Wide Shelf Research Articles

Internal Tides at the Coast: Energy Flux of Baroclinic Tides Propagating into the Deep Ocean in the Presence of Supercritical Shelf Topography

Abstract The generation of internal tides at coastal margins is an important mechanism for the loss of energy from the barotropic tide. Although some previous studies attempted to quantify energy loss from the barotropic tides into the deep ocean, global estimates are complicated by the coastal geometry and spatially and temporally variable stratification. Here, we explore the effects of supercritical, finite amplitude bottom topography, which is difficult to solve analytically. We conduct a suite of 2D linear numerical simulations of the barotropic tide interacting with a uniform alongshore coastal shelf, representing the tidal forcing by a body force derived from the vertical displacement of the isopycnals by the gravest coastal trapped wave (of which a Kelvin wave is a close approximation). We explore the effects of latitude, topographic parameters, and nonuniform stratification on the baroclinic tidal energy flux propagating into the deep ocean away from the shelf. By varying the pycnocline depth and thickness, we extend previous studies of shallow and infinitesimally thin pycnoclines to include deep permanent pycnoclines. We find that scaling laws previously derived in terms of continental shelf width and depth for shallow and thin pycnoclines generally hold for the deeper and thicker pycnoclines considered in this study. We also find that baroclinic tidal energy flux is more sensitive to topographic than stratification parameters. Interestingly, we find that the slope of the shelf itself is an important parameter but not the width of the continental slope in the case of these steep topographies. Significance Statement The objective of this study is to better understand how vertical density stratification, which can vary seasonally in the ocean, affects the interaction of tides with steep coastal topography and the generation of waves that travel away from the coast in the ocean interior. These waves in the interior can travel over long distances, carrying energy offshore into the deep ocean. Our results suggest that the amount of energy in these internal waves is more sensitive to changes in topography and latitude than to the vertical density profile. The scaling laws found in this study suggest which parameters are important for calculating global estimates of the energy lost from the tide to the ocean interior at the coastal margins.

A systemic and comprehensive assessment of coastal hazard changes: method and application to France and its overseas territories

Abstract. In the context of climate change, height and frequency variations in extreme sea levels (ESLs) are studied using deterministic and probabilistic approaches. However, this type of approach does not highlight the dynamic effects (waves, currents) generated by metocean events (storms, cyclones, long swells, and tsunamis) beyond their effects on sea levels. In particular, ESL estimates are calculated by considering the main determining physical factors but cannot include all the effects of these factors. Ultimately, this can lead to confusion between ESL and hazard. This article proposes a systemic assessment method to analyze coastal hazard changes at regional scales, integrating parameters influencing sea levels, as well as factors describing the geomorphological context (length and shape of the coast, width of the continental shelf), metocean events, and the marine environment (e.g., coral reef state and sea ice extent). French mainland and overseas territories were selected to apply the method. The present study highlights the need to consider not only the sea level variability, but also the current and future characteristics of metocean events. The long, concave coasts bordered by a wide continental shelf appear particularly sensitive to variations in the intensity or trajectory of metocean events. Coral reef degradation in the tropics and the decrease in seasonal sea ice extent in the polar regions can also significantly change the nearshore hydrodynamics and impacts on the shoreline. These results help us to predict the types of hazard (shoreline erosion, rapid submersion, and/or permanent flooding) that will increase the most in different coastal zones.

Analysis of the Influence of Peripheral Light Shelves on Building Lighting

Due to the design problems in modern architecture, it often appears that although the sunlight of the building is sufficient, the lighting quality of the building interior is not good. Due to the reasons of lighting, shading and interior layout, the phenomenon of glare and uneven distribution of light often occurs in buildings. Therefore, it is recommended to use light shelves to reduce problems such as glare and improve the indoor light environment comfort. The effects of six different widths and five different heights of peripheral light shelves on DF, UDI, GA and Uniformity Ratio of Daylighting are analyzed. The results show that the width and height of light shelves have no improvement effect on DF. The height of the light shelves has an improvement effect on indoor UDI, but the width does not; The width and height of light shelves can improve GA and Uniformity Ratio of Daylighting.

