Erosive water level regime and climatic variability forcing of beach-dune systems on south-western Vancouver Island, British Columbia, Canada

Increases in the frequency and magnitude of extreme water levels and storm surges are correlated with known indices of climatic variability (CV), including the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), along some areas of the British Columbia coast. Since a shift to a positive PDO regime in 1977, the effects of ENSO events have been more frequent, persistent, and intense. Teleconnected impacts include more frequent storms, higher surges, and enhanced coastal erosion. The response of oceanographic forcing mechanisms (i.e. tide, surge, wave height, wave period) to CV events and their role in coastal erosion remain unclear, particularly in western Canada. As a first step in exploring the interactions between ocean-atmosphere forcing and beach-dune responses, this paper assembles the historic erosive total water level (TWL) regime and explores relations with observed high magnitude storms that have occurred in the Tofino-Ucluelet region (Wickaninnish Bay) on the west coast of Vancouver Island, British Columbia, Canada. Extreme events where TWL exceeded an erosional threshold (i.e. elevation of the beach-foredune junction) of 5·5m aCD are examined to identify dominant forcing mechanisms and to classify a regime that describes erosive events driven principally by wave conditions (61·5%), followed by surge (21·8%), and tidal (16·7%) effects. Furthermore, teleconnections between regional CV phenomena, extreme storm events and, by association, coastal erosion, are explored. Despite regional sea level rise (eustatic and steric), rapid crustal uplift rates have resulted in a falling relative sea level and, in some sedimentary systems, shoreline progradation at rates approaching +1·5m a^-1 over recent decades. Foredune erosion occurs locally with a recurrence interval of approximately 1·53years followed by rapid rebuilding due to high onshore sand supply and often in the presence of large woody debris and rapidly colonizing vegetation in the backshore.