Dune Morphology and Sediment Budget Responses to Varying Vegetation Cover and Restoration: Humboldt Bay National Wildlife Refuge, Northern California.

This study examines morphodynamic and sedimentation responses of a stretch of coastal foredune undergoing removal of invasive vegetation (Ammophila arenaria) to restore ecosystem dynamics at Humboldt Bay National Wildlife Refuge. Seasonal topographic and vegetation transect surveys and historical aerial photography are analyzed to assess interannual to decadal geomorphic responses of the foredune and sediment budget changes. Relationships between sedimentation and geomorphic change are explored between dominant vegetation cover types as possible. The foredune maintained a near balanced position (+0.004 m a-1) between 1939 and 2014 across the study site. However, there is a general N to S trend from progradation to retreat of the foredune with a maximum change of +1.34 m a-1 in the northern A. arenaria dominated areas to a max retreat of -0.49 m a-1 in the southern sites with a lower and more hummocky foredune dominated by native plants. From 2004 to 2014, percent active sand surface and average aerial change of blowouts remained relatively stable across the study site, with average change values of -0.97% and +0.01% respectively. Positive statistical correlations exist between seasonal beach and foredune volume changes across all sites, yet no significant differences are observed in total volumetric change over the observation period or volume changes within beach and foredune between different vegetation cover types. Survey and aerial photography results suggest that the increased density of A. arenaria has contributed to foredune stabilization over recent decades. However, there is no observed significant difference in seasonal sand volume change in relation to differing dominant vegetation covers. Rather, strong positive correlations exist between seasonal beach volumes and foredune sedimentation, which suggests that foredune sediment budgets may be driven primarily by littoral and aeolian supply variations. Future research will explore vegetation-sedimentation relationships at finer temporal scales and across various vegetation restoration treatments using terrestrial laser scanning and Structure-from-Motion photogrammetry to examine the effects on dune morphodynamics.