On the west coast of Canada, large woody debris (LWD) consists largely of historical escape logs from the coastal logging industry. In areas with competent wind regimes, LWD can trap appreciable amounts of windblown sand in the backshore, which can alter beach–foredune sediment budgets and initiate incipient dune formation. As this additional store of sediment must be reworked first during high water events, it provides an important buffer that reduces erosion of established foredunes.
This research examines how large woody debris (LWD) alters the sediment budgets and geomorphology of beach-dune systems. LWD is common on many beaches in western North America and most of it is a historic artefact of the logging industry. This material, and other flotsam, aids in dune initiation and growth, stablization and progradation of beaches, and provides an enhanced store of sediment in the backshore that buffers dune ecosystems from storm surge flooding and wave attack.
Working with three study sites in NE Haida Gwaii, we used coincident high spatial resolution LiDAR data and digital orthophotographs to derive DEMs for distinct ground cover classes (sand, LWD). From this we could quantify relative storage capacities of LWD and sand in the backshore. We found that significant amounts of sand are stored within and around LWD on beaches in the study region. Also, the relative storage capacity of these features is reflected in the backshore morphology of each site, with sediment transport further into the backshore dependent upon the morphology and relative in-filling of the log debris jam. With this additional sediment storage, log debris could enhance development of large incipient dunes in the backshore thereby buffering against increasing storminess and gradual sea-level rise in the region.
This research is sponsored by Dr. Walker's ongoing NSERC Discovery Grant.
Heathfield, DK, Walker, IJ (2011). Analysis of coastal dune dynamics, shoreline position, and large woody debris at Wickaninnish Bay, Pacific Rim National Park, British Columbia. Canadian Journal of Earth Sciences, 48(7): 1185-1198.
Eamer, JBR, Walker, IJ (2010). Quantifying sand storage capacity of large woody debris on beaches using LiDAR. Geomorphology, 118, 33-47.