Quantifying sand storage capacity of large woody debris on beaches using LiDAR

The sedimentological role of large woody debris (LWD) on beaches is understudied and is relevant for the morphodynamics of sandy, high-energy beach-dune systems of the northeast Pacific Ocean. On the west coast of Canada, this debris 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 study examines the morphology and sand storage capacity of three backshore LWD deposits of varying morphologies on northeastern Graham Island, Haida Gwaii (Queen Charlotte Islands) British Columbia, Canada. A new method was developed using coincident high spatial resolution LiDAR data and digital orthophotographs to derive DEMs for distinct ground cover classes (sand, LWD). These DEMs were then used to quantify relative storage capacities of LWD and sand in the backshore. Significant amounts of sand are stored within and around LWD on beaches in the study region. Existing storage quantities (above HHWMT) range from 9.19×10⁴ to 1.39×10⁵kgm^-1 beach width or ~1.14 to 1.60m storage depth. The same LWD deposits have a further potential storage capacity ranging from 1.04 to 1.70×10⁴kgm^-1 beach width or ~0.21 to 0.28m depth. 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. As the use of precise LiDAR data is increasing in geomorphic research, the development of a verifiable methodology for estimating beach-dune storage volumes is timely.