Climates and sea-level controls on chemical weathering in the northern shelf of the Okhotsk Sea in the past 23 ka

The Okhotsk Sea, a subarctic marginal sea with a wide shelf and abundant weathering products, provides ample information about the chemical weathering in the geological past. However, insufficient investigation in the Okhotsk Sea, especially in its shelf, limits our knowledge of the intensity of chemical weathering in high-latitudes. In this study, high-resolution geochemical elements compositions of the core LV87-54-1 were measured to reveal the provenance change in the Northern Okhotsk Sea Shelf since the Last Glacial Maximum (LGM) and to discuss the influential factors of the deep-sea weathering records. The elemental ratios suggested that the sediment provenance remained stable during 23–11.4 ka, mainly transported from the Okhotsk-Chukotka volcanic belt; whereas the sediment input from the Kamchatka Peninsula to the study area has been enhanced since 11.4 ka. During 23–14 ka, the chemical index of alteration (CIA) was observed to align with climatic changes in the Northern Hemisphere. Synchronized climatic deterioration (i.e., cooling and drying) in the Northern Hemisphere mainly contributed to the reduced alteration of the sediments eroded from the Okhotsk-Chukotka volcanic belt. During 14–11.4 ka, the reworking of older weathered sediments caused by sea-level rise overwhelmed climates in controlling silicate weathering, resulting in an increased alteration of sediments. During the Holocene, the core sediment witnessed a lower weathering degree, compared to the LGM–Deglaciation. We attribute this to an increased sediment input from the Kamchatka Peninsula that supplied more weaker weathered sediments to the study area.

Three decades of nearshore surveys reveal long-term patterns in gray whale habitat use, distribution, and abundance in the Northern California Current

The nearshore waters of the Northern California Current support an important seasonal foraging ground for Pacific Coast Feeding Group (PCFG) gray whales. We examine gray whale distribution, habitat use, and abundance over 31 years (1992–2022) using standardized nearshore (< 5 km from shore) surveys spanning a large swath of the PCFG foraging range. Specifically, we generated density surface models, which incorporate detection probability into generalized additive models to assess environmental correlates of gray whale distribution and predict abundance over time. We illustrate the importance of coastal upwelling dynamics, whereby increased upwelling only yields higher gray whale density if interspersed with relaxation events, likely because this combination optimizes influx and retention of nutrients to support recruitment and aggregation of gray whale prey. Several habitat features influence gray whale distribution, including substrate, shelf width, prominent capes, and river estuaries. However, the influence of these features differs between regions, revealing heterogeneity in habitat preferences throughout the PCFG foraging range. Predicted gray whale abundance fluctuated throughout our study period, but without clear directional trends, unlike previous abundance estimates based on mark-recapture models. This study highlights the value of long-term monitoring, shedding light on the impacts of variable environmental conditions on an iconic nearshore marine predator.

SEAFRAME time series from the Southern Shelf reveal a neglected major upwelling mechanism

ABSTRACT In this era of climate change, it is important to have a reference state. For this purpose, an array of 12 port-based SEAFRAME gauges was established by Professor Geoffrey Lennon, through the Flinders University of South Australia, around the Australian coastline between 1991 and 1993. In addition to changes in sea level, they also record air and water temperature and wind velocity. We provide a method to extract the daily temperature anomaly between atmospheric and oceanic temperature recorded in the SEAFRAME gauge system. Thus, a fine-resolution record of the coastal ocean temperature structure can be obtained, which is a direct index of the vertical coastal heat flux that characterises both downwelling and upwelling. Results are presented for four gauges on the Southern Shelf for 2005, and for the ocean temperature anomaly time series during 2001–2010 at Portland, which monitors the structure of the Bonney Coast upwelling. This time series is also compared with that from an ECMWF model, and with that at Esperance which monitors an upwelling of similar strength on a wider shelf. The analysis reveals a second upwelling process in addition to the Ekman upwelling, which we have called the Flinders upwelling. The Ekman upwelling is due to a vertical velocity at the base of the mixed layer. The Flinders upwelling is due to a vertical velocity at the top of the mixed layer, which has been previously ignored. Along the Southern Shelf, the SEAFRAME data suggest that the Flinders upwelling is the dominant process.

