Date of Award
Master of Science in Geology
The shoreline of Narragansett Bay, R.I. was analyzed for erosion and accretion rates using photogrammetric techniques. Vertical aerial photographs were used to map the 1938 and 1975 shorelines. Comparative mapping of the shorelines was done utilizing a zoom-transfer scope which enabled elimination of photographic distortion. A digital planimeter was used to make areal measurements of erosion and accretion. The 360 km of shoreline was first mapped and divided into segments according to its composition: beach, dune, cliff, or man-made structure. These segments were then measured for changes in area. Changes in beach area were presented in conjunction with shoreline surficial composition.
Areas of high erosion and accretion rates are discussed in relation to probably causal factors, such as relative erosional resistance of beach material, wave fetch, wind characteristics, bathymetry, tidal current velocity data, and local river discharge.
Areas of greatest sediment movement during the study period were cuspate shoreforms. The greatest amounts of shoreline change not engineered by man were found at McCurry and Sandy Points, on Aquidneck Island. This change is attributed to the migration of the shoreforms and is measured at a maximum of 1.7 m/yr of erosion and accretion for McCurry Point and 1.5 m/yr and 0.6 m/yr of erosion and accretion respectively for Sandy Point. Areas of little or no erosion usually occurred in protected coves, on bedrock beaches, and at man-made engineering structures. Approximately 30% of the shoreline of the bay showed little or no erosion from 1938-1975. Average erosion for those beach areas exhibiting change was 0.3 cm/yr.
A sediment budget analysis was conducted to determine the volume of sediment eroded from and added to the shoreline and to determine what percentage of eroded sediment was redeposited along the shoreline. Values were calculated for total sediment volumes and for volume percentage of cobbles and pebble, gravel, sand and silt-sized particles. Of all the sediment eroded from the shoreline, 40% was redeposited. Sand-sized material showed the highest shoreline redeposition rate for both outwash and till shorelines. Silt-sized particles were not redeposited.
Dein, Margaret Gertrude, "A QUANTITATIVE, PHOTOGRAMMETRIC ANALYSIS OF NARRAGANSETT BAY, RHODE ISLAND SHORELINE CHANGES, 1938-1975" (1981). Open Access Master's Theses. Paper 2021.