SHORELINE CHANGE AT THOMPSON ISLAND,BOSTON HARBOR,MASSACHUSETTS, 1938-1977

A photogrammetric and sediment analysis is presented to illustrate the relationship between beach erosion and seacliff recession on Thompson Island, Boston Harbor, Massachusetts. Aerial photographs taken in 1938, 1952, 1963, and 1977 were measured to determine rates of shoreline change around the island. The 39-year average rate of beach erosion is 0.3 m/yr ± 2% with an average rate of cliff recession at 0.2 m/yr ± 2%. Rates of beach erosion between six orientations that reflect principal wave approach to the island were not found to be significantly different (0.05) for the 39-year period. The rank order correlation between beach erosion and cliff recession for these six orientations was moderately well correlated (0.63). To determine possible controls of cliff erosion, 58 sediment samples were collected from glacial cliffs along the shoreline. The textural composition was determined, and then tested with discriminant function analysis. Partial correlation analysis between beach erosion and cliff recession holding a surrogate for sediment size constant improved the rank order from 0.63 to 0.84. The results indicate that coarser-grained cliffs recede at faster rates, but with less erosion occurring on adjacent beaches. Alternatively, finer-grained cliffs recede at slower rates, but with greater erosion occurring on adjacent beaches.