Analysis of coastal dune dynamics, shoreline position, and large woody debris at Wickaninnish Bay, Pacific Rim National Park, British Columbia
Large woody debris (LWD) and colonizing vegetation alter the sediment budgets and stability of coastal dune systems. In British Columbia, LWD on beaches consists largely of historical escape logs from the coastal logging industry. In areas with strong wind regimes and high sand supply, LWD can trap appreciable amounts of windblown sand in the backshore, which can enhance foredune development and stabilization (roles typically played by vegetation) on stable or prograding shorelines. This additional store of sediment provides an important buffer that reduces erosion of established foredunes and backshore ecosystems. This study examines trends in LWD and vegetation coverage and associated geomor- phic changes within the Long Beach unit of Pacific Rim National Park Reserve derived from aerial photography since the early 1970s. Over this time LWD has been reworked seasonally to interannually and, at Wickaninnish Bay, has declined in areal coverage by 61%. Despite this decline, LWD is found extensively within established foredunes and swales in the study area. In combination with vegetation colonization, this has promoted shoreline advance rates as rapid as 1.5 m·a -1 .A t Schooner Cove and Wickaninnish Beach, vegetation colonization is occurring rapidly and has reduced active sand surfaces of large, transgressive dunes landward of the foredune by ~28% over the 34 year observation period. This may reflect both park protection initiatives (i.e., reduced foot and vehicular traffic) and a warming and wetter climate regime on the British Columbia coast over the study period and suggests increasing future stabilization of dune systems in the area. Resume : D'importants debris de bois et une vegetation colonisatrice alterent les bilans sedimentaires et la stabilite des sys- temes de dunes littorales. En Colombie-Britannique, ces importants debris de bois sont en general constitues de billots ega- res provenant de l'industrie forestiere de la cote. La ou les vents sont tres forts et que beaucoup de sable est disponible, les debris peuvent pieger de grandes quantites de sable souffle par le vent dans l'arriere-plage, ce qui peut encourager le deve- loppement et la stabilisation d'avant-dunes (des roles habituellement reserves a la vegetation) sur des littoraux stables ou d'accretion. Cette reserve additionnelle de sediments fournit un tampon important qui reduit l'erosion des avant-dunes et des ecosystemes d'arriere-plage etablis. La presente etude examine les importants debris de bois et la couverture vegetale ainsi