Setting an Environmental Baseline for the Deep-Sea Slope Offshore Northwestern Cuba (Southeastern Gulf of Mexico) Using Sediments and Nematode Diversity

Deep-sea offshore northwestern Cuba is the less studied zone of the Gulf of Mexico (GoM). Our study aimed to set an environmental baseline and investigate a potential west-east gradient of sediment properties and nematode diversity along the northwestern Cuba. Sediments were collected by multicorer at nine sites in the insular slope between 974 and 1682 m depth. Sediment texture and composition showed a west-east gradient caused likely by narrowing of shelf width and increasing of downslope transport of terrigenous material. We found clear signatures of heavy metal pollution likely derived from Havana City but also from open-sky mining (Castellanos mining complex) and port dredging operations (Mariel). Nematode assemblages were dominated by the deep-sea genera: Acantholaimus, Metadasynemella, Desmodorella, Cervonema, Daptonema, Halalaimus, and Pselionema. α-diversity was about 20–50 genera and γ-diversity of about 100 genera. The β-diversity was substantial likely because small-scale patchiness of resources and heavy metals stress that increased assemblage variability. Individual weight decreased with depth indicating food-supply limitation in the deep sea; also, heavy metals were negatively correlated with weight suggesting deleterious effects on the growing. Our findings indicate that heavy metal pollution has reached deep-sea sediments and corroborate the long-range effects of anthropogenic activities on deep sea. Taxonomic diversity and biological traits (life strategy and trophic composition) constituted a powerful bioindicator of benthic health and benchmark for future potential disturbances in the region. Northwestern Cuba slope is still an understudied portion of the GoM and more research is needed to fully understand its biodiversity and biogeochemical patterns.

Widespread and increasing near-bottom hypoxia in the coastal ocean off the United States Pacific Northwest

The 2021 summer upwelling season off the United States Pacific Northwest coast was unusually strong leading to widespread near-bottom, low-oxygen waters. During summer 2021, an unprecedented number of ship- and underwater glider-based measurements of dissolved oxygen were made in this region. Near-bottom hypoxia, that is dissolved oxygen less than 61 µmol kg−1 and harmful to marine animals, was observed over nearly half of the continental shelf inshore of the 200-m isobath, covering 15,500 square kilometers. A mid-shelf ribbon with near-bottom, dissolved oxygen less than 50 µmol kg−1 extended for 450 km off north-central Oregon and Washington. Spatial patterns in near-bottom oxygen are related to the continental shelf width and other features of the region. Maps of near-bottom oxygen since 1950 show a consistent trend toward lower oxygen levels over time. The fraction of near-bottom water inshore of the 200-m isobath that is hypoxic on average during the summer upwelling season increases over time from nearly absent (2%) in 1950–1980, to 24% in 2009–2018, compared with 56% during the anomalously strong upwelling conditions in 2021. Widespread and increasing near-bottom hypoxia is consistent with increased upwelling-favorable wind forcing under climate change.

Multi-Timescale Characteristics of Southwestern Australia Nearshore Surface Current and Its Response to ENSO Revealed by High-Frequency Radar

