Net Transport

Excerpt

Net transport refers to the difference between the total upcoast and the total downcoast movement of sand approximately parallel to the shore over a specified period, often one year (see entries on Cross-Shore Sediment Transport and Longshore Sediment Transport). Waves approaching the shore at an oblique angle, such that the crests of the breakers are at an angle to the shoreline, generate longshore currents that convey the mobilized sand along the shore (see entry on Waves). In many locales, the wave approach direction varies over time such that this transport can be in either direction relative to the shore. Consequently, sand may move downcoast (positive direction) for an interval and then, as wave conditions change or the lesser effects of tidal or wind-driven currents intervene, reverse and move upcoast (negative direction). Summing the volumes of sand transport over the period of interest and taking into account their signs yields net transport. The customary units of net transport are volumetric rates (cubic meters per year). The terms littoral drift and longshore drift are occasionally used to mean either net transport or gross transport (see entry on Gross Transport) so caution must be taken in their interpretation. …