robotics. Here, I show the mechanical and electrical upgrades made to turn the BlueROV2 Heavy into an RAUV. I also present the work done to develop an internal temperature monitoring system that ensures the RAUV’s batteries do not overheat during charging. I characterize the response speed of our thermistors and verify that the temperature monitoring system works with a simple experiment. Through the work here, I develop an RAUV platform that we can use to test the wireless, inductive charging of RAUV batteries by a docking station and the management of RAUV batteries underwater. In doing so, we move towards creating a RAUV that can one day operate long-term in KNO.
Numerical modeling of wave dynamics at Ulupa‘u crater: Validation of SWAN and XBEACH with field observations
(University of Hawai'i at Manoa, 2025) Dillon, Camryn Legier; Huang, Zhenhua; Ocean & Resources Engineering
Accurately modeling wave conditions is essential for assessing the impact of artificial coastal structures on wave dynamics and coastal resilience. Before predicting the effects of proposed modifications, it is critical to validate numerical models against existing site conditions to ensure reliability. This thesis focuses on the validation of two wave models, SWAN and the nonhydrostatic version of XBeach, by comparing modeled wave transformations with field data collected offshore of Ulupaʻu Crater, Oʻahu, Hawaiʻi. The primary objective is to assess the accuracy of SWAN in simulating deepwater to transitional water wave transformations by comparing model output to Acoustic Doppler Current Profiler (ADCP) data collected at the site. This validation will determine if nesting SWAN with XBeach is an effective approach for wave modeling over large spatial domains. Additionally, the thesis aims to calibrate the XBeach model with the existing reef environment by validating wave transformations within the proposed artificial reef deployment site using pressure sensor data. The validation process involved integrating site-specific bathymetry and offshore wave conditions into the XBeach and SWAN models. Model parameters, including bottom friction and nonhydrostatic effects, were adjusted to best replicate the observed wave transformations. Spectral analysis and time-series comparisons were used to assess model performance, identifying key processes such as energy dissipation across the existing reef system. Successfully calibrating these models to represent existing site conditions establishes a foundation for future simulations for hybrid reef installations such as those being pursued under the Rapid Resilient Reefs for Coastal Defense (R3D) project. Future simulations will incorporate these artificial structures to assess their effectiveness in wave attenuation and coastal protection. This research provides a framework for using numerical modeling to inform coastal engineering decisions, ensuring that artificial reef designs are optimized for both ecological and hydrodynamic benefits.
Nonhydrostatic XBeach Simulation of Wave Transformations in a Fringing Reef Environment: Validation using Field Observations
(University of Hawai'i at Manoa, 2024) White, Charlotte E.; Huang, Zhenhua; Ocean & Resources Engineering
Modeling wave transformations in nearshore fringing reef environments is an evolving field, especially for engineering applications. This study focuses on wave transformation at Waimānalo Beach, a location that features a shallow fringing reef and sloping seafloor. To study the wave characteristics at the site, the nonhydrostatic version of XBeach (nonhXB), a two-dimension, depth-integrated numerical model is implemented. The offshore boundary wave conditions are provided by the Simulating Waves Nearshore (SWAN) model. The bathymetry for the nonhXB model is prepared using US Army Corps of Engineers (USACE) 1-m resolution LiDAR data, while the bathymetry for the SWAN model is prepared with the NOAA 3-m resolution Continuously Updated Digital Elevation Model. This study adopts an approach by integrating in-situ field observations at Waimānalo Beach, as a method of calibrating the nonhXB model setup for the site. Waves at two nearshore locations were measured using pressure transducers. The results of the field survey reveal infragravity (IG) waves present at both pressure sensor locations. Using spectral input from a nearby offshore wave buoy, the ability of nonhXB to model IG waves was explored. It was found that the simulated results aligned reasonably well with those of the field observations, including IG waves. Finally, the study examined how bottom roughness affects the presence of the IG waves. It was concluded that increasing the bottom roughness decreases the magnitude of the IG waves, as well as the probability of waves breaking at the site.