The surface currents in coastal areas are closely related to the ecological environment and human activities, and are influenced by both local and remote factors of different timescales, resulting in complex genesis and multi-timescale characteristics. In this research, 9-year-long, hourly high-frequency radar (HFR) surface current observations are utilized together with satellite remote sensing reanalysis products and mooring data, and based on the Empirical Orthogonal Function (EOF) and correlation analysis, we revealed the multi-timescale characteristics of the surface currents in Fremantle Sea (32°S), Southwestern Australia, and explored the corresponding driving factors as well as the impact of El Niño-Southern Oscillation (ENSO) on the nearshore currents. Results show that the currents on the slope are dominated by the southward Leeuwin Current (LC), and the currents within the shelf are dominated by winds, which are subject to obvious diurnal and seasonal variations. The strong bathymetry variation there, from a wide shelf in the north to a narrow shelf in this study region, also plays an important role, resulting in the frequent occurrence of nearshore eddies. In addition, the near-zonal winds south of 30°S in winter contribute to the interannual variability of the Leeuwin Current at Fremantle, especially in 2011, when the onshore shelf circulation is particularly strong because of the climatic factors, together with the wind-driven offshore circulation, which results in significant and long-lasting eddies. The southward Leeuwin Current along Southwestern Australia shows a strong response to interannual climatic variability. During La Niña years, the equatorial thermal anomalies generate the westward anomalies in winds and equatorial currents, which in turn strengthen the Leeuwin Current and trigger the cross-shelf current as well as downwelling within the shelf at Fremantle, whereas during El Niño years, the climate anomalies and the response of coastal currents are opposite. This paper provides insights into the multi-timescale nature of coastal surface currents and the relative importance of different driving mechanisms. It also demonstrates the potential of HFR to reveal the response of nearshore currents to climate anomalies when combined with other multivariate data. Meanwhile, the methodology adopted in this research is applicable to other coastal regions with long-term available HFR observations.

Western Boundary Current–Subtropical Continental Shelf Interactions

Western boundary currents (WBCs) adjacent to subtropical continental shelves (STCSs; between ~25° and 35° latitude; Figure 1) transport heat, nutrients, and biota poleward along the western margins of major ocean basins, interacting with the continental margins and influencing their physics and biology. Eddies and meanders along the shelf edge upwell deep, nutrient-laden water that can be advected onto the adjacent shelves with a corresponding export of particle-rich shelf water (e.g., Lee et al., 1991; Kimura et al., 1997; Campos et al., 2000; Roughan and Middleton, 2002, 2004; Lutjeharms, 2006; Savidge and Savidge, 2014). Despite their similarities, the various STCS regions display key differences with respect to boundary current strength and variability, shelf width and geometry, and trophic structure. Comparative analyses of the physical forcing and biological responses among STCS have the potential to reveal common underlying properties, forcing mechanisms, and sensitivities to climatic perturbations that are not possible to elucidate with region-specific studies. This kind of fundamental understanding of relationships between physics and biological responses is critical to predicting consequences of environmental change across a wide range of spatiotemporal scales.

Algal cover as a driver of diversity in communities associated with mussel assemblages across eastern Pacific ecoregions

AbstractResearch on intertidal mussel assemblages and associated communities has revealed that complexity and structure are influenced by environmental heterogeneity and local‐scale factors affecting recruitment. Research in situ in eastern and western Pacific intertidal ecosystems has suggested drivers of species diversity and community structure encompassing large geographic scales, however, there are major gaps in geographic coverage. Our aim is to fill some of these gaps by analyzing macrofaunal functional group diversity and effects of environmental factors on intertidal mussel communities from three distinct marine ecoregions in the southern and northern hemispheres. We identified the effects of algal cover and environmental heterogeneity on species richness and evenness, and we modeled factors effecting mussel layer complexity from assemblages in three marine ecoregions. We analyzed macrofaunal species diversity within one of the austral ecoregions based on the width of the coastal shelf. Species richness was highest in samples from the northern hemisphere while evenness was highest in samples from the southern hemisphere. Similarity in functional group structure for all communities sampled was ≤55% (Bray–Curtis dissimilarity) and ≤35% (Chao–Jaccard dissimilarity). Wave exposure had a significant effect on shell length and complexity of mussel matrices on rocky bench platforms. The presence of algal cover had a strong effect on species richness in mussel matrices regardless of complexity, while algal canopies had no effect on species evenness. Overall, this study provides significant new insight on the community complexity of mussel beds in parts of the world which have been poorly studied in this regard.

