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.
In some special locations, because of physical barriers or climatic anomalies, waves approach from only one direction. In this case, the net transport can be identical to the gross transport. In most instances, however, this is not the case. Studies of the potential for longshore transport, based upon directional measurements of nearshore waves at a large number of coastal locations, have shown that the net transport is often a very small difference between large values of positive and negative transport. For example, at Oceanside Harbor in California, during the entire year of 1980, the net transport potential observed was less than 1% of the gross transport (Castel and Seymour 1986).
- Castel D, Seymour RJ (1986) Coastal data information program longshore sand transport report, February 1978–December 1981. Institute of Marine Resources, University of California, IMR Reference No. 86-2. 216 ppGoogle Scholar