Nearshore Currents Over a Barred Beach
Nearshore Currents Over a Barred Beach

Author: Antonio Fernando Garcez Faria

Publisher:

Published: 1997

Total Pages: 182

ISBN-13:

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The objective of this dissertation is to develop numerical models and compare their predictions with data acquired during the DUCK94 experiment in order to improve our physical understanding of the hydrodynamic processes governing the vertical and cross shore distributions of both longshore and cross shore currents over a barred beach. The vertical structure of the mean longshore current is found to be well described by a logarithmic profile and a relationship between bed shear stress and bottom roughness, including the influence of ripples and mega-ripples, was also found. The vertical structure of the mean cross shore current (undertow) is modeled using an eddy viscosity closure scheme to solve for the turbulent shear stress and includes contributions from breaking wave rollers. These models of the vertical profiles of longshore and cross shore mean currents are combined to formulate a quasi three dimensional model to describe the cross shore distribution of the longshore current. This model includes turbulent mixing due to the cross shore advection of mean momentum of the longshore current by the mean cross shore current and contributions from wave rollers.

A Simple Quasi-three Dimensional Model of Longshore Currents Over Arbitrary Profile
A Simple Quasi-three Dimensional Model of Longshore Currents Over Arbitrary Profile

Author: Antonio Fernando Garcez Faria

Publisher:

Published: 1995

Total Pages: 52

ISBN-13:

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The long shore current maximum observed in the trough of a barred beach during the nearshore dynamics experiment DELILAH at Duck, North Carolina, is not predicted by present theory. The simplest longshore curren models balance cross-shore changes in the alongshore wave momentum (radiation stress) with the alongshore bottom shear stress. Waves break over the bar, reform in the trough and again break on the foreshore resulting in changes in the radiation stress, which predicts two jets, one over the bar and the other at the foreshore, which does not agree with the observed current maximum in the trough. The advection of the momentum of the longshore current by mean cross-shore currents as a source of momentum mixing is investigated. The longshore current is strongest toward the surface and decreasing to zero at the bottom. The cross-shore mean current has an onshore transport in the wave crest/trough region and an offshore transport beneath (undertow). The net interaction can induce significant lateral mixing of the alongshore momentum of the mean currents, which is shown using a simplified three- dimension model of nearshore currents to explain much of the differences with observations.

Topics in Longshore Currents
Topics in Longshore Currents

Author: John Casey Church

Publisher:

Published: 1993

Total Pages: 140

ISBN-13:

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The momentum equation governing mean longshore currents on straight beaches is a balance of forcing from the momentum transfer of the oscillatory wave motion, turbulent momentum transfer (mixing), and bottom stress. Of these, the wave's contribution is well understood, but the remaining two are not, principally due to the complicated hydrodynamics of the surf-zone. Addressing the bottom stress term, a longshore current model is developed which includes a modification of the bottom stress due to the effects of breaking-wave induced turbulence. A one-dimensional turbulent kinetic energy equation is used to model this breaking-wave induced turbulence, producing a spatially varying bottom friction coefficient. The modeled longshore current cross-shore profiles show improved agreement with field observations. In a second bottom stress study, vertical profiles of mean longshore currents are examined using field data obtained with vertically stacked electromagnetic current meters with the goal of measuring the bottom stress and its associated drag coefficient. The profiles are observed to become vertically uniform whenever the ratio of wave height to depth exceeds 0.3, indicating that nearly all of the waves passing a given location are breaking. Finally, horizontal turbulent momentum transfer (mixing) is examined for the case of shear instabilities of the longshore current.

Advances In Coastal And Ocean Engineering, Vol 2
Advances In Coastal And Ocean Engineering, Vol 2

Author: Philip L-f Liu

Publisher: World Scientific

Published: 1996-07-03

Total Pages: 290

ISBN-13: 9814499838

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This volume contains six papers discussing coastal processes, and physical and numerical modeling.In the first paper, Svendsen and Putrevu give an extensive review on the state of understanding of surf-zone hydrodynamics, including subjects such as wave breaking, wave-induced currents, and instability of nearshore currents and infragravity waves. They point out that the most urgent need is to develop an adequate theory for wave breaking and broken waves in the surf zone.One of the methods for studying the complex coastal processes is to perform laboratory experiments. However, physical models are always plagued by scale and laboratory effects, because the coastal process involves many different length and time scales. In the second paper, Kamphuis presents a detailed discussion on the sources and implications of the scale and laboratory effects on physical modeling.The third and the fourth papers are two parts of the discussion on the mathematical modeling of the meso-tidal barrier island coasts. To understand the dynamics of coastal inlet systems, one can either rely on empirical knowledge and construct various forms of empirical and semi-empirical models (Part I), or develop a set of mathematical models based on the physical processes (Part II). Although these models do not provide the details of the dynamics, they give valuable knowledge of the equilibrium-state relationships. de Vriend and Ribberink give a detailed review on two models, Initial Sedimentation/Erosion models and Medium-Term Morphodynamic models. They have also presented many examples of applications.In the fifth paper, Houston gives a brief review on different methods to mitigate beach loss caused by storms or persistent long-term erosion. He then describes, in detail, the method of beach nourishment, which is also called a beach fill. This paper discusses the information that must be collected to design a beach fill and that should be monitored after the completion of the project.The last paper of this volume shifts our attention to the design of offshore structures, such as gravity structures, floating barges and tankers. Chakrabarti discusses the effects of the uniform and shear currents on fixed and floating structures.

Advances in Coastal and Ocean Engineering
Advances in Coastal and Ocean Engineering

Author: Philip L. F. Liu

Publisher: World Scientific

Published: 1996

Total Pages: 290

ISBN-13: 9810224109

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This volume contains six papers discussing coastal processes, and physical and numerical modeling.In the first paper, Svendsen and Putrevu give an extensive review on the state of understanding of surf-zone hydrodynamics, including subjects such as wave breaking, wave-induced currents, and instability of nearshore currents and infragravity waves. They point out that the most urgent need is to develop an adequate theory for wave breaking and broken waves in the surf zone.One of the methods for studying the complex coastal processes is to perform laboratory experiments. However, physical models are always plagued by scale and laboratory effects, because the coastal process involves many different length and time scales. In the second paper, Kamphuis presents a detailed discussion on the sources and implications of the scale and laboratory effects on physical modeling.The third and the fourth papers are two parts of the discussion on the mathematical modeling of the meso-tidal barrier island coasts. To understand the dynamics of coastal inlet systems, one can either rely on empirical knowledge and construct various forms of empirical and semi-empirical models (Part I), or develop a set of mathematical models based on the physical processes (Part II). Although these models do not provide the details of the dynamics, they give valuable knowledge of the equilibrium-state relationships. de Vriend and Ribberink give a detailed review on two models, Initial Sedimentation/Erosion models and Medium-Term Morphodynamic models. They have also presented many examples of applications.In the fifth paper, Houston gives a brief review on different methods to mitigate beach loss caused by storms or persistent long-term erosion. He then describes, in detail, the method of beach nourishment, which is also called a beach fill. This paper discusses the information that must be collected to design a beach fill and that should be monitored after the completion of the project.The last paper of this volume shifts our attention to the design of offshore structures, such as gravity structures, floating barges and tankers. Chakrabarti discusses the effects of the uniform and shear currents on fixed and floating structures.