Coastal vulnerability assessment along the coast of Kerala, India, based on physical, geological, and socio-economic parameters

The increase in population and rapid economic development activities increase vulnerability along the coastal areas. Coastal zones are among the planet’s most dynamic regions and are susceptible to natural and anthropogenic hazards. To comprehend how risks and hazards are distributed spatially along the coast, coastal vulnerability assessment is crucial. This study aims at identifying the vulnerable zones along the coast of Kerala, India. For this, the coastal vulnerability index (CVI) is calculated by considering the physical, geological, and socio-economic parameters. The physical-geological parameters considered are nearshore bed slope, shoreline change rate, coastal elevation, tidal range, and continental shelf width. The socio-economic parameters include population density, road network, and land use/land cover. Analytical hierarchy process (AHP) is used for obtaining weights for physical-geological and socio-economic parameters. Coastal vulnerability results show that 82.40 km of the coastal stretch has a very high CVI value along the coast of Kerala, accounting for 13.87% of the total coastline. The most vulnerable stretches are along the coast of Kozhikode, Malappuram, Thrissur, Ernakulam, Alappuzha, and Trivandrum districts while Kasargod and Kannur districts have no coastal stretches with very high CVI. High population density, low nearshore bed slope, low coastal elevation, high erosion rates, built-up areas, and roads near to the coastline are some of the factors contributing to very high CVI in coastal districts of Kerala.

Bayesian Hierarchical Modeling for Probabilistic Estimation of Tsunami Amplitude From Far‐Field Earthquake Sources

AbstractEvaluation of tsunami disaster risk for a coastal region requires reliable estimation of tsunami hazard, for example, wave amplitude close to the shore. Observed tsunami data are scarce and have poor spatial coverage, and for this reason probabilistic tsunami hazard analysis (PTHA) traditionally relies on numerical simulation of “synthetic” tsunami generation and propagation toward the coast. Such an approach has been extensively studied in the past and it is widely recognized as an important disaster‐risk mitigation tool. PTHA can not only provide less uncertain and spatially coherent hazard estimates in comparison with classical empirical data analysis which is restricted at the tide gauge stations, but also local inundation information. In this paper, we explore a purely statistical alternative to traditional PTHA for evaluation of tsunami amplitude hazard. Here, we use tide gauge measurements of tsunami amplitude along the western United States, specifically California and Oregon, and develop a spatial Bayesian hierarchical model (BHM) to assess tsunami hazard from far‐field earthquake sources at various recurrence intervals. The configuration of our model incorporates latent Gaussian fields that utilize information on the distance between tide gauges as well as on the continental shelf width, that is, a covariate linked to potential dissipative effects on wave energy as the tsunami travels over shallow water. Through our BHM, we produce spatially continuous probabilistic maps of far‐field tsunami hazard which can aid comprehensive tsunami disaster risk reduction and management.

Protocol for a web survey experiment studying the feasibility of asking respondents to capture and submit photos of the books they have at home and the resulting data quality.

This document presents the protocol of a study conducted as a part of the WEB DATA OPP project, which is funded by the H2020 program. The study aimed to investigate different aspects of the collection of images through web surveys. To do this, we implemented a mobile web survey in an opt-in online panel in Spain. The survey had various questions, some of which were about the books that the participants have at their main residence. The questions related to books were asked in three different ways: regular survey questions showing visual examples of how different numbers of books fit in a 74 centimetre wide shelf depending on their thickness, regular survey questions without the visual examples, and questions where participants were asked to send photos of the books at their home. This report explains how the study was designed and conducted. It covers important aspects such as the experimental design, the questionnaire used, the characteristics of the participants, ethical considerations, and plans for disseminating the results.

Physical mechanisms affecting phytoplankton variability along the Chilean coast

Chile has high phytoplankton production due to being a classic example of an Eastern Boundary Upwelling System. Monthly averaged chlorophyll-α (Chl) and physical parameters (sea surface temperature, precipitation rate, southerly and westerly winds) were studied off the Chilean coast from 2002 to 2018, in order to understand the primary production along this important ocean margin. The coastal margin was split into three zones and ten sub-sections. The Northern Zone had a low phytoplankton production with small seasonal variability, except in its north. This pattern is due to a narrow shelf, weak winds, lack of precipitation and relatively stable weather conditions driven by the Southeast Pacific Subtropical Anticyclone (SPSA). The Central Zone presented a seasonally varying production, with a high Chl concentration in summer and early spring. This is linked to the SPSA movement and sunlight reduction during the winter. A high Chl activity is seen in the Southern Zone despite this Zone being at the limits of the SPSA effect, leading to weak longshore winds only during the warm season. Overall, this study has demonstrated the importance of shelf width and the upwelling driven by the presence or absence of the SPSA for ocean primary production. Thus, the most productive region is from 35°S to 45°S owing to both variables being present.

