Longshore sand transport on beaches

Abstract

Simultaneous field measurements of wave and current parameters in the surf zone and the resulting longshore transport of sand have been made on two beaches under a variety of conditions. The direction and flux of wave energy was measured from an array of digital wave sensors placed in and near the surf zone. Quantitative measurements of the longshore sand transport rate were obtained from the time history of the center of gravity of sand tracer. The measurements have been used to test two models for the prediction of the longshore transport rate of sand. The first model gives the immersed weight longshore transport rate of sand, Il, as proportional to the longshore component of wave energy flux (power), Il = K(ECn)b sin αb cos αb, where E is the energy density, Cn is the wave group velocity, and αb is the breaker angle. The second model assumes that the waves provide the power to move and support the sand and that the superimposed longshore current 〈νl〉 provides a longshore component that results in the longshore transport of sand according to the relationship Il = K′ (ECn)b cos αb〈νl〉/um, where um is the magnitude of the maximum horizontal component of orbital velocity near the bottom under the breaking wave, assumed to be proportional to the rate of energy dissipation by friction on the beach bed. The measurements show that both models successfully predict the sand transport rate, with values of the dimensionless coefficients K = 0.77 and K′ = 0.28. The coherence of the models implies that they are interrelated, their common solution giving the relation as 〈νl〉 = K″ um sin αb, where K″ is a dimensionless constant equal to 2.7. This relation can be obtained directly by equating the longshore current and the longshore component of the momentum flux (radiation stress) of the breaking waves. Thus, the coherence of the models appears to be based on the generation of the longshore currents by the longshore radiation stress. The models will not be equivalent if 〈νl〉 owes its origin to some other generating mechanism such as tides or winds.

Variability and trends in the directional wave climate of the Southern Hemisphere

“…The strength and direction of these currents are dependent on the direction of the waves off the continental shelf. For example, the most widely used equation for longshore sediment fluxes, Q S , is known as the CERC equation (Komar and Inman, 1970;Komar, 1998):…”

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“…Conocer el espesor de este «río de arena» es de gran interés para calcular el transporte sedimentario longitudinal (Komar e Inman, 1970;Inman et al, 1980;Kraus, 1985;Sunamura y Kraus, 1985) y el flujo sedimentario asociado a los ciclos de marea (King, 1951;Otvos, 1965y Williams, 1971) y a los temporales (Nicholls y Orlando, 1993).…”

“…Sunamura y Kraus (1985) desarrollaron un modelo basado en las variaciones de la fuerza de rozamiento de los granos, obteniendo una relación teóri-ca entre la profundidad de mezcla (Z) y la altura de la ola en rompiente (H b ): Z = 0,027 H b . King (1951), Otvos (1965), Komar e Inman (1970), Williams (1971) y Jackson y Nordstrom (1993) midieron la profundidad de removilización durante un ciclo mareal en playas micro y mesomareales.…”

Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain

“…Waves and currents were monitored by a pressure transducer and an electromagnetic current meter respectively. Following Komar and Inman (1970), fluorescent tracers served to investigate longshore and cross-shore transport and disturbance depth.…”

“…In the laboratory, the cores were split and then cut into 2-cm slices which were dried in the open air and weighed on a balance with a precision of 0.01 g. In each of the seven slices selected for the study, marked fluorescent grains were manually counted under an ultraviolet lamp and the results evaluated by means of the spatial integration method (Komar and Inman 1970). The location of the centroid-i.e.…”