THE RELATIONSHIP BETWEEN HYDRODYNAMIC AND MORPHOLOGIC CHANGES AT SUNSET BEACH
(University of Hawai'i at Manoa, 2024) Shepherd, Merritt Anne; Stopa, Justin E.; Ocean & Resources Engineering
In recent years, Sunset Beach on O´ahu’s North Shore has experienced multiple erosion events. Consequently, the US Army Crops of Engineers has collected datasets describing Sunset Beach through aerial photographic surveys and beach cameras to resolve the coastline variability from 2020-2021. The aerial and coastline imagery were processed to determine beach area and volume time series. The constructed beach time series indicates large seasonal variations and year-to-year variations, driven by energetic winter waves and calm summer waves. Analysis of adjacent beach sections shows high variability and often opposing beach behavior consistent with long-shore sediment transport. This study investigates the relationship between ocean hydrodynamics and beach morphology using a wave hindcast. The relationship, quantified by correlation coefficients, between the offshore wave conditions and the beach area is weak (r=0.4). The effect of the antecedent beach condition was found to have a large range from 4 to 53 days and it does not drastically enhance the relationship between the offshore wave conditions and beach response. Overall, the shoreline observations reveal a high spatial variability across the beach which might be due to the heterogeneous nature of the offshore reefs, nearshore rock structures, or important sub-littoral cell circulation patterns. The observations demonstrate that the beach min/maximum sizes lag the wave seasonality by two months and the summer-time beach recovery greatly varies (area difference between the two summer: 1500 m2) in the 1.5-year time series.
A COMPARATIVE STUDY OF THE TSUNAMIS FROM THE 2021 AND 2023 LOYALTY ISLANDS Mw 7.7 THRUST-FAULT AND NORMAL-FAULT EARTHQUAKES
(University of Hawai'i at Manoa, 2024) Robert, William Henry; Cheung, Kwok F.; Ocean & Resources Engineering
The Vanuatu Subduction Zone, which runs along the San Cristobal Trench and New Hebrides Trench, is an active tectonic region with a history of moderate to large tsunamigenic earthquakes. While over 40 earthquakes of Mw 7.0 or stronger have occurred along the New Hebrides trench within the past quarter-century, the subduction dynamics, tsunamigenic potential, and coastal risk of tsunamigenic earthquakes emanating from the southern trench remain understudied. We compare the tsunami signals from the 2021 and 2023 Mw 7.7 thrust-fault and normal-fault earthquakes along the South New Hebrides Trench. The 2021 megathrust earthquake was determined to have a relatively shallow slip patch with a focal depth of 10.0 km. The 2023 outer-rise normal-faulting earthquake occurred westward of the 2021 event at a depth of 18.1 km. The comparable moment magnitude and proximity of their epicenters provide an opportunity to examine the influence of fault mechanisms and bathymetric features at the Southern New Hebrides Trench on the tsunami waveform throughout the Southwest Pacific. The preferred finite-fault rupture models for each earthquake event were determined through iterative inversion of teleseismic P-waves and forward modeling of the tsunami waveform to match measurements at DART and coastal tide gauge stations. Analysis of the computed tsunami waveforms reveals that the 2021 and 2023 tsunamis are primarily influenced by the location of the seafloor deformation with respect to major bathymetric features in the near-field. The width of seafloor deformation and source water depth influence the subsequent period of tsunami waves, which has a strong effect on far-field energy lobes guided by seamounts and small islands, but the overall wave patterns exhibit similarities because the effects of bathymetry and resonance dominate at a larger scale.
EXPERIMENTAL INVESTIGATION OF THIN-WALLED CYLINDRICAL CANTILEVER BEAMS UNDERGOING VORTEX-INDUCED VIBRATIONS
(University of Hawai'i at Manoa, 2024) Encke, Clara Vanessa; Gedikli, Ersegun Deniz; Ocean & Resources Engineering
The main objective of this Master’s thesis is to present the execution and results of an experimental study on the dynamic response of three low-mass ratio cylindrical cantilever beams experiencing vortex-induced vibrations. The tested cylinders are hollow, sealed, and made out of polycarbonate with low mass ratios m* of 0.761, 0.830, and 0.922 and damping ratios between 0.037, 0.051, and 0.045. The motion is being analyzed by using high-speed cameras to document the motion under UV lights for better image quality. ProAnalyst software is being used to translate the videos into motion-tracking data in two directions In-Line (IL) and Cross-Flow (CF). Our findings indicate that the IL: CF frequency ratios conform to the conventional 2:1 frequency ratio. Nonetheless, we also observed 1:1 and 3:1 frequency ratios at low and high reduced velocities, accompanied by predominantly small positive and negative lift coefficients in phase with velocity values. The amplitude response aligns with previous literature, although our study shows that the in-line amplitudes reach higher values due to the cylinders’ low mass and damping ratios. Specifically, at reduced velocities below 3, one of the cylinders experienced higher in-line amplitudes than cross-flow.
