On the use of variance images for runup and shoreline detection

•use of variance images for runup and shoreline detection;•theoretical justification and experimental confirmation through time stack images;•study performed for 3 beaches in Spain and New Zealand;•images for the analysis include low and high tide conditions;•good agreement in the estimates from time stacks and variance images

The use of variance images for runup and shoreline detection is analyzed in this work. A methodology based on the variance images is first motivated with synthetic time stacks and then confirmed with real images. A total of 55 time stack images from five different cameras at three video stations (in Spain and New Zealand) were digitized and used for the study. These images include different tide and wave conditions. From the time stack images we: 1) obtained the mean shoreline position and maximum runup manually digitizing the water/sand interface and, 2) computed the variance function. The variance function is shown to have a particular cross-shore shape, with two characteristic positions which allow us to satisfactorily recover the mean shoreline position and maximum runup obtained in the first step. Variance images were shown to be useful both for detecting mean shoreline, and for evaluating runup, providing a potential alternative for timex images. The method did not work well when runup occurred within the groundwater seepage face.