Longshore and seasonal variations in beach sand, Humboldt County, California : Implications for bulk longshore transport direction

Subaerial beach sediments were sampled at 80 stations along 62 kilometers of the central Humboldt County shoreline. Samples were taken during winter and summer to represent both the fully eroded and fully accreted conditions of the beach. Beach samples were analyzed for the grain size distribution parameters of mode, mean, standard deviation, and skewness. The 1.75 phi to 2.75 phi size fractions were recombined and analyzed for both heavy mineral to light mineral ratios and for heavy mineralogy. Three samples each from the Mad and Eel Rivers, which are the chief sediment contributors to the study area, were subjected to a similar petrographic analysis. Results of the grain size analyses and of the analyses of the ratio of heavy minerals to light minerals indicate that the net annual longshore drift is generally northward, and that the Eel River is the chief source of sediment in the study area. The entrance channel to Humboldt Bay appears to be selectively trapping coarse particles (500 microns) as sediments are passed through the channel system by longshore drift. Heavy/light ratios along the beach show strong seasonal differences probably related to selective sorting associated with cross-shore sand movements. Heavy mineral comparisons indicate that river sediments undergo a selective sorting in transport to the beach which obscures the provenance of beach materials.