AN AUTONOMOUS SAMPLER FOR IN-SITU VERTICAL BENTHIC BIOGEOCHEMICAL FLUXES DETECTION
(University of Hawai'i at Manoa, 2024) Lamoonkit, Jomphol; Briggs, Ellen; Ocean & Resources Engineering
Benthic environments in the coastal ocean, such as seagrass meadows, have gained attention for their capacity to store organic carbon in their systems such as the organic tissue of seagrass. Nevertheless, there is still room to study their influence on vertical biogeochemical fluxes between pore water beneath the sediment layer and the overlying seawater. The ability to monitor the vertical biogeochemical fluxes of seawater in the vegetated sedimentary environment could improve our understanding of the role of the vegetated benthic environment on the carbon cycle. Moreover, accurate and rapid measurement of benthic vertical biogeochemical fluxes could also contribute to the monitoring efforts for marine-based Carbon Dioxide Removal (mCDR) research in order to verify the subtle changes of seawater chemistry in the benthic regions.This study aims to contribute to the biogeochemical monitoring efforts through an in-situ sampler system called the Benthic Alternating Autonomous Sampler (BAAS). The instrument is designed to be deployed on the seabed and to alternately sample between the sediment pore water and overlying water. The system is autonomously controlled by a microcontroller (Arduino). The design integrates Rhizon screen sections that allow retrieval of water samples from various sediment grain sizes (≥ 0.15 μm). In this study, optical pH and oxygen sensors, and temperature probes were integrated with the BAAS system as one such example and to demonstrate proof of concept. The BAAS system is intentionally designed to be able to interface with numerous different flow-through sensors. A benchtop system is demonstrated in this study to explore the flow rate under different sediment types, carryover volume, alternation properties, and sensor readings, which would provide the critical basis for in-situ sensor development. This prototype, bench version of the in-situ sampler apparatus would enable us to study the effect of sediment types and sampling depth on the performance of the sampler. Furthermore, the open-source platform would facilitate other researchers' ability to replicate the system easily. This sampler system would also open up possibilities for studying benthic sedimentary fluxes, providing insights into ocean ecosystems and seawater chemistry.
Fluid-Structure Interaction Analysis of an Oscillating Wave Surge Energy Converter using LS-Dyna
(University of Hawai'i at Manoa, 2023) Pappas, Kyle; Gedikli, Ersegun D.; Ocean & Resources Engineering
Three-Dimensional two-way coupled fluid structure interaction analysis requires a complex strategy utilizing the finite element method (FEM) for large matrix computations. Two FEM solvers in LS Dyna are utilized to conduct a structural analysis of the Hawai’i Wave Surge Energy Converter (HAWSEC) for a particular wave condition case study that is directly compared with experimental results. The HAWSEC is a hollow, surface piercing, bottom secured, flap type oscillating wave surge energy converter designed for nearshore applications. The Arbitrary Lagrangian Eulerian (ALE) solver produces high fidelity solutions utilizing an explicit solver to couple structural mechanics with the fluid domain. The Incompressible Computational Fluid Dynamics (ICFD) solver utilizes an implicit solver with larger timesteps, making use of a Newton loop to converge the structural part with the fluid part. While both solvers accurately produce the force and pitch angle of the flap, the ICFD solver stands out for its low computation time, and ease of modeling the FSI boundary. Leakage control issues in the ALE simulations are addressed to adequately contain most of the air inside of the flap. Applying a plastic kinematic material to the aluminum flap allows for stress and strain contours to be observed in either solver. While intuitive stress and strain contours are observed in the ICFD simulations, the ALE simulations present questionable results that may require further refinement in leakage control. It is therefore suggested to use the ICFD solver for this type of problem where the structure is hollow and very small time steps are not necessary. Utilizing accurate two-way coupled FSI simulations may streamline design processes for wave energy converter technology, reducing development costs, allowing for faster optimizations, and increasing reliability of the structure.
Using Nonhydrostatic XBeach to Simulate Wave Transformations in Fringing Reef Environments
(University of Hawaii at Manoa, 2023) Chase, Jonathan; Huang, Zhenhua; Ocean & Resources Engineering
Nearshore modelling in application for engineering disciplines is still in a state ofdevelopment due to high computational requirements and is difficult to calibrate consistently with field measurements. The scope of this study is to assess the feasibility and limitations of using the Nonhydrostatic version of XBeach (NonhXB, a two-dimensional (2D), phase resolved, depth integrated numerical model) to study the effects of nearshore wave transformations in a fringing reef environment. This study focused on finding the optimal grid size to simulate the breaker zone characteristics in a fringing reef environment by comparing NonhXB results alongside a dataset from a large scale wave flume test. The purpose of this study is to recommend a set of parameters which can be used to quantify the probability of wave breaking and provide a reasonable estimate of the breaker zone width; these parameters include grid resolution, breaker steepness parameter, reform steepness parameter and eddy viscosity. These recommended parameters should be further clarified by field observations in future applications.
