Beach cusp morphodynamics on a composite beach observed using UAV structure from motion

Beach cusps are often more prominent on beaches composed of coarse or mixed sediments, as opposed to sand alone. For cusps on mixed sediment beaches, one of the most important morphological characteristics to quantify is the surface sediment texture distribution, differentiating between sand and gravel units on the beachface. Despite this, most research methods to date have been unable to accurately quantify textural zonation, and instead focus on the bed level change as a whole across the beachface. However, new combinations of sensors and technology, such as drone-mounted cameras and Structure from Motion approaches allows for the direct coupling of beach morphological evolution to surface sediment texture. In this contribution we apply image segmentation algorithms to drone imagery in order to classify the extent of surface sand and gravels, and compare this to bed level changes in digital elevation models. The explicit coupling of sediment texture and morphological response shows the beach cusp features to be largely accretionary, with gravel horns initially forming on a sandy beachface. The gravel horns are seen to progressively grow in size, creating well developed beach cusps. As accretion continues, the sandy cusps bays are filled with gravel sediments, resulting in the formation of an alongshore-uniform regular berm feature. This study highlights the utility of drone based sensors to provide a full suite of morphological and textural information about mixed sediment beaches, not achievable with traditional survey methods. Moving forward, this level of information can be used to investigate and understand the relative importance of the two sediment fractions in controlling the form and behaviour of mixed sediment beach cusps