GEOLOGIC INFLUENCE ON SHORELINE CHANGE IN A LOW-ENERGY COASTAL ENVIRONMENT: NARRAGANSETT BAY, RI

Narragansett Bay, situated in eastern Rhode Island, covers 342 km² with 875 km of shoreline. The Bay is microtidal (mean range:1.05 m Newport, 1.34 m Providence; spring range: 1.17 m, 1.47 m respectively). The Bay is mixed-energy wave/tide dominated, and can be considered a low-energy coast line. Bay shore zone types and percentages are: Beach plain/barrier spit – 25%, Glacial stratified material bluff – 10, Till bluff – 23, Meta-sedimentary bedrock – 8, Igneous/other meta bedrock – 5, Discontinuous bedrock – 1, Shore protection structure – 28.

Shoreline change data were created by digitizing the wet/dry line on georeferenced aerial images from 1939 and 1975 and on orthophotographs obtained in 2002/2003. A shoreline change analysis was conducted using the Digital Shoreline Analysis System (DSAS) for ArcMap™ (Theiler et al., 2005). The dataset was analyzed to determine areas of significant change differentiated by geologic shoreline type. Results indicate that 70% of the shoreline is eroding and 30% accreting at maximum distances of - 126 m (-2 m•yr^-1) and +77 m (+1.2 m•yr^-1) respectively. Areas of greatest natural change were beach plain and barrier spit including cuspate shoreforms, and bluffs of stratified glacial material. Lowest changes were till bluffs; no measurable changes were detected for bedrock shorelines. Shoreline change is influenced by the frequency and intensity of tropical and extratropical cyclones and by shoreline exposure to maximum fetch distances of 20 km from the southeast and northeast. Maximum storm surge recorded in the Bay (2.88 m above MHHW Newport, 3.86 m Providence) occurred during the New England Hurricane of September 1938 resulting in deeper depths to carry larger waves onto exposed shorelines. However, extratropical cyclone surges can approach those of category 1 hurricanes.

Results of the shoreline change analysis reveal statistical differences exist among erosion rates for different geologic shoreline types, indicating their importance as a tool for predicting and evaluating the spatial distribution of future coastal erosion hazards. The pressure for development, coupled with significant shoreline change quantified by this study, highlight the critical need for evaluation of historical shoreline change in low-energy coastal environments and their response to storm events.