Cobble Tracking Observations at Torrey Pines State Beach, CA, USA

Cobbles provide a nature-based method to help generate beach stability. However, few detailed cobble movement observations exist. This study deployed 344 radio-frequency identification tagged cobbles in a California beach in November-December 2020. Coincident LiDAR surveys quantified beach morphology. Cobbles were mapped daily for 10 days and then similar to monthly until January 2023. Cobble detection rates ranged 17%-92%, and generally declined with time (with slight increases during winter months). Cobble movement exhibited complex patterns, sometimes moving in opposing directions during the same time period. Large winter waves (up to similar to 4 m) resulted in average displacements of 40 m per month between November 2020 and April 2021. From April 2021 to August 2022, most cobbles were located high on the beach and net alongshore movements were relatively low, despite several moderate size (similar to 2 m) wave events. The largest wave event in January 2023 (>4 m) moved cobbles to the highest elevations of the study period. The initial release location and accommodation space in the back beach influenced cobble movement and final position. Cobbles high on the beach were relatively stable compared to lower elevation cobbles, and more than half of all cobble detections were within 50 m of the initial release location. However, three cobbles moved >500 m. The probability distribution of displacement was approximately exponential. Statistically, alongshore cobble spreading followed a non-Gaussian subdiffusive process. Despite myriad sources of noise, the results suggest that cobble shape and mass were related to maximum cobble displacement. Overall, displacements increased with incident wave energy and depended on elevation relative to total water level. Plain Language Summary Cobbles may provide a nature-based method to help generate beach stability and prevent backshore erosion. However, few detailed observations of cobble motion exist. To better understand cobble dynamics, this study installed radio-frequency identification tags inside cobble to allow individual cobble tracking. Three hundred forty four tagged cobbles were released in a California beach and tracked for 26 months. Cobble movement exhibited complex patterns, sometimes moving in opposing directions during the same time period. Cobble displacement increased with wave energy and detection rates generally decreased with time. Large winter waves resulted in average displacements of 40 m per month. Over the study period, three cobbles moved >500 m. However, many cobbles moved to, or remained at, higher, more stable elevations on the furthest inland portion of the beach. The largest wave event moved cobbles to the highest observed elevations. Cobbles high on the back of the beach were relatively stable compared to lower elevation cobbles. More than half of all cobble detections were within 50 m of the initial release location. The initial release location and accommodation space on the back beach influenced cobble movement and final position. The results suggest that shape and mass influence cobble movement.