Field observations of sand transport by shoaling waves

Abstract

A study of wave-generated currents and associated sand transport in the offshore zone of southern California has modified several previously-recognized theories concerning sedimentary processes in this region. A set of passing waves generates oscillating currents at the sea floor which have a spectrum of velocities whose average strengths over broad shelves are predicted accurately by classical wave theory. Long period swells and offshore breezes cause a net transport of bottom water towards the beach, while short period waves and onshore winds are associated with neutral or seaward flow. Ripple marks on the shallow sea floor, whose dimensions are a function of sediment size and current velcity, allow all available sand sizes to be moved with equal ease. A ripple remains in the same position even though sediment and water may be differentially transported. Most bottom sand transport occurs in the lowest few centimeters of the water column at a rate which varies with the square of current strength. Fine sand-grain size populations migrate in the direction of the greatest current velocities as a unit without being sorted. During winter in southern California, rip currents transport beach sand to the offshore where it is confined by predominant seaward oscillations caused by steep waves and strong winds. Rips are not active in calm summer months, and long period swells produce shoreward surge asymmetry which replenishes the beach with sediment.