Vertical Profiles of Longshore Currents PDF Download
Are you looking for read ebook online? Search for your book and save it on your Kindle device, PC, phones or tablets. Download Vertical Profiles of Longshore Currents PDF full book. Access full book title Vertical Profiles of Longshore Currents by Carlos Manuel da Costa Ventura Soares. Download full books in PDF and EPUB format.
Author: Carlos Manuel da Costa Ventura Soares
Publisher:
ISBN:
Category : Ocean circulation
Languages : en
Pages : 84
Get Book Here
Book Description
Author: Carlos Manuel da Costa Ventura Soares
Publisher:
ISBN:
Category : Ocean circulation
Languages : en
Pages : 84
Get Book Here
Book Description
Author: Carlos Manuel da Soares Costa Ventura
Publisher:
ISBN:
Category :
Languages : en
Pages : 65
Get Book Here
Book Description
The vertical structure of the mean longshore current is examined on three strong current days during the DUCK94 experiment and it is found well described by a logarithmic profile (mean correlation coefficient for all 22 profiles, 0.98). This hypothesis works better in the trough where turbulent bottom boundary layer processes are predominant than over the bar, where breaking wave induced turbulence generated at the surface modifies the profile. A relationship between near bottom vertical velocity profiles and bottom roughness was found. The bed shear stress coefficient varied by an order of magnitude across the surf zone (0.001-0.05). For the three days considered, it is concluded that the bed shear stress coefficient increased with increasing bottom roughness, and therefore is an important parameter to characterize the bottom boundary layer. The influence of the wind is parameterized by an exponential approximation that works well in 50% of the analyzed profiles. This parameterization assumes that the residual Data Normal distribution of data is only due to the wind and does not consider the alongshore component of mass transport velocity, which is assumed small. These results should be treated carefully since there were obtained for only three days over a barred beach with strong longshore currents and cannot be generalized without further studies.
Author: John Casey Church
Publisher:
ISBN:
Category : Littoral drift
Languages : en
Pages : 140
Get Book Here
Book Description
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.
Author: Antonio Fernando Garcez Faria
Publisher:
ISBN:
Category : Science
Languages : en
Pages : 182
Get Book Here
Book Description
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.
Author: Antonio Fernando Garcez Faria
Publisher:
ISBN:
Category : Ocean circulation
Languages : en
Pages : 52
Get Book Here
Book Description
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.
Author: Guy Allen Meadows
Publisher:
ISBN:
Category : Littoral drift
Languages : en
Pages : 200
Get Book Here
Book Description
This field investigation was conducted to obtain simultaneous and continuous measurements of the horizontal, vertical and temporal variability of the longshore current flow field. The present study has resulted in a two-dimensional mapping, across the surf zone and with depth, of the longshore current field. The vertical structure of the mean longshore current flow field is nearly uniform with depth, with a narrow bottom boundary layer and sharp velocity gradients at the water-sediment interface. This investigation has also shown that the total longshore current velocity vector, at any point across the surf zone, is composed of three distinct velocity components. These components are: (1) a steady longshore current velocity component; (2) a long-period fluctuating velocity component which tends to be out-of-phase with the incident wave field and; (3) a short-period fluctuating longshore current velocity component which tends to be in-phase with the incident wave field. The results of this study have further indicated that neither the deterministic radiation stress approach to the prediction of longshore currents, nor a probabilistic formulation, provide adequate prediction of the magnitude or distribution of the longshore current velocity across the surf zone. In addition, the existence of a low velocity zone in the longshore current flow field has been isolated over the submarine bar. It appears that existing analytical formulations for longshore current flow prediction must be re-evaluated in light of the findings of this study. (Author).
