800 research outputs found
Crafts in the Southern Mountains
No full textThis article appeared in the 1931, November/December issue of "Handicrafter" magazine. It traces author "P.B."'s travels to various craft centers in the southern Appalachian mountains. Mentioned in the article are Allanstand Cottage Industries, The Spinning Wheel, Clementine Douglas, Winogene Redding, Evelyn Bishop, Mrs. Stone, Blue Ridge Weavers, Penland's Weaving Institute, Penland Weavers and Potters, Crossnore School, Mrs. H. N. Johnson, Pi Beta Phi Fraternity School, and Berea College's Fireside Industries. The author only gives a brief impression of his or her visit to each place. It is likely that author P.B. is Paul Bernat, editor of "Handicrafter" magazine
Deterministic and Stochastic Modelling of Ocean Surface Waves
No full textPredicting the mean wave statistics in the nearshore, for instance the significant wave height, has predominantly been the domain of operational stochastic wave models based on the radiative transport (or energy balance) equation. Although reasonably successful in the nearshore, these models were originally developed for oceanic scales, and necessarily neglect or parametrise processes that are only significant in shallow water, such as the linear processes of interference and diffraction, or the nonlinear triad wave-wave interactions and dissipation due to wave breaking. In this dissertation we investigate the possibility of predicting the wave statistics on small scales in strongly non-linear conditions, such as found in the surfzone, using the recently developed Surface WAves till SHore (SWASH) model, whereas on larger scales we pursue a generalisation of existing stochastic models by incorporating coherent effects, hereby extending these models to include interference and diffractive effects.Hydraulic EngineeringCivil Engineering and Geoscience
Coherent interference and diffraction in random waves
No full textWave fields traveling through a varying medium (e.g. topography, currents), can develop well-defined focal zones and caustics, where the wave field is highly coherent and wave statistics vary rapidly. However, the presence and evolution of such coherent structures in the wave field are not resolved in a quasi-homogeneous description of the wave field (e.g. the radiative transport equation), and a more general description of the wave statistics (and its evolution) is needed. In this work we demonstrate with numerical examples that, when using a (recently developed) transport equation for the second order inhomeogeneous wave statistics that accounts for cross-variance contributions, we can resolve coherent structures in wave fields such as those typically found in focal and diffraction zones. What this shows is, that in a statistical sense, diffraction is essentially an interference phenomenon that can be readily resolved if cross-phase information in the transport equations is retained.Hydraulic EngineeringCivil Engineering and Geoscience
Non-hydrostatic modelling of large scale tsunamis
No full textThe Indian Ocean Tsunami has once again revived the discussion in the tsunami modelling community if the non-linear shallow water equations are a valid model for the propagation of tsunamis. It is suggested that the mechanism of frequency dispersion which is absent in these equations might be important in the correct modelling of large scale tsunamis. In this master Thesis a non-hydrostatic numerical model based upon the scheme proposed by Stelling and Zijlema (2003) is constructed and it is investigated if it can be an effective and efficient way to include the effect of frequency dispersion in the modelling of tsunamis in their propagation and run-up. The non-hydrostatic algorithm is incorporated into the existing explicit shallow water solver of XBeach. In this way the model is extended to allow for shorter wave propagation. The main reason for doing this was to show that the employed non-hydrostatic scheme can be easily incorporated as a simple add-on. The depth averaged formulation of the XBeach model prevented an easy extension towards multiple layers but, for a single layer, the addition of the non-hydrostatic pressures was indeed straightforward. No large modifications to the existing code where required. The numerical model is based on the application of mehrstellen verfahren for the pressure gradients in the vertical. This makes it possible to exactly set the surface pressure to zero which is important for the correct modelling of surface waves. The advective terms have been included in a momentum conservative way based on Stelling and Duinmeijer (2002). This allows for the correct modelling of braking waves. The resulting 2DV model is validated with analytical solutions available for: (i) an oscillating basin (ii) the propagation of a solitary waves (iii) the run-up of long waves on a beach and (iv) the dambreak solution. Furthermore the model is verified using experimental data by Synolakis (1987) on the run-up of solitary waves on a plane beach. In all cases it is concluded that the results are satisfactory. The 2DV model is subsequently expanded into a 3D model which is validated