Grain size variability on a rip-channeled beach

Abstract Grain size is an important variable when predicting beach morphodynamics. Beaches, to the eye, seem relatively uniform in grain size and morphodynamic modeling efforts usually assume a single mean grain size for an entire beach environment. Therefore, estimating grain size is traditionally done by collecting only a few samples and averaging to characterize the mean grain size of the whole beach. However, some studies have shown that even small variations in grain size can have a significant effect on model results when predicting beach morphology changes. Here, a mobile digital imaging system (DIS) has been developed for surveying spatial and temporal variation in grain size across a beach following the ideas of Rubin (2004). Using an off-the-shelf camera and underwater housing, macro photographs are taken of sand across a beach, which produce estimates of mean grain size that are highly correlated with estimates from sieves (R 2  = 0.92). High resolution maps of mean surface grain size are produced using the DIS (with ~ 1000 images over a 300 × 500 m area), which suggest that large variations in grain size exist (0.2–0.7 mm over tens of meters with accuracies of ~±0.03 mm) and that there is a correlation between spatial grain size variations and morphological variability.