STORM-INDUCED NEARSHORE BAR MOVEMENT AND SWITCHING ON THE NORTH SHORE OF PRINCE EDWARD ISLAND

The North Shore of Prince Edward Island is a transgressive shoreline where sand resides on the shoreface, in flood- and ebb-tidal deltas, in beaches and in coastal dunes. Up to five nearshore bars, roughly shore-parallel, are characteristic features of this coast. The bars are partially sand-starved, with lag gravel in the troughs, and are prone to migration under the influence of storm-induced waves and currents. This paper investigates net changes in bar crest position during a 15 month interval including two intense fall storms. The first storm occurred in late October 2000, accompanied by storm-force northeast winds and significant wave heights up to 7.4 m. The second occurred in early November 2001, with storm-force north to northwest winds and record high water levels. Impacts at the coast included major infrastructure damage, lowering of upper beachface profiles, trimming of dunes and erosion of underlying glacial sediments.

The 2000 storm was predated by vertical aerial photography flown in August 2000. The 2001 storm was postdated within a month by IKONOS satellite imagery with 1 m resolution panchromatic data and four bands of multispectral imagery at 4 m resolution. The aerial photographs were scanned to achieve a ground resolution better than 1 m, georeferenced and mosaicked. The IKONOS imagery was geocorrected to the same vectors and processed to discern nearshore features.

Significant local reorganisation of the multiple bar system occurred during the study. There is clear evidence for bar switching, in which crest realignment occurs in transition zones marked by discontinuities or bifurcations. This behaviour, previously observed in sweep bathymetric surveys at one location in the study area, has been described from a number of other regions, but remains poorly understood. Although these observations of bar migration are clearly aliased, variable bar numbers across transition zones and repetitive nearshore profile surveys suggest the possibility of slow net offshore migration. During this study, up to 100 m of migration occurred, with bars shifting both seaward and landward. There were marked changes in sinuosity and in the alongshore position of crest bifurcations. The highly variable migration rates and episodes of bar switching indicate a strongly non-linear response to wave forcing.