with a 3D version of the oscillating basin and verified with the Berkhoff shoal which includes shoaling, refraction and diffraction of waves. A surprising result is that the model using only a single layer is able to satisfactorily reproduce the measurements. The numerical model is applied to two tsunami benchmark tests conducted by Briggs (1995). The first test consists of the run-up of solitary waves on a vertical wall while the second deals with the run-up of solitary waves on a conical island. From the first test it is concluded that the model can correctly model these types of waves using only a single layer. Furthermore, when compared to hydrostatic solutions, the model is a dramatic improvement. The over steepening, typical of the non-linear shallow water equations, does not occur. From the results of the second test it is concluded that the model can accurately predict the inundation heights. However, very fine grids where needed due to the excessive numerical diffusion introduced by the upwind approximations. It can be concluded that the non-hydrostatic model by Stelling and Zijlema can indeed be an attractive way to include frequency dispersion into large scale tsunami propagation models. It is anticipated that the non-hydrostatic terms add about fifty percent to the duration of a simulation.Civil Engineering and Geoscience
Validation of X-Band radar derived hydrodynamic phenomena
No full textThe coastal area is a dynamic environment that suffers from human interventions and climate change. This requires extensive research and monitoring of the coastal areas. Measurement systems play a key role in the monitoring. Current velocities are for example often measured to investigate local hydrodynamics. At the moment the ADCP is often used to measure current velocities. The disadvantage of this measurement system is the lack of spatial information. New innovative measurement systems are developed to overcome this disadvantage. The X-Band radar is an example of such a measurement system. The X-Band radar can be used to survey water surfaces resulting in images with wave information. These images can be used to derive current, depth and wave information. The main advantage of the X-Band radar is the spatial extend. The radar images have a range over several kilometres. The aim of this study is to validate X-Band derived current velocities. SeaDarQ is an algorithm that can be used to process the X-Band radar images and derive depth, current and wave information. The technique used in the SeaDarQ algorithm was developed by (Young et al., 1985). A 3D Fourier Transformation is used to gain the wave information from the images. Subsequently the linear dispersion relation, which describes the fixed relation between wavenumber and wave frequency as a function of the depth and the current, is fitted through the obtained wave information to derive the depth and the current. The algorithm is analysed to find the valid depth for the derived currents. The analysis has revealed that the current is fitted using the short waves obtained from the images. The active depth of a wave depends on the ratio between the wave length and water depth. A long wave is active over a larger part of the water column than a short wave. The consequence of using the short waves for the current fit is therefore that the derived for the top part of the water column. A case study is done to validate the current output. A X-Band radar is installed near the Sand Motor, which is a large scale nourishment at the Dutch coast at ter Heijde. The velocity output from point of the SeaDarQ output field is compared with ADCP measurements during different metocean conditions. It can be concluded from the case study that the X-Band derived velocities are indeed valid for the top part of the water column. The measurement period includes calm periods as well as a severe storm. Wind from sea and from land is observed including wind speeds between zero and twenty meter per second. The wave height varied between 28 cm and 5 meters. The data comparison revealed that SeaDarQ is not able to derive current velocities with wind speeds lower than 2 m/s or heavy rain conditions. All other circumstances resulted in accurate velocity output. Also during stratified conditions, resulting from a fresh plume passing by, the SeaDarQ output is accurate. SeaDarQ can cope with the broad variety of conditions due to the current fitting procedure in which only the short waves are included. The short waves are less easily affected by the bottom than long waves due to their small active depth. The short waves follow therefor the linear dispersion relation very well. The same reasoning holds for the stratified conditions. The stratification does not affect the short waves that much due to the small active depth. The short waves follow the linear dispersion relation therefore very well during these conditions. This new measurement system can derive accurate current velocities and can therefor become a powerful measurement system in the future.Coastal EngineeringHydraulic EngineeringCivil Engineering and Geoscience
The Evolution of Inhomogeneous Wave Statistics through a Variable Medium