Seasonal Wave Climate Anomalies On The North Shore Indicative Of Erosion Conditions
(University of Hawaii at Manoa, 2022) Storey, Andrew; Stopa, Justin E.; Ocean & Resources Engineering
In recent years, ocean inundation has impacted local infrastructure at Sunset Beach, O`ahu especially when the beach is highly eroded. The objective of this thesis is to explain and identify potential drivers of erosion. Given a lack of sediment observations at Sunset Beach, this study uses a combination of numerical modeling, buoy observations, sediment characteristics, US Army Corps of Engineers Honolulu District’s remote sensing observations, and anecdotal evidence from local community observers to identify the erosion drivers. The seasonal cycle of the wave environment which drives the beach dynamics is dramatic, with large waves in December-March and small waves in May-August. This study relates seasonal wind, wave, and water level anomalies to the recent erosion and negligible erosion years using the various datasets. The wave and wind fields have the largest deviations from the climatological seasonal cycle in the May-August preceding the dramatic erosion which occurs during September-December. The largest and most significant changes are related to the local trade winds variability varying +14% to -23% from the climatological reference. These findings suggest the summer conditions help (or hinder) the beach recovery to endure (or suffer) the average erosive wave conditions experienced in the winter months.
Detecting Spinner Dolphin (Stenella longirostris) Clicks In Noisy And Low Sampling Rate Hydrophone Recordings
(University of Hawaii at Manoa, 2021) Manabe, Kei; Nosal, Eva-Marie; Ocean & Resources Engineering
Development of automated detection algorithms for cetacean vocalizations is important to facilitate marine mammal research. This thesis focuses on click train detection in cases in which sampling rates are too low to capture the full bandwidth of the clicks, and in which impulsive noise confounds current detection methods. We develop an algorithm to detect/classify odontocete click trains based on the regular timing of clicks; the method relies on the slowly- varying nature of Inter-Click Intervals (ICIs) within a click train. The algorithm is refined and evaluated using simulated data. It is motivated and applied to recordings of spinner dolphins collected in Hawaii. Performance is quantified using receiver-operating and precision-recall curves for both simulated and real data. While the method shows promise (including the ability to separate multiple clicking animals) for click trains with stable ICI and in relatively low-noise conditions, the performance on the spinner dolphin dataset is marginal.
Morphodynamic Changes Due To Calm/moderate Wave Forcing: A Case Study Of Waikīkī Beach
(University of Hawaii at Manoa, 2021) Kalksma, Julianne; Stopa, Justin E.; Fletcher, Charles H.; Ocean & Resources Engineering
Sea level rise, erosion, and the wave climate influence Waikīkī Beach on the South Shoreof O‘ahu which is a popular beach in metropolitan Honolulu. In response to recent erosion events and on-going beach nourishments, weekly surveys have been collected for the past 3 years, from April 2018 through December 2020, to better understand coastal morphology. Local studies found detailed two-dimensional morphological structures; however, no direct relationships between the offshore driving ocean conditions and Waikīkī Beach have been established. Consequently, the purpose of this study is to relate the offshore wave conditions to sand movement. We use a wave hindcast to quantify relationships between the sand volume and various wave parameters. We find that the two dominant wave parameters driving changes in the sand volume are the wave direction from swells generated in the Southern Ocean (Dps) and Easterly wind wave height (Hse) relative to many other parameters of the wave climate. Both wave sources are active throughout the year and we are unable to discern seasonal beach changes. We find the antecedent wave condition influences the beach state and the previous 50 weeks might affect the present beach state. The spatial relationship between the wave parameters demonstrates clear geophysical oscillations in the sand motion which supports that Dps and Hse are influencing the nearshore dynamics and resultant beach morphology.
