Observations of surface cobbles at two southern California beaches

Abstract

Southern California beaches are often sandy, but with largely undocumented cobble patches and berms. We describe the first multi-year, spatially extensive observations of surface cobbles at southern California beaches. The variation (spatial and temporal) of surface cobble distribution and backshore (e.g. upper foreshore) cobble morphology (slope, vertical extent, and elevation) are characterized using 11 years (2008–2018) of observations spanning 4.8 km of shoreline at Cardiff and Torrey Pines State beaches. Quarterly Global Positioning System surveys of beach elevation and visually identified sediment type (either sand or cobble) are used to create 1076 cross-shore profiles. Cobbles were not exposed continuously at either of these predominantly sandy beaches. The 202 cross-shore profiles with backshore cobbles and sandy foreshores are used for morphology analysis. Consistent with previous studies, cobble and sandy morphologies varied seasonally, and cobbles were most exposed during winter when wave energy increased. Extensive (vertical extent ≥ 2 m) backshore cobbles were usually fronted by low elevation, low slope sandy foreshores. At Torrey Pines cobble patches were occasionally exposed mid-profile on primarily sandy summer beaches. Persistent (2008–2015) backshore cobbles at Cardiff were apparently buried in 2016 by effects of a 2012 sand nourishment. Formation of year-round cobble piles on these seasonally sandy beaches may result from chronically reduced sand input owing to anthropogenic controls on river flooding and coastal cliff erosion (e.g. dams and seawalls).