Longshore drift: Trapped in an expected universe

On the basis of a review of current practice in coastal science and engineering with regard to quantitative determination of longshore drift, we conclude that coastal scientists and engineers have been trapped in an expected universe of longshore-transport sand volumes without critical assessment of assumptions made in pioneering studies. As a result, workers in this field have come to accept a range of sand volumes and a range of techniques to measure or predict these volumes, stabilized by opinion of the leading experts. The shortcomings of previous studies and subsequent practice, however, indicate that these transport volumes and techniques may well be wrong. Certainly at present we have no dependable, verifiable, and consistent field measure of net or gross instantaneous or annual longshore transport volumes against which predictions can be compared. Both field measurements and mathematical model results, especially as used for applied purposes and expressed in annual terms, are suspect for a number of reasons. These include questionable assumptions that lag far behind our current understanding of shoreface processes, ``fudge factor'' coefficients for equations, the lack of storm transport measurement, and a cascade of uncertainties that moderate current practice. Once determined, annual longshore-transport sand volumes tend to be long-lived and in all cases are unverifiable by field measurement. More fundamentally, the question is raised whether an earth surface system as complex and variable as longshore transport on beaches can ever be quantitatively modeled or measured successfully. Qualitative mathematical modeling remains a valid and useful approach to understanding the nature of sedimentary processes.