Labral Support Shelf Acetabuloplasty for Late Presenting Perthes Disease: Outcomes in Indian Patients.

The treatment of late-presenting Perthes disease with extrusion is controversial and debatable. One of the options available is the labral shelf acetabuloplasty (LSA). The aim of the study was to evaluate the results of LSA in late-presenting Perthes disease in terms of clinic-radiological outcome measures. A retrospective analysis of prospectively collected data of patients with late presenting Perthes disease (Elizabethtown stage 2B onwards) treated by LSA by 2 experienced paediatric orthopaedic surgeons was performed. Data was collected of clinical parameters such as hip range of motion(ROM) and Harris Hip score and radiological parameters such as acetabular height, width and volume, shelf width, Centre Edge angle(CEA) and the lateral extrusion. Thirty-five patients (28 males and 7 females) treated between 2012 to 2019 were analyzed. Majority were in Elizabethtown stage 3A (23) followed by 2B and 3B (12 each). At a mean follow up of 36months, the hip ROM and the Harris Hip Score (from 65 ± 3.5 to 81.33 ± 7.12) improved significantly and there was a statistically significant improvement in terms of all radiological parameters. Majority of the hips were in Stulberg grade 3 (20) followed by grade 1 and 2 (7 each) and Stulberg 4 (1). There were no major complications in any of the patients of the series. Labral support shelf acetabuloplasty is a valuable surgery for late presenting Perthes disease and helps in maintenance of good coverage and allows restoration of range of motion over time.

Wide shelves as discriminators between hinterland tectonic and intra-basin rifting signals

Decoding interwoven tectonic and climatic drivers in global source-to-sink systems remains a major challenge in Earth science. This study demonstrates that the very wide shelf in the northern South China Sea acted as a critical discriminator of signals related to hinterland tectonism and basin rifting. The rifting process is the main driver of variations of detrital zircon (DZ) signals in the succession of the wide shelf. DZ data show an increasing proportion of intrabasinal sediment source that is in phase with seafloor spreading and crustal extension (ca. 32–16 Ma), and then a gradual increase of extrabasinal source until the Pleistocene, in step with post-rifting thermal subsidence. These DZ variations are free of the strongest Cenozoic tectonism, the uplift of the eastern Tibet-Himalayan orogen since ca. 23 Ma, although this regional uplift dominated the DZ-signal variations in the entire deep-water basinal areas of the northern South China Sea basins fed by the Central Canyon of the Qiongdongnan Basin. Moreover, rising of global sea-level driven by the Miocene Climatic Optimum had little impact on sediment delivery in the shelf-to-basin system. The strong decoupling of DZ-variation trends between the shelf and the deep basin in this study underscores the importance of wide shelves in distinguishing different allogenic drivers in global source-to-sink systems.

Incidence and Reflection of Offshore Subinertial and Barotropic Pressure Signals in Wide Shelf Seas

