Foredune morphodynamics and sediment budgets at seasonal to decadal scales: Humboldt Bay National Wildlife Refuge, California, USA

Coastal foredunes are shore-parallel ridges that form in the backshore and their morphodynamics are controlled partly by seasonal and spatial variations in the coastal (onshore) sediment budget that, in turn, are driven by oceanic and atmospheric processes and interactions, including regional wave and wind regimes, climatic variability events (e.g., ENSO), sediment availability, beach characteristics (e.g., width, slope), and vegetation type and cover in the backshore. Previous studies on shoreline change in Northern California report only broad rates of erosion and accretion related to regional meteorological regimes. This study presents a more detailed, multi-decadal to seasonal account of shoreline response and foredune morphodynamics along a 2.5 km stretch of coast in the Humboldt Bay National Wildlife Refuge (HBNWR). Analysis of historical aerial photography (1939–2014) reveals trends in shoreline position that are coupled with more detailed assessments of foredune morphodynamics and seasonal scale volumetric changes from cross-shore topographic profiles``. These findings set the historical context of foredune morphodynamics and allow exploration of the implications of seasonal meteorological variation on long-term (75-year) foredune evolution and development at the HBNWR. DSAS describes maximum foredune progradation in the north (up to +0.51 m a⁻¹) and maximum foredune retreat in the south (up to −0.49 m a⁻¹). Aerial photograph analysis (2004–2014) shows statistically significant larger erosive features in the southern zone than in the northern and central zones. Seasonal volume calculations from cross-shore profiles indicate statistically significant differences in alongshore transect elevation and foredune volume, with larger elevations and volumes in the northern and central zones than in the southern. Combined with evidence of seasonal bidirectional littoral drift, these data support a north to south gradient in sediment availability, foredune position and resulting stages of established foredune development. Seasonal storm energies and climate forcing events introduce variability in erosive patterns but support the persistence of alongshore developmental stages. Future research should explore foredune morphodynamics on a smaller spatial scale and changes related to the presence/absence of multiple vegetation assemblages.