Mass wasting and coastal erosion on Yukon Coast and Herschel Island

Erosion rates along permafrost coastlines are among the fastest in the world, despite the fact that they are only ice free for 3-4 months of the year. Yearly coastal erosion rates of up to 20 m were recorded along ice rich and unconsolidated coasts of the Beaufort and Laptev Sea. Coastal erosion can thus cause rapid land loss and release large amounts sediments, which can alter near-shore ecosystems. Mass-wasting processes such as active-layer detachments, retrogressive thaw slumping and block failures frequently occur along the coasts of Yukon Coastal Plain and Herschel Island. They can significantly impact coastal dynamics and sediment delivery on the shore. In our study we use high resolution digital elevation models (DEMs) to observe short term coastal erosion along Yukon Coast and Herschel Island. DEMs were acquired from LIDAR surveys during the AIRMETH campaigns in 2012 and 2013. The DEMs were processed to obtain a horizontal resolution of 1 meter and compared to identify erosion and accumulation events. Our results show that erosion behaviour is simple and relatively linear at low-elevation coasts (up to 10 m height), where we recorded yearly coastline retreat from 0 to 20 m. Coastal erosion behaviour becomes diverse and slower at higher-elevation coasts, where mass-wasting processes are more active. Among these mass-wasting processes, retrogressive thaw slumping is particularly important. Activated material can be accumulated at the slump outlets or can be transported along the coast by longshore drift. Such material accumulations caused up to 42 m of coastline progradation. Significant accumulation events were identified also due to block failures (up to 20 m of coastline progradation). Although they are generally short-lived features, they can occur frequently and can influence coastline digitalisations. Coastline observations are therefore not indicating the volume loss that is occurring due to mass wasting. We observe discrepancy between planimetric (coastline movement) and volumetric (recorded by DEMs) coastal erosion on Herschel Island. Exploring the relationship between both measures of coastal erosion would enable better estimates of released sediments from the coasts characterised by mass wasting