Paper No. 52-17
Presentation Time: 1:00 PM
QUANTIFYING AND EXAMINING THE RELATIONSHIP BETWEEN HISTORICAL LARGE WOODY DEBRIS, SHORELINE POSITION, AND VEGETATION COLONIZATION ON A SANDY BEACH USING HISTORICAL AERIAL PHOTOGRAPHY
Large woody debris (LWD) plays important roles in the ecology and geomorphology of sandy beach morphodynamics and is common on almost all beaches throughout the Pacific Northwest. In British Columbia, LWD is comprised mainly of historical escape logs from marine boom transportation. With improvements in log transportation methods and a general decline in coastal logging, the total amount of LWD in coastal systems is decreasing, yet in many areas it continues to play a significant role in beach-dune sediment budgets and shoreline stabilization trends. This study quantifies the historical coverage of LWD at a study site (Pacheedaht Beach) near Port Renfrew on SW Vancouver Island, British Columbia, Canada to test the hypothesis that there is a positive relationship between the areal coverage of LWD and shoreline progradation rates. The study presents a simple, efficient, and statistically robust remote sensing method using the USGS Digital Shoreline Analysis System (DSAS) with a supervised classification method to analyze surface cover and shoreline position changes from historical aerial photography between 1968 and 2013. Over this interval, LWD coverage declined by 46% (-16 146 m2), which supports other findings on LWD decline in southern British Columbia, and the intervals of greatest LWD decline were associated with strong El Niño seasons and/or storm activity. Despite gradual declines in LWD, shoreline position prograded 70.6 m (1.6 m·a–1) and vegetation colonized seaward by 51.8 m (1.1 m·a–1) atop former dune and LWD deposits. In conclusion, this study shows that measurable spatial and temporal relationship exist between LWD cover, shoreline position, and vegetation colonization rates at this site, although positive relations do not always exist between LWD cover and shoreline position, which begs further investigation into regional tectonic effects on coastal emergence, variable sediment inputs by large rivers draining logged basins in the vicinity, and human activity (i.e. log removal from beaches).