How natural are the sediments on our beaches? Characterising urban anthropogenic mixed beaches in Scotland

Mixed beaches containing anthropogenic geomaterials such as bricks, concrete, glass, ceramics, tarmac and slag are globally understudied. These materials enter natural beach systems through erosion or dumping, and rising sea levels and increased storminess are expected to intensify coastal erosion, reworking and re‐exposing previously dumped debris. Studying such beaches is vital to understanding and managing the impacts of anthropogenic geomaterials on coastal environments. This study investigated six beaches with clear bimodal sediment distributions in the Firth of Forth, Scotland, examining surface sediment composition and geomorphology using grain‐size analysis, Wolman pebble counts and visual classification. Anthropogenic geomaterials were found in both coarse (>11 mm to 256 mm) and fine (>2 mm to 11 mm) sediment at all sites, comprising 2 to 48% (average 22%) of coarse sediment weight, with brick being dominant at five beaches. Anthropogenic geomaterials averaged 22% of coarse and <1% of fine sediments and influenced grain‐size distributions in both fractions. The largest median grain size correlated moderately with total percentage weight of anthropogenic geomaterials and consistently appeared in the mid to upper beach zones for all studied beaches. The degree of surface incorporation from coarse to fine fractions was limited (max ratio 0.22), generally increasing (except Granton) along an inner‐to‐outer estuary morphodynamic gradient. Digital Terrain Models from Unmanned Aerial Vehicle surveys showed varied shoreface widths (25 to 50 m) and similar surface slopes (0.09 to 0.11) across sites, with no clear morphological impact from anthropogenic geomaterials. These beaches are geomorphologically comparable to each other and to Mixed Sand and Gravel beaches. Based on their morphological and sedimentological characteristics, these are classified as a variant: Anthropogenic Mixed Sand and Gravel beaches. These findings provide essential first‐order insights into the nature of anthropogenic mixed beaches and establish a foundation for further research into their evolution under sea level rise and shifting hydrodynamic regimes.