No full textThe interaction of ocean waves with variable currents and topography in coastal areas can result in inhomogeneous statistics because of coherent interferences, which affect wave-driven circulation and transport processes. Stochastic wave models, invariably based on some form of the radiative transfer equation (or action balance), do not account for these effects. The present work develops and discusses a generalization of the radiative transfer equation that includes the effects of coherent interferences on wave statistics. Using multiple scales, the study approximates the transport equation for the (complete) second-order wave correlation matrix. The resulting model transports the coupled-mode spectrum (a form of the Wigner distribution) and accounts for the generation and propagation of coherent interferences in a variable medium. The authors validate the model through comparison with analytic solutions and laboratory observations, discuss the differences with the radiative transfer equation and the limitations of this approximation, and illustrate its ability to resolve coherent interference structures in wave fields such as those typically found in refractive focal zones and around obstacles.Hydraulic EngineeringCivil Engineering and Geoscience
Topography-induced focusing of random waves
No full textRefraction of narrow-band surface waves in coastal areas can result in wave-focal zones where due to interference, wave statistics vary rapidly and on similar length scales as those of individual waves. However such interference patterns, or wave coherence, are not accounted for in conventional stochastic wave models that are based on the energy balance equation or radiative transfer equation. In this work we present a quasi-coherent theory, which is an extension of the radiative transfer equation and quasi-homogeneous theory. We show that this new stochastic modelling approach can resolve rapid variations in wave statistics that occur in the vicinity of a wave caustic. The results compare favourably to those obtained from ensemble averages calculated with a deterministic phase resolving model (SWASH) and, in a focal zone, constitute a significant improvement over those obtained with a conventional stochastic wave model based on an energy balance equation (SWAN).Hydraulic EngineeringCivil Engineering and Geoscience
Stochastic modeling of coherent wave fields over variable depth
No full textRefractive focusing of swell waves can result in fast-scale variations in the wave statistics because of wave interference, which cannot be resolved by stochastic wave models based on the radiative transport equation. Quasi-coherent statistical theory does account for such statistical interferences and the associated wave inhomogeneities, but the theory has thus far been presented in a form that appears incompatible with models based on the radiative transfer equation (RTE). Moreover, the quasi-coherent theory has never been tested against field data, and it is not clear how the coherent information inherent to such models can be used for better understanding coastal wave and circulation dynamics. This study therefore revisits the derivation of quasi-coherent theory to formulate it into a radiative transport equation with a forcing term that accounts for the inhomogeneous part of the wave field. This paper shows how the model can be nested within (or otherwise used in conjunction with) quasi-homogeneous wave models based on the RTE. Through comparison to laboratory data, numerical simulations of a deterministic model, and field observations of waves propagating over a nearshore canyon head, the predictive capability of the model is validated. The authors discuss the interference patterns predicted by the model through evaluation of a complex cross-correlation function and highlight the differences with quasi-homogeneous predictions. These results show that quasi-coherent theory can extend models based on the RTE to resolve coherent interference patterns and standing wave features in coastal areas, which are believed to be important in nearshore circulation and sediment transport.Hydraulic EngineeringCivil Engineering and Geoscience
Simulating nearshore wave transformation with non-hydrostatic wave-flow modelling
No full textHydraulic EngineeringCivil Engineering and Geoscience
Non-hydrostatic modelling of infragravity waves using SWASH
No full textThis paper presents numerical modelling of the nearshore transformation of infragravity waves induced by bichromatic wave groups over a horizontal and a sloping bottom. The non-hydrostatic model SWASH is assessed by comparing model predictions with analytical solutions over a horizontal bottom and with detailed laboratory observations for a sloping bottom. Good agreement between model predictions and data is found throughout the domain for bound infragravity waves. Furthermore the model predicts greater outgoing free infragravity wave-heights for steeper slope regimes which is consistent with the measurements. The model however tends to overestimate the magnitude of the outgoing infragravity waves.Hydraulic EngineeringCivil Engineering and Geoscience
- …
