Analysis of Cliff Retreat and Shoreline Erosion: Thompson Island, Massachusetts, U.S.A.
Jones JR, Cameron B, Fisher JJ. 1993. Analysis of Cliff Retreat and Shoreline Erosion: Thompson Island, Massachusetts, U.S.A.. Journal of Coastal Research (JCR). 9(1):87-96
Shoreline and cliff erosion rates between 1938 and 1977 were determined by photogrammetric analysis (1938, 1952, 1963, 1977 data) for an island in Boston Harbor. These data were used to evaluate the influence of sediment properties on the erosion rates of glacial cliffs with respect to most-frequent storm-approach directions. Analysis of variance indicated that sediment textures and shear strengths differed according to geographic orientation. Regression analysis showed that percent gravel and sand:mud ratios accounted for about 70% of the variation in shear strength. Thus, the most resistant cliffs should have the average sand:mud ratios, low gravel content, and high shear strengths. The highest cliff-erosion rates were generally associated with cliffs with the highest sand:mud ratios, the lowest shear strengths, an orientation which subjected them to frequent storms. The highest erosion rates (0.4 to 0.6 m/yr) were in the northwest and southeast quadrants, but not in the storm-dominated northeast quadrant. The northeast-facing cliffs had the lowest sand:mud ratio and relatively high shear strengths. Erosion rates were highest for southeast-facing cliffs, which had a high sand:mud ratio and the lowest average shear-strength measurements. Over the 39-year period, the island's mean cliff recession was about 0.3 m/yr (> 250,000 cu m of sediment), while shoreline erosion was about 0.4 m/yr (> 200,000 cu m of sediment). These estimates indicate that nearly half a million cu m of materials have been removed from the island since 1938. The Boston Harbor Islands may be a significant sediment source for offshore areas. Prediction of cliff erosion rates may be more complex than prediction of beach erosion rates. In addition to dominant storm directions, sediment properties must be considered when predicting long- term erosion rates. (Author's abstract)
- Type
- Journal Article
- Authors
- Jones, J; Cameron, Barry; Fisher, John
- Units
- BOHA
- Keywords
- Aerial Photography, Boston Harbor, Cliff erosion, Erodability, Erosion forecasting, erosion rates, glacial sediments, Massachusetts, Sediment texture, Sediment Transport, Shear strength, shore erosion, Statistical Analysis, Storm effects