AN ASSESSMENT OF GEOMORPHIC THRESHOLDS FOR COASTAL BLUFFS ALONG THE SOUTHEASTERN SHORE OF LAKE ONTARIO

MONTESI, James P., and PINET, Paul R., Dept. of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346-1398

The southeastern shoreline of Lake Ontario is lined with a series of alternating glacial drumlins and baymouth barriers. The relative interplay of wave notching, slumping, and gullying determine the morphological character of the steep coastal bluffs that are cut into the north-south-trending drumlins. These bluffs undergo distinct phases of landform development, progressing systematically through young, mature, old, and terminal stages. Geomorphic thresholds keyed mainly to bluff height and hence sediment supply likely separate each of these morphologic stages. These thresholds mark a metastable condition under which the coastal bluff undergoes a significant, abrupt transformation in its morphological state without an obvious change in the driving environmental forces. Because lakeshores are prime sites for residential development, identifying bluff slopes that are poised metastably at geomorphic thresholds is critical for site evaluation and engineering construction.

A field study designed to identify geomorphic threshold values was conducted at 13 Lake Ontario bluff sites. The bluff height and slope angle, beach width, and the dominant erosional processes (gullying or slumping) were measured at 10-m intervals across the breadth of each bluff. The height of the bluff face where gullying superseded slumping was noted and is used as a topographic indicator of a geomorphic threshold. Strong inverse relationships were found between bluff height and slope, and between bluff slope and length (the horizontal distance between the bluff toe and the headwall) for all lakeshore bluffs regardless of their stage of geomorphic development. Also, a strong positive relationship exists between bluff height and length. All of these correlations are in good agreement with our proposed Lake Ontario model of bluff development. A mean threshold value of 22.8 m (S.D. = 1.4; range = 20 - 28 m) was calculated for the 13 lakeshore bluffs. This suggests that any bluff sections with topographic heights of around 23 m are potentially metastable and hence extremely hazardous, and may require special stabilization measures in order to mitigate catastrophic erosion.

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