1,721,021 research outputs found
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Improved wave runup forecasts using remote observations and numerical models
Get PDFThe accuracy of flood forecasting models depends crucially on understanding wave runup. I use theory, insitu and remote observations, numerical modeling, computer vision, and deep learning to (1) investigate numerically the runup dependence on bathymetry and incident wave conditions, (2) improve video-based bathymetry estimates, and (3) characterize infragravity waves in 10m depth, for use in boundary conditions of runup models. Implementation into operational runup observing systems and models is ongoing. A numerical modeling (SWASH) study used 138 hindcast historical storm waves, two offshore boundary conditions, and 24 representative eroded beach bathymetries from a Southern California beach. The runup 2% exceedance level varied by more than 30% in response to changes in bathymetry or infragravity wave boundary conditions. An empirical parameterization trained on this dataset includes both a foreshore beach slope beta_f and an effective mid-surfzone slope beta_eff (Chapter 2, Lange et al. 2022). Subaqueous bathymetry is usually unknown because of the large expense of insitu jetski surveys, but beta_f and beta_eff can be estimated approximately and cost-effectively from images. The new 2-slope runup models have smaller errors than 1-slope models, but lack generality and have fundamentally limited accuracy. I show that useful bathymetry can be extracted from video collected during a single 17-minute quadcopter hover. The existing cBathy algorithm is extended with a crest-tracking algorithm that significantly reduces large cBathy errors near the breakpoint. The crest-tracking algorithm uses a deep-learning neural network to annotate timestacks for celerity estimates, and the depth inversion includes a nonlinear correction. This approach reduces RMSE surfzone depth errors to 0.17m, from ~0.81m with cBathy (Chapter 3, Lange et al. 2023, revision submitted to Coastal Engineering). The infragravity offshore (~10m depth) boundary condition is another potential error source in the runup model estimates. Several years of observations show that free infragravity waves are often much larger (up to x10) than the bound waves often used as a boundary condition. A parameterization of the incident-free IG wave field is combined with the predicted boundwave energy into a sea surface elevation timeseries of the incident IG energy suitable for use in numerical models (Chapter 4)
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From Subsurface to Shoreline: Impacts of Sea Level Rise on Coastal Groundwater, Reef Dissipation, and Beach Morphology
Get PDFCoastal systems face unprecedented challenges from sea level rise (SLR), threatening natural mechanisms that dissipate ocean wave energy and regulate shoreline stability. This dissertation integrates three critical lines of investigation to assess how SLR disrupts coastal protection systems across spatial and temporal scales. First, groundwater observation and modeling reveals that sea level rise is projected to exacerbate coastal flooding through subsurface emergence in Imperial Beach, California by the end of the century. Second, hydrodynamic analysis of a reef in American Samoa demonstrates that SLR reduces reef dissipation of nearshore sea-swell wave energy. If reefs are unable to dissipate incident sea-swell energy due to SLR, places like American Samoa will need to prepare for dramatic increases in wave run-up and nearshore wave forces. Third, satellite-derived shoreline analyses of O‘ahu, Hawai‘i, demonstrate that interannual sea level variability drives interannual beach width change for many of O‘ahu’s beaches. The increased shoreline sea-swell variability due to SLR modeled in American Samoa may drive increases in sandy beach shoreline erosion around many shorelines of O‘ahu. These findings collectively highlight the need for adaptive strategies that address groundwater flooding, reef degradation, and sediment dynamics as interconnected components of coastal resilience
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Infragravity Runup Methods: 1D or 2D?
Get PDFInfragravity (IG) waves are long period ocean waves (25 to 250 seconds) generated by nonlinear, long period variations in the momentum flux of incident sea swell (short period) wave groups in shallow water. IG waves are ubiquitous on the shoreline, can dominate the runup during storms, and contributes to wave driven flooding. Methods for predicting IG waves are a current topic for research in Nearshore Physical Oceanography. Typical methods are either empirical parametrizations or numerical wave models. In chapter 1, we test the nonhydrostatic numerical wave model SWASH for predicting infragravity waves by simulating waves from offshore measurements in 6m depth, along a 1D transect to shore, and comparing to LiDAR observations of wave runup and overtopping in Imperial Beach, CA. SWASH (1D) demonstrates high skill for predicting wave by wave runup and overtopping. In chapter 2, we challenge the 1D assumption inherent in chapter 1 by examining the impacts of infragravity edge waves, which propagate alongshore (in 2D) rather than cross-shore, over 2 months at Torrey Pines. Edge waves can be up to 25% of the runup, and are extremely significant for alongshore velocities. In contradiction, SWASH 1D, which lacks edge waves, overpredicts, showing that edge waves are one of several dynamics that may contribute to errors in 1D methodologies. In the appendix, SWASH 2D is used to simulate infragravity edge waves, but emergent dynamics demonstrate a need for further development of modeling methodologies for considering the impacts of edge waves
Effect of irregular, abrupt topography on near-inertial currents and waves
