Longshore Drift Rate Calculator

Longshore drift is the shore-parallel flux of sand driven by waves that break at an oblique angle to the beach. The CERC formula treats the problem energetically: the longshore component of wave power at breaking, Pl = (rho g Hb squared / 16) times Cb times sin(2 alpha), is assumed to be converted into sediment motion with a fixed efficiency K. Breaker celerity Cb comes from shallow-water theory as the square root of g times Hb divided by the breaker index gamma, conventionally 0.78. Dividing the immersed weight rate K times Pl by the submerged specific weight of the bed, (rho_s minus rho_w) times g times (1 minus porosity), converts it to a bulk volume. Quartz sand takes rho_s near 2650 kg per cubic meter and porosity near 0.40, against seawater at about 1025 kg per cubic meter. The sin(2 alpha) term peaks at a breaker angle of 45 degrees and vanishes at both 0 and 90 degrees, so transport is acutely sensitive to angle in the 5 to 30 degree band that most natural beaches occupy. Reported net rates span under 10,000 cubic meters per year on sheltered coasts to well over 500,000 on exposed ones. The main limitations are that K is not truly constant, falling from roughly 0.77 for fine sand toward 0.1 on gravel beaches, and that the formula omits grain size, beach slope, and wave period entirely. It also assumes unlimited sediment supply, so it overpredicts on rock platforms. Treat the output as an order-of-magnitude estimate requiring field calibration.

Systematic study of littoral transport began with wartime and postwar amphibious engineering. The US Army Corps of Engineers established the Beach Erosion Board in 1930, and its successor the Coastal Engineering Research Center consolidated the energy-flux approach into the Shore Protection Manual, whose editions of 1973, 1977, and 1984 fixed what practitioners still call the CERC or SPM formula. The physical basis was assembled in the preceding two decades: Per Bruun, Paul Bagnold, and others linked transport to available wave energy, while Robert Inman and Douglas Inman with Thorndike Bagnold established the immersed-weight formulation that made the coefficient K dimensionless and therefore transferable between sites. The theoretical underpinning for the driving current came from Michael Longuet-Higgins and Rodney Stewart, who introduced the concept of radiation stress in the early 1960s and showed in 1970 how the along-shore momentum flux of breaking waves generates the longshore current. Field calibration followed through tracer and impoundment experiments, notably at Silver Strand and Point Mugu in California, which anchored K near 0.39 when significant wave height is used. Recognizing the formula's insensitivity to sediment properties, J. William Kamphuis published an alternative in 1991 that explicitly includes median grain size, beach slope, and wave period. Both remain in use, and the Corps replaced the Shore Protection Manual with the online Coastal Engineering Manual beginning in 2002.