LONGSHORE TRANSPORT PREDICTION — SPM 1973 EQUATION

Abstract

The 1973 Shore Protection Manual (SPM) predicts longshore transport rates that are 83% higher than its 1966 predecessor, for the same wave conditions. This upward revision is the result of concurrent increases due to (1) deletion of all laboratory data used to establish the 1966 prediction, (2) addition of Komar's (1969) field observations, and (3) limiting energy flux values computed from previously unused data obtained at Santa Barbara (Johnson, 1952). A derivation based on conservation of energy shows that P. is the longshore component of the energy flux confined between two wave orthogonals spaced a unit distance apart in the longshore direction, and that a term previously identified as the onshore component of energy flux is identical with the total energy flux in the direction of wave travel between these orthogonals. Use of submerged weight transport rates has no engineering benefit at the present time because: (1) all available data are in terms of volume rates, (2) conversion to submerged weight requires estimates of the void ratio and sand grain density which have been assumed constant in practice, and (3) the engineering problem needs volume rates which would require reconversion back to volume rates if an immersed weight prediction were established.