Spit and Barrier Island Migration in the Southeastern Canadian Beaufort Sea

Abstract

The coastline of the southeastern Canadian Beaufort Sea is essentially composed of eroding bluffs of unconsolidated Quaternary sediments, complex embayments formed by the breaching of thermokarst lakes, and spits and barrier islands which are experiencing rapid landward migration. Comparison of air photographs shows that between 1950 and 1985 spits were retreating landward at an average rate of 1.7 m a ^-1 while offshore barrier islands were migrating onshore at a rate of 3.1 m a ^-1. "Detached spits," which represent a type of barrier island almost attached to the mainland, migrated landward at a rate of 2.0 m a ^-1. These differences in migration rates are explained in terms of sediment supply, morphology and physical processes responsible for the landward migration of the coastal landforms. Regression analyses of retreat rates as a function of longshore sediment supply revealed that sediment availability is a significant parameter controlling spit and barrier island landward migration. Analyses of retreat rates as a function of incoming deep-water wave power showed that barrier islands are retreating more rapidly than spits even if they are exposed to the same level of wave energy. Direct sediment supply from adjacent bluffs and the presence of coastal dunes effectively limit spit erosion and retreat. In the case of barrier islands, the lack of significant longshore sediment supply is favourable to a net sediment deficit and results in higher retreat rates. Spits are retreating in response to beachface erosion and storm overwashing where low crestal elevation permits swash incursion across the crest. Barrier islands are more often and more extensively overwashed and overwashing represents the main physical process responsible for their migration.