Advanced Monitoring of Subsurface Sediment Structures Within Gravel Berms

Abstract

Berms composed of surface gravel and underlying sand and gravel mix (gravel berms) naturally form on beaches and can help mitigate coastal erosion and flooding. Previous studies suggest that subsurface sediments influence gravel berm behavior, although detailed investigations remain limited. Here, we present a novel integrated field approach to quantitatively monitor gravel berm subsurface sediments. From March to October 2023, surface and subsurface sediments were characterized using combined methods including mechanical excavation, image-based grain size analysis, GNSS, and ground penetrating radar (GPR) surveys, and compared with gravel berm topography at two beaches (South Carlsbad and Torrey Pines) in southern California with contrasting sediment characteristics. South Carlsbad consistently exhibited backshore gravel exposure and limited seasonal sand volume changes, whereas Torrey Pines exhibited varying backshore gravel exposure and more pronounced seasonal sand volume fluctuations. At both beaches, the gravel berms consistently had a surface pure-gravel (PG) layer overlying a mixed sand-gravel layer, with the PG layer thickness decreasing seaward. At Torrey Pines, the upper gravel berm profile changed seasonally from spring to late summer as sand gradually accumulated within the berm (resulting in a PG layer thickness decrease) and varied subsurface sediment composition. In contrast, in South Carlsbad, both upper gravel berms and subsurface sediment structures exhibited seasonal consistency, with little change in PG layer thickness. The combined approach enables detailed and repeatable assessment of gravel berm subsurface sediments, offering further insights into the links between internal sediment structure and surface morphology. The present results inform a new conceptual model of seasonal gravel berm evolution.