Impacts of El Niño on the sediment balance of a transgressive dune-beach system

Understanding long-term evolution of sandy coasts requires in-depth analysis of the sediment balance from the shoreface to the beach and dune. While storms typically erode the subaerial beach, they can also contribute sediment from deeper waters to the coastal budget. Here, we explore the impacts of El Niño-driven storms on the sediment balance across the entire shore-beach-dune profile. Satellite-derived shorelines (1984-2020) were combined with sixty-six topo-bathymetric surveys (2014-2020), along a 1 km stretch of southern Ensenada beach (Baja California). Forcing conditions were characterized using hourly wave and wind data. Multi-decadal shoreline data reveal that high-energy El Niño events have led to punctuated landward coastline translation. Interannual topo-bathymetric surveys show an active 8 to 9.5 m-high dune ridge along the upper part of the profile, migrating landward at a rate of 5 m/yr. This migration gradually decoupled the dune from the beach, occasionally creating space across the dune toe area for the formation of incipient dunes that eventually merged with the main ridge. The sediment budget analysis indicates a one-way landward transfer of sediment from the winter-berm to the dune. Interestingly, the overall shore-beach-dune sand budget remained positively balanced, with an increase of ≈40 m³/m related to sediment availability from deeper waters and onshore transport from the lower shoreface. Shoreline trends and sediment budgets illustrate a long-term transgressive system capable of maintaining sedimentary balance in the mid-term. Increased storm activity and sea-level rise are expected to accelerate beach-dune transgression, which could retain sediment balance if offshore inputs persist.