Longshore Current on a Barred Beach: Field Measurements and Calculation

Measurements of the longshore current on a barred beach made during the 1990 Duck Experiment on Low-Frequency and Incident-Band Longshore and Across-Shore Hydrodynamics (DELILAH) field data collection project conducted at Duck, North Carolina, revealed an unexpected and persistent broad peak in the current velocity in the trough between the nearshore bar and the shore. This paper introduces longshore current and associated wave measurements from DELILAH together with a numerical model capable of describing the field observations. An existing numerical model of the mean current is modified to include a general transport equation for the mean turbulent kinetic energy created by wave breaking, and Reynolds stress components needed to calculate the longshore current and mean water level are reexpressed to include the turbulent momentum transport. In comparison to predictions from the original model, the modified model produces much improved agreement with the measured current velocity on the barred profiles in the field measurements and with the velocity profile and mean water level generated on a uniformly sloping laboratory beach by monochromatic waves. Three forms of a bottom friction coefficient are examined, and the open-channel-flow Manning friction coefficient is selected because of best agreement and consistency. Values of the Manning friction coefficient required to calibrate the model agree with values normally assigned to the related bottom roughness in open-channel flow.

[1]  S. Pawka.  Island shadows in wave directional spectra , 1983 .

[2]  I. A. Svendsen,et al.  WAVE CHARACTERISTICS IN THE SURF ZONE , 1978 .

[3]  Edward B. Thornton,et al.  Surf Zone Longshore Currents and Random Waves: Field Data and Models , 1986 .

[4]  Douglas L. Inman,et al.  Wave ‘set-down’ and set-Up , 1968 .

[5]  W. Dorn.  SET-UP AND RUN-UP IN SHOALING BREAKERS , 1976 .

[6]  W. Rodi,et al.  Predictions of Heat and Mass Transfer in Open Channels , 1978 .

[7]  W. Dally.  Random breaking waves: A closed-form solution for planar beaches , 1990 .

[8]  R. Holman,et al.  Infragravity Waves, Longshore Currents, and Linear Sand Bar Formation , 1991 .

[9]  M. Longuet-Higgins,et al.  Radiation stresses in water waves; a physical discussion, with applications , 1964 .

[10]  Nicholas C. Kraus,et al.  Numerical Model of Longshore Current for Bar and Trough Beaches , 1991 .

[11]  R. J. Sobey,et al.  Cross-shore wave transformation and mean flow circulation , 1990 .

[12]  P. J. Visser,et al.  The proper longshore current in a wave basin , 1982 .

[13]  J. A. Roelvink,et al.  Bar-generating cross-shore flow mechanisms on a beach Barre provoquant des mecanismes d' ecoulement perpendiculaires a la plage , 1989 .

[14]  Ivar G. Jonsson,et al.  WAVE BOUNDARY LAYERS AMD FRICTION FACTORS , 1966 .

[15]  Edward B. Thornton,et al.  Transformation of wave height distribution , 1983 .

[16]  K. Nadaoka.  A Fundamental Study on Shoaling and Velocity Field Structure of Water Waves in the Nearshore Zone , 1986 .

[17]  Howard N. Southgate,et al.  Transition Zone Width and Implications for Modeling Surfzone Hydrodynamics , 1991 .

[18]  S. Baum,et al.  A NUMERICAL INVESTIGATION OF THE LONGSHORE CURRENT PROFILE FOR MULTIPLE BAR/TROUGH BEACHES , 1986 .

[19]  James J. McGuirk,et al.  A depth-averaged mathematical model for the near field of side discharges into open-channel flow , 1978, Journal of Fluid Mechanics.

[20]  Jørgen Fredsøe,et al.  Modelling of undertow by a one-equation turbulence model , 1991 .

[21]  I. James.  A non-linear theory of longshore currents , 1974 .

[22]  P. J. Visser,et al.  UNIFORM LONGSHORE CURRENT MEASUREMENTS AND CALCULATIONS , 1984 .

[23]  E. Thornton.  VARIATION OP LONGSHORE CURRENT ACROSS THE SURF ZONE , 1970 .

[24]  H. G. Wind,et al.  Cross-shore mean flow in the surf zone , 1986 .

[25]  R. Holman,et al.  Shear instabilities of the mean longshore current: 1. Theory , 1989 .

[26]  W. Birkemeier,et al.  Shear instabilities of the mean longshore current: 2. Field observations , 1989 .

[27]  P. Bradshaw,et al.  Turbulence Models and Their Application in Hydraulics. By W. RODI. International Association for Hydraulic Research, Delft, 1980. Paperback US $15. , 1983, Journal of Fluid Mechanics.

[28]  A. Townsend.  The Structure of Turbulent Shear Flow , 1975 .

[29]  William A. Birkemeier,et al.  Beach and nearshore survey data, 1981-1984, CERC Field Research Facility / by Peter A. Howd, William A. Birkemeier, Coastal Engineering Research Center ; prepared for Department of the Army, US Army Corps of Engineers. , 1987 .

[30]  M. Longuet-Higgins.  Longshore currents generated by obliquely incident sea waves: 1 , 1970 .

[31]  P. Visser.  Laboratory measurements of uniform longshore currents , 1991 .

[32]  J. Battjes,et al.  Modeling of Turbulence in the Surf Zone , 1975 .

[33]  I. A. Svendsen.  Analysis of surf zone turbulence , 1987 .

[34]  C. Mei.  The applied dynamics of ocean surface waves , 1983 .

[35]  M. S. Longuet-Higgins,et al.  Longshore currents generated by obliquely incident sea waves: 2 , 1970 .

[36]  I. A. Svendsen.  Wave Heights and Set-up in a Surf Zone , 1983 .

[37]  Robert G. Dean,et al.  Wave height variation across beaches of arbitrary profile , 1985 .

[38]  V. T. Chow.  Open-channel hydraulics , 1959 .