A Fatigue Analysis Of The No-WEC Mooring System At The U.S. Navy Wave Energy Test Site Off O‘ahu, Hawai‘i
(University of Hawaii at Manoa, 2020) Morrow, Cameron Sean; Huang, Zhenhua; Ocean & Resources Engineering
The Wave Energy Test Site (WETS), off the coast of Marine Corps Base Hawai‘i, provides a unique location for the full-scale validation of Wave Energy Conversion (WEC) devices in the USA. WETS has three separate berths, allowing for the simultaneous testing of three WEC devices with up to 1 MW power transmission to shore. Two WEC devices have been tested at WETS (two deployments each), and many other devices are planned to be deployed in the coming years. Since the 2014 installation of the mooring systems at the 60 m (Site A) and 80 m (Site B) berths, some of the original mooring lines have failed– the lines failed due to this lack of pretension in the mooring lines during much longer than anticipated period during which no WEC was deployed, which resulted in substantial link-to-link collisions and failure of joining links in catenary "thrash zone" of the chain. The new mooring system at the two deeper WETS berths is a 3-point spread catenary system consisting of 2.75" ground chain and 4” (60m berth)/3.5” (80m berth) riser chains, leading from the anchors to three separate surface buoys. From the surface buoys, hawsers connect a WEC device to the rest of the mooring system. When no WEC is deployed, a no-WEC hawser system keeps the system in tension to reduce fatigue damage and wear on the moorings. This study focuses on understanding the fatigue damage to the mooring chain at the 60 m berth when the mooring system is in the no-WEC configuration. This analysis is desirable for understanding the fatigue for the no-WEC configuration during the extended WETS idle periods. For the fatigue damage analysis, typical sea states are identified based on analyzing a 41-year wave hindcast and validated with 20 years of buoy data. The fatigue damage analysis is based on a frequency-domain analysis of the no-WEC mooring system’s responses to typical sea states. The long-term fatigue damage calculation is performed by considering the probability of occurrence of these typical short-term sea states. The long-term fatigue analysis shows that the new mooring lines installed at the 60 m berth at WETS can withstand failure due to fatigue for at least 41 years when they are in the no-WEC configuration, which is well past the estimated decommission date for WETS.
Relationships Between Tsunami Size And Earthquake Magnitude Improved By Fault Parameters
(University of Hawaii at Manoa, 2020) Sun, Lin; Cheung, Kwok Fai; Ocean & Resources Engineering
Megathrust earthquakes are the main source of tsunamis. The rupture at the plate interface deforms the seafloor, displacing seawater over a large region. The earthquake magnitude is not the only factor that affects the tsunami amplitude. A tsunami earthquake, which produces a much larger tsunami than what can be inferred from the seismic energy release, exemplifies this phenomenon. This thesis examines relationships between tsunami size and key geophysical attributes such as fault depth, fault dip, fault size, rigidity, and water depth, besides moment magnitude. The parametric study involves four sets of simplified megathrust-ocean models with an elastic planar-fault solution to define the earth surface deformation and a non-hydrostatic model to describe the resulting tsunami. The first set of models contains a flat seafloor to provide a baseline for comparison. The second set includes a flat seafloor abutting a 2° slope, and by varying the fault depth, fault dip, and water depth, explores the contributions from wave shoaling and wave energy anisotropy to peak tsunami amplitude. The third set utilizes the same topography to demonstrate effects of reduced rigidity or fault size for the same seismic moment. The fourth set examines the combined effects of the geophysical parameters as well as their trade-off. The results highlight the importance of depth-dependent fault rigidity and size in describing the two orders of magnitude variability in observed peak tsunami amplitude for given moment magnitude.
Uncertainties Of Multi-decadal Buoy And Altimeter Observations
(University of Hawaii at Manoa, 2020) Leyva, David James; Stopa, Justin E.; Ocean & Resources Engineering
The impacts of climate change are evident globally. Changes to the sea state, including ocean wave heights, are important for the long-term design of oceanic infrastructure as well as resource extraction. Previous studies have estimated trends of wave height from multi-decade satellite altimetry and wave buoy observations but they do not robustly quantify uncertainties in trends. Changes to buoy hulls, payloads, and processors can introduce step changes and thus may influence year-to-year variability in the buoy time series which can create spurious climate signals. We find that standard approaches to identify step-changes in buoy time series are highly dependent on the reference time series. This is because identifying step-changes is most effective when analyzing a difference series, which is derived from the original time series minus a reference series. However, in the absence of a reliable reference series this method becomes subjective due to differences in reference series, (different hindcast products or different altimeter products). Therefore we adopt another approach which estimates uncertainties in multi-decadal wave height trends of buoy observations by extrapolating co-located altimeter-buoy wave height residuals. Synthetic wave height time series are created by randomly applying altimeter-buoy residuals to the original buoy time series through Monte Carlo simulations. The uncertainties of the buoy wave height trends are on the order of millimeters per year. This is relatively small when compared to the magnitudes of the buoy wave height trends which are on the order of centimeters per year and suggests that changes to buoy configurations are not having a large impact on the overall buoy trends. These synthetic time series provide a unique perspective into understanding wave climate and represent the uncertainty of buoy time series based on the differences between buoy and altimeters.

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