Abstract The response of a wide shelf to subinertial and barotropic offshore pressure signals from the shelf edge was investigated. By relaxing the semigeostrophic approximation, an elliptical wave structure equation was formulated and solved with the integral transform method. It was found that when the imposed offshore signal has an along-shelf length scale similar to the shelf width, it can efficiently break the potential vorticity barrier and propagate toward the coast, producing a significant coastal sea level setup. Thereafter, the pressure signal reflects from the coast or the sloping topography, producing a transient eddy and propagates to the downshelf. The intensities of the coastal setup and the eddy increase as the along-shelf scale of the subinertial signal decreases or when its time scale is close to the inertial period. For a signal with longer time scale, the eddy is insignificant. The nature of the shelf response is controlled by the shelf conductivity κ ≡ r/(fsB), in which r is the Rayleigh friction coefficient, f is the Coriolis parameter, s is the shelf slope, and B is the shelf width, respectively. For a given offshore signal, coastal setup increases with κ. For large κ, the eddy energy is concentrated at low modes, producing a large eddy, whereas a small κ produces a small eddy. The proposed theory can explain coastal sea level fluctuations under eddy impingement in the Mid-Atlantic Bight or other similar areas. Significance Statement Coastal sea level and shelf circulation are greatly affected by offshore pressure signals, e.g., mesoscale eddy impingements or boundary current fluctuations. It is often assumed that the along-shelf length scale of the forcing is much larger than the shelf width, i.e., the semigeostrophic approximation. Here in this study, we found this approximation significantly underestimates the shelf–ocean interaction. A general shelf wave equation was developed that relaxed the semigeostrophic approximation and was solved analytically with a novel mathematical method. The solution can characterize the shelf response to subinertial offshore forcing at arbitrary spatiotemporal scales. It was found that for a subinertial signal with scale close to or smaller than the shelf width, significant coastal sea level setup and transient eddy can be formed, which was consistent with realistic phenomena. The new theory could promote the understanding of coastal sea level variations and along-/cross-shelf transports at synoptic and intermediate scales.

Intercomparison of hydrostatic and nonhydrostatic modeling for tsunami inundation mapping

Nonhydrostatic modeling has emerged as an effective tool for seismological and tsunami research for over a decade, but its general application in hazard mapping and engineering design remains a topic of discussion. The approach incorporates the depth-averaged vertical velocity and nonhydrostatic pressure in the nonlinear shallow-water equations that provide a Poisson-type equation via the conservation of mass for quasi-three-dimensional flows. After the 2011 Tohoku tsunami, the State of Hawaii augmented the existing inundation maps to account for probable maximum tsunamis from Mw 9.3 and 9.6 Aleutian earthquakes. The use of both hydrostatic and nonhydrostatic modeling with a common set of telescopic computational grids covering 1330 km of shorelines facilitates a thorough intercomparison under distinct extreme events over a range of tropical island terrain and bathymetry. Including vertical flow dynamics can enhance the formation of a slowly attenuating trough behind the leading crest across the ocean as well as drawdown of receding water over steep nearshore slopes. The nonhydrostatic approach consistently gives lower predictions of the offshore tsunami amplitude due to frequency dispersion but can produce more severe coastal surges from resonance of the leading crest and trough over insular slopes as well as trapping of tsunami waves by wide shelves. Despite the potential for underestimating coastal surges, the lack of vertical inertia in hydrostatic models can result in substantially larger runup over steep terrain. The tsunami processes leading to inundation are complex with a strong dependence on the waveform and topography that can be well elucidated by the nonhydrostatic approach.

Controls on continental shelf width: A machine learning approach

The continental shelf edge marks the transition between shallow- and deep-water environments, and records the cumulative influence of subaerial and submarine processes. The width of the shelf therefore dictates where, and how efficiently, particulates are transferred into the deep-ocean. Previous studies have shown that shelf width is heavily influenced by eustasy and tectonics through geological time, however the dominant subaerial and submarine controls on shelf width during the present-day highstand remains unquantified on a continental scale. Using 15 climatic, hydrographic, oceanographic and tectonic variables, coupled with machine learning algorithms, we assess the dominant predictors of shelf width along 51,000 km of the continental margin of the Americas. Results show that the thickness of sediment accumulated on the shelf and the seismicity of the nearest onshore catchment best predict shelf width, with thinner sediment accumulations and more seismically-active catchments resulting in narrower shelves. Tectonics is therefore a first order predictor of shelf width, with the narrowest shelves associated with tectonically-active, steep and low-accommodation margins. These findings support the view that tectonics is the dominant control on shelf width variation across the Earth's surface in the present-day, and indicate that tectonics will modify the width of the shelf during all sea-level stands.