Geomorphic and sediment volume responses of a coastal dune complex following invasive vegetation removal

This paper documents application of an established geostatistical methodology to detect significant changes in a foredune-transgressive dune complex where Parks Canada Agency (PCA) implemented a dynamic restoration program to remove invasive marram grasses (Ammophila spp.) and enhance dynamic dune habitat for an endangered species. Detailed topographic surveys of a 10 320 m² site in the Wickaninnish Dunes in Pacific Rim National Park, British Columbia, Canada for the first year post-treatment are compared to a pre-restoration LiDAR baseline survey. The method incorporates inherent spatial structure in measured elevation datasets at the sub-landscape scale and models statistically significant change surfaces within distinct, linked geomorphic units (beach, foredune, transgressive dune complex). Seasonal and annual responses within the complex are discussed and interpreted. All geomorphic units experienced positive sediment budgets following restoration treatment. The beach experienced the highest differential volumetric change (+1656 m³) and net sediment influx (+834 m³, 0·19 m³ m^-2) mostly from supply to the supratidal beach and incipient dune. This sediment influx occurred independent of the restoration effort and was available as a buffer against wave erosion and as supply to the landward dunes. The foredune received +200 m³ (0·13 m³ m^-2) and its seaward profile returned to a similar pre-restoration form following erosion at the crest from vegetation removal and scarping by high water events. Sediment bypassing and minimal change was evident at the mid-stoss slope with appreciable extension of depositional lobes in the lee. The transgressive dune complex experienced high accretion following restoration activity (+201 m³) and over the year (+284 m³, 0·07 m³ m^-2) mostly from depositional lobes from the foredune, precipitation ridge growth along the downwind boundary, and growth of existing lobes within the complex. Further integration of this methodology to detect significant geomorphic changes is recommended, particularly for applications where sampling densities are limited or logistically defined.