Infra-gravity Waves and Cross-shore Transport -- A Conceptual Study

Infra-gravity waves are generally known as small-amplitude waves of periods between 25 seconds and 5 minutes. They originate from the presence of wave groups in the open ocean waves and can move freely after being released near the surf zone where they can be further fueled with energy from the spatially varying break point of swell waves . As these waves approach the shore, the relative importance of the infra-gravity wave signal increases, and its impact on the shorter waves gets stronger. In addition, infra-gravity waves drive strong cross-shore currents, which lead to significant back-and-forth motion of the underlying sea water. This strong cross-shore motion has been made visible by recent field studies, where significant cross-shore movement was detected and found to be correlated with the infra-gravity wave signal. In the present work, the connection between infra-gravity waves is explored further using linear wave theory and an established numerical nearshore wave model (BOSZ). It is shown that in all cases, the presence of infra-gravity waves leads to strong cross-shore motion. This behavior can be understood by considering the infra-gravity waves as separate free waves, and then following the fluid particle trajectories excited by these waves. As it is shown, these trajectories have a very large horizontal extent which -- if not separated from the main gravity wave field -- appears as a large, but often not directly visible back-and-forth motion, underlying the more readily observable gravity ocean waves.Comment: 13 pages, 5 figure

Generation and propagation of ship-borne waves - Solutions from a Boussinesq-type model

Ship-borne waves are of significant interest for the design of port and waterway infrastructure and the maintenance of its surrounding environment. Computation of these nonlinear and dispersive waves has mainly been focusing on their near-field generation as a fluid-body interaction problem. This study presents an approach for the computation of ship waves generated by a moving pressure disturbance with phase-resolving and depth-averaged equations. To support a wide range of applicability, the paper deals with the evolution of the vessel wedge compared to an analytical solution for sub-to supercritical speeds and the assessment of wave patterns from a broad range of pressure term dimensions, including cross-references to findings in other studies. The conducted numerical experiments showcase the typical response of a Boussinesq-type model to a simplified moving pressure disturbance and identify the main factors and criteria for ship-wave propagation in the far-field of a vessel. Finally, a unique field dataset underlines the capability of an extended Boussinesq-type model to compute the propagation of vessel waves over an irregular bathymetry.Hamburg Port Authority (HPA)Japanese Society for the Promotion of Sciences (JSPS)EC/FP7/Marie Curie International Outgoing Fellowshi

Considering socio-political framings when analyzing coastal climate change effects can prevent maldevelopment on small islands

Adapting to climate change and sea level rise is challenging on small islands. False adaptation can lead to adverse impacts on natural and societal dynamics. Therefore, an interdisciplinary perspective on the interaction of natural dynamics, societal demands, and political decisions is crucial. In this sense, this study scrutinizes coastal processes and socio-political dimensions of erosion on the reef island Fuvahmulah, the Maldives. The national government and Fuvahmulah’s population have an opposed perception and attribution of the drivers and processes behind Fuvahmulah’s most pressing coastal issue – coastal erosion. To review these perceptions, natural dynamics are recreated with process-based methods and discussed regarding present and projected marine pressures. Population surveys and interviews with actors in coastal development complement the physical insights into erosion on Fuvahmulah and describe the socio-political dimension of climate change adaptation on small islands. This interdisciplinary approach demonstrates how small-islands’ adaptive capacities are typically impaired and disclose the potential of local knowledge to overcome maldevelopment

Benchmarking of Numerical Models for Wave Overtopping at Dikes with Shallow Mildly Sloping Foreshores: Accuracy versus Speed

To accurately predict the consequences of nearshore waves, coastal engineers often employ numerical models. A variety of these models, broadly classified as either phase-resolving or phase-averaged, exist; each with strengths and limitations owing to the physical schematization of processes within them. Models which resolve the vertical flow structure or the full wave spectrum (i.e. sea-swell (SS) and infragravity (IG) waves) are considered more accurate, but also more computationally demanding than those with approximations. Here, we assess the speed-accuracy trade-off of six well-known wave models for overtopping (q), under shallow foreshore conditions. The results demonstrate that: i) q is underestimated by an order of magnitude when IG waves are neglected; ii) using more computationally-demanding models does not guarantee more accurate results; and iii) with empirical corrections to account for IG waves, phase-averaged models like SWAN can perform on par, if not better than, phase-resolving models but with far less computational effort

Forecasts of Wave-Induced Coastal Hazards in the United States Pacific Islands: Past, Present, and the Future

This paper summarizes the existing coastal hazard forecast methods of PacIOOS, such as wave-induced run-up, by focusing on the critical components that need to be addressed in order to improve these forecasts and make them more accurate and available to broader coastal communities. We then propose that a horizontally, two-dimensional numerical modeling approach method should be adopted for developing future wave-induced coastal forecasts. To reach a future in which real-time two-dimensional model-based forecasts are a reality, we identify existing technologies that could lead to improvements, such as: (i) more accurate, accessible and frequently updated bathymetry and topography datasets; (ii) increased computational and software capabilities; and, (iii) more accurate sea level datasets. These advances, combined with crowdsourced-based model-data validation, will result in faster and more accurate forecasting tools that could greatly benefit coastal communities in need of more efficient risk mitigation programs

Boussinesq-type model for nearshore wave processes in fringing reef environment

Ph.D. University of Hawaii at Manoa 2010.Includes bibliographical references.The extended lagoons and steep flanks of most fringing reefs produce unique coastal processes that are challenging to numerical wave models developed for continental shelf conditions. This dissertation describes the formulation and validation of a coastal wave model applicable to fringing reef environment. The governing Boussinesq-type equations, which include a continuity and a momentum equation with conserved variables, contain the conservative form of the nonlinear shallow-water equations for shock capturing. The finite volume method with a Godunov-type scheme provides a conservative numerical procedure compatible to the present governing equations. A fifth-order TVD (Total Variation Diminishing) reconstruction procedure evaluates the inter-cell variables, while a directional splitting scheme with a Riemann solver supplies the inter-cell flux and bathymetry source terms in the two-dimensional horizontal plane. Time integration of the governing equations provides the conserved variables, which in turn provide the flow velocities through a linear system of equations derived from the dispersive terms in the momentum equations. The model handles wave breaking through momentum conservation based on the Riemann solver without the use of predefined empirical coefficients for energy dissipation. A series of numerical experiments verify the dispersion characteristics of the model. The computed results show very good agreement with laboratory data for wave propagation over a submerged bar, wave breaking and runup on plane beaches as well as wave transformation over fringing reefs. The model accurately describes transition between supercritical and subcritical flows as well as development of dispersive waves in the processes

Sudden wave flooding on steep rock shores: a clear but hidden danger

It is shown that very steep coastal profiles can give rise to unexpectedly large wave events at the coast. We conduct a statistical analysis of runs from a nearshore Boussinesq-type model to demonstrate that under certain wave conditions, which a casual observer would perceive as calm, the likelihood of large run-up events is uncharacteristically high. The data computed by the Boussinesq-type model show that sea states with lower overall wave steepness favor higher run-up. Under these wave conditions, more of the available wave energy reaches the shore, since less wave breaking occurs, which can create a false sense of security for beach-goers