Foredune morphodynamics and seasonal sediment budget patterns: Humboldt Bay National Wildlife Refuge, Northern California, USA




Rader, Alana Marie

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Delivery of sediment to beach-dune complexes along the northern California coast, as elsewhere, is controlled by littoral and aeolian processes governed largely by oceanic and meteorological conditions such as wind speed and direction, wave characteristics and water level fluctuations. Furthermore, patterns of sediment deposition on foredunes are controlled by the zonation, density and physical structure of dominant vegetation assemblages. This study explores the link between varying oceanic, meteorological and ecological patterns and coastal foredune morphodynamics at a site within the Humboldt Bay National Wildlife Refuge (HBNWR) near Arcata, CA, to provide coastal managers a local context of foredune erosion and accretion. At a site within the HBNWR a 75-year north to south alongshore gradient in foredune response was observed during the study period. Foredunes in the north experience seaward progradation (up to +0.51 m a-1) and greater sediment volumes then southern foredunes, characterized by foredune retreat (up to -0.49 m a-1) and larger erosive feature areas. Seasonal signatures of a previously observed bi-directional littoral drift partially inform the interpretation of an alongshore gradient in foredune position. In the summer, wind and wave directions were out of the NNW, combined with north to south littoral drift and significant sediment input into the northern beaches. During the winter, the dominant drift direction was from the south to the north, accompanied by large waves, high water levels and beach erosion. Following a comprehensive morpho-ecological model of foredune evolution (Hesp, 1988; 2002), greater foredune volumes, dense vegetation and seaward progradation are indicative of stage 1 foredunes. Transitioning to the south, lower vegetation densities and seaward retreat support a classification of stage 3 developed foredunes, characterized by shorter, more hummocky morphologies. Meteorological patterns and disturbance to vegetation concurrently influence foredune response and recovery to erosive wind, wave and water level events. As such, seasonal to interannual patterns of foredune morphodynamics may be altered following periods of both environmental and human induced vegetation disturbance (i.e., seasonal phenology, dynamic restoration). At a section of foredune in the northern HBNWR, a dynamic restoration project was implemented with the first stages of vegetation removal occurring in August, 2015. In a year following vegetation disturbance through preliminary stages of restoration an annual sediment budget examination indicates net accretion on the foredune (+0.54 m3 m-2) while net erosion occurred on the beach (-0.38 m3 m-2). At smaller seasonal scales site-wide erosion occurred in the winter due to high-water and wave run-up recorded during intense storms. Summer monitoring reveals site-wide accretion due to beach rebuilding, heightened aeolian activity and an increase in vegetation cover. As such, seasonal sediment budgets that influence longer-term patterns of foredune development may be primarily controlled by the amount of sediment available on the beach for aeolian transport and secondarily by localized presence/absence of vegetation. Results of this study provide insight into the impact of continued coastal disturbance on foredune morphodynamics, around which a framework for future vegetation management projects may be implemented.



Coastal, Geography, Geomorphology, Coastal erosion, LiDAR