Get PDFAn impulsive or rotary wind stress excites inertial oscillations (IOs) in the ocean mixed layer. In the open ocean, IOs behave as uniform slabs rotating anti-cyclonically. However, IOs can become divergent by features, such as the spatial variability of the winds, latitudinal variations in the Coriolis frequency, f, and coastal topography, pumping the base of the mixed layer and exciting near-inertial waves (NIWs). Studies have found IOs in the mixed layer weaken near coastal topography due to the reflection of NIWs excited by the IOs impinging on a boundary; however, few studies have considered island topography. Here, the encounter between wind-generated near-inertial oscillations (NIOs) and islands, with particular focus on the island chain of Palau, is examined using observational and analytical techniques, and numerical modelling In the first chapter, mean surface current (0-50 meters) observations from a 10-month field deployment of moored Acoustic Doppler Current Profilers (ADCPs) are used to examine how near-inertial oscillations (NIOs) are modified by coastal boundaries around the island chain of Palau. At moorings farthest from Palau, near-inertial surface currents are intermittent and clockwise rotational, suggestive of wind-generated NIOs. Closer to topography, near-inertial currents become rectilinear, with enhanced energy at the northern and southern tips of the meridionally elongated island. EOF analysis reveals that most of the NIO variance is explained by coherent flow across the breadth of the island (approximately 150 km), suggestive of a slab-like NIO response to local wind stress, which is strongly modulated as the island topography blocks the flow. Vorticity estimates from a cluster of moorings at the northern tip of Palau further reveal that near-inertial vorticity generation increases during bouts of strong near-inertial currents. The second chapter dives deeper into the dynamics of these observations using analytical techniques in conjunction with a linear, inviscid, 1.5-layer reduced gravity model of idealized, circular island topography. Longuet-Higgins (1970) first presented the analytical solutions of IOs around a circular and elliptical island. Here, I revisit this work to understand the limitations and relevance of the solution to the ocean. The reduced gravity model is used to assess the solution under more realistic ocean conditions, most notably when a wind forcing is used to excite IOs and when free waves are allowed to propagate. The numerical model shows the response of IOs around circular islands is composed of three primary components: the radiation of Poincaré waves, a blocking IO response, and an island trapped wave (ITW). Finally, the third chapter presents results from the 1.5-layer reduced gravity model to explain the observed spatial variability of inertial currents around Palau (Chapter 1) and understand their contribution to the generation of vorticity. First, results from Chapter 2 are extended to an elliptical island, which more similarly reflects the geometry of Palau than a circular island. Second, non-linear and viscous terms are considered to examine the generation of vorticity around an ellipse. Third, a Palau shaped land mask is used to understand how features specific to Palau, such as a deep channel separating the main island, Babeldaob, from the northern region, Velasco Reef, impact the behavior of IOs. Finally, because Palau is situated near the equator, the effect of latitudinal variations of f on the behavior of IOs, Poincaré waves, and ITWs around circular islands is explored.Ph.D
Internal tide scattering at midocean topography
Get PDFThe scattering of mode-one M2 internal tides from 1) idealized Gaussian topography and 2) the Line Islands Ridge is examined with a primitive equation numerical model. Internal tide scattering at topography leads to a loss of energy to mixing and to a redistribution of energy flux in space, frequency, and mode number. Scattering from idealized ridges focuses wave energy directly downstream, while scattering from seamounts produces azimuthal energy dispersion. Scattering to higher modes occurs in the lee of near- and supercritical seamounts and ridges. The Mellor-Yamada level-2.5 submodel parameterizes turbulent mixing. For the near- and supercritical ridges with realistic stratification, elevated mixing is found over the leading edge of the topography and along a tidal beam up to the first surface bounce. A transition from a beam structure near the topography to a low-mode structure further away occurs due to an increased contribution from the mode-one internal tide as it refracts around the topography and not due to turbulent dissipation. At the Line Islands Ridge, runs with baroclinic and barotropic forcing are performed to distinguish scattered from locally-generated internal tides. Spatial and modal distributions of energy density and flux show internal tide scattering dominates at Hutchinson Seamount, while higher modes are generated locally at Sculpin Ridge. Hutchinson Seamount's slopes are steeper over a greater continuous area than Sculpin Ridge, which make Hutchinson Seamount a strong scatterer. Overall 37% of the incident mode-one energy flux is lost due to scattering into modes 2-5 (19%), dissipation by the model's turbulence parameterization (15%), and nonlinear transfer to the M. internal tide (3%). Two TOPEX groundtracks pass through the model domain roughly normal to the ridge topography and confirm the general features of the modal and spatial distribution found in the model.Ph.D
Structure and variability of internal tides in Mamala Bay, O'ahu, Hawai'i
Get PDFThis thesis demonstrates, from current and temperature observations, that approximately 60% of the observed semidiurnal internal tide variance in Mamala Bay, located off the southern shore of Oʻahu, Hawaiʻi, is phase-locked to the surface tide. It is further demonstrated, using a three-dimensional numerical model, that the observed internal tide is non-locally generated in contrast to the converging barotropic tidal flow hypothesis of Hamilton [1995]. The coherent semidiurnal internal tide is observed primarily at 240 m depth in the central regions of the Bay and is partially described by a standing wave. The observations and model results support the conclusion that coherent internal wave energy generated in the deep (500-1000 m) waters of the Kauaʻi and Kaiwi Channels, as well as offshore, gets focused and amplified at the head of Mamala Bay. A westward net flux of baroclinic energy suggests the primary internal tide energy source emanates from the east side of the Bay in the Kaiwi Channel.M.S
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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