Author: Carolina Burnette
Publisher:
ISBN:
Category : Ocean currents -- Florida -- Melbourne Beach -- Statistics -- Evaluation
Languages : en
Pages :
Get Book Here
Book Description
A seasonal and long-term analysis of the vertical structure of currents in the nearshore is conducted to determine the role of the wind in driving currents and consequently affecting littoral transport processes. Approximately ten years (January, 2002 – October, 2011) of nearshore current profiles are examined using the data collected with an Acoustic Doppler Current Profiler (ADCP) installed off of Spessard Holland North Beach Park located in Melbourne Beach, Florida. Additionally, wind data collected with a directional anemometer from September, 2002, until October, 2008, are used to further characterize the long-term hydrodynamic forcing. With the shoreline oriented nominally 17o west of magnetic north, both the current profiles and the wind vectors have been rendered into longshore and cross-shore components. The water level record from a NOAA tide station located at the Trident Pier at nearby Port Canaveral is utilized in establishing the water depth and conditioning the data for statistical analysis. Monthly mean vertical profiles reveal that during the winter months the surface currents are usually toward the south, and toward the north in the summer. In spring and fall, they are mixed, demonstrating a clear seasonality in both direction and intensity of the longshore current. Subjecting the longshore and cross-shore current data to Empirical Orthogonal Function Analysis reveals that the first spatial Eigenfunction accounts for more than 98% of the variability in the vertical profile of the longshore current, and more than 86% of the variability in the profile of the cross-shore current. However, there is a rotation of the current to the right (clockwise) with the rotation angle increasing and the variance decreasing with depth below the surface. The spiral structure of the water column follows a surface Ekman veering, but for very shallow water. The upper layer of the current is almost aligned with the direction of the wind. Monthly correlations between 2-hour average time series of longshore current and 2-hour average time series of wind speed reveal the seasonal patterns of the wind and longshore current in which the upper layer of the water column is highly correlated with the longshore component of the wind speed for most of the year and slightly less correlated for the lower layer of the water column. Most importantly, on average, wave height (Hmo) is larger when the longshore current is heading to the south (Hmo=0.95 m) than when the current is going to the north (Hmo=0.73 m). Additionally, there is a stronger correlation between southerly directed currents and incident wave energy flux than northerly directed currents and wave energy flux. These results indicate that the net long-term north-to-south sediment transport known to characterize the region is heavily influenced by wind-driven currents.
Author: Dennis James Whitford
Publisher:
ISBN:
Category : Banks (Oceanography)
Languages : en
Pages : 238
Get Book Here
Book Description
Previous investigations of longshore currents have included simplifying assumptions and restriction (such as a planar beach, a steady and depth uniform flow, spatially-variant bed shear stress and turbulent momentum exchange, and the exclusion of surface wind stress. These assumptions are quantitatively investigated by calculating the relative importance of each term in the longshore momentum balance with an emphasis on the relative importance of wind forcing across the barred nearshore. Wind and wave forcing of longshore currents across a barred beach are examined using both a numerical model and field measurements. A local momentum balance was measured at various locations across the surf zone during the SUPERDUCK experiment held at the USACE CERC Field Research Facility, Duck, N.C. in October 1986. A moveable sled was instrumented with pressure, current, and wind sensors to measure the various terms in the longshore momentum equation. Stability-dependent atmospheric drag coefficients for the surf zone are determined from wind stress measurements acquired just beyond the surf zone and wind speed measurements acquired from an anemometer atop the 9 m sled mast. Breaking waves were visually identified and electronically marked on the data tapes. Keywords: Ocean currents; Air water interactions; Nearshore surf zone; Wind stress; Theses. (EDC).
Author: Ferris Webster
Publisher:
ISBN:
Category :
Languages : en
Pages : 14
Get Book Here
Book Description
Author: Jorgen Fredsoe
Publisher: World Scientific Publishing Company
ISBN: 9814365688
Category : Technology & Engineering
Languages : en
Pages : 390
Get Book Here
Book Description
This book treats the subject of sediment transport in the marine environment, covering transport of noncohesive sediment by waves and currents in- and outside the surf zone. It can be read independently, but a background in hydraulics and basic wave mechanics is required.The primary aim of the book is to describe the physical processes of sediment transport and how to represent them in mathematical models. The book can be divided in two main parts; in the first, the relevant hydrodynamic theory is described. This part contains a review of elementary theory for water waves, chapters on the turbulent wave boundary layer and the turbulent interaction between waves and currents, and finally, surf zone hydrodynamics and wave driven currents.The second part covers sediment transport and morphological development.The part on sediment transport introduces the basic concepts (critical bed shear stress, bed load, suspended load and sheet layer, near-bed concentration, effect of sloping bed); it treats suspended sediment in waves and current and in the surf zone, and current and wave-generated bed forms. Finally, the modelling of cross-shore and long-shore sediment transport is described together with the development of coastal profiles and coastlines.
