Longshore Sediment Transport

Excerpt

Longshore transport refers to the cumulative movement of beach and nearshore sand parallel to the shore by the combined action of tides, wind, and waves and the shore-parallel currents produced by them. These forces usually result in an almost continuous movement of sand either in suspension or in bedload flows (see entry on Cross-Shore Sediment Transport). This occurs in a complex, three-dimensional pattern, varying rapidly with time. At any moment, some sand in the area of interest may have an upcoast component while other sand is moving generally downcoast. The separation of the total transport into components parallel and perpendicular to the shore is artificial and is done as a convenience leading to a simpler understanding of a very complex environment. To be meaningful, the rate of longshore transport must be averaged over intervals of at least many wave periods and is typically predicted or measured over much longer times, ranging up to a year (see entries on Gross Transport and Net Transport). It is also summed algebraically, at least conceptually, in the direction perpendicular to the shoreline. The result is that the transport rate is defined either as the volume of sand passing a point on the shore in a unit time (Watts, 1953), or the immersed weight of sand passing per unit time (Inman and Bagnold, 1963). The latter method accounts for the average density of the sand, which can vary significantly between beaches comprised principally of carbonate rather than quartz sand. …