The Effect of Future Sea Level Rise Along the Southern Rhode Island Coast

Abstract

The barrier spit and headland shoreline of southern Rhode Island is presently migrating landward under the effects of storm-wave driven frontal erosion, while coastal lagoon and upland shorelines are being displaced landward by rising sea level. Relative sea level curves for southern Rhode Island projected to the year 2100 were constructed by adding a local isostasy rate of 0.15 ± 0.04 cm·yr-1 to the range of eustatic sea level predictions determined by the United States Environmental Protection Agency (EPA) (Hoffman, 1984). The EPA mid-range high scenario used in this study predicts a 216 cm global rise by 2100, and when local isostasy is added, a 235 cm rise by 2100 for southern Rhode Island. Historic frontal erosion rates were extrapolated to the years 2020 and 2100 to map future barrier and headland shoreline position, while lagoon and upland shorelines for the same years were derived by inundating the present landscape with a 60 and 244 cm (2 and 8 ft) mean sea level rise. Frontal erosion will account for 49 ha of land loss by 2020 and 163 ha by 2100 between Watch Hill point and Point Judith. Inundation from a 60 cm (2 ft) mean sea level rise will submerge 87 ha of upland, while 645 ha will be submerged by a 244 cm (8 ft) relative sea level rise. During the predicted relative sea level rise, Federal Emergency Management Agency (FEMA) V- and A-zones will expand in area and experience an increase in storm-surge water depth over present values. By 2020, map analysis indicates that the area flooded (FEMA A- and V-zones) along the south shore of Rhode Island by the 100-year event will be 2132 ha, an increase of 10% over the present area. Combined V- and A-zone area will expand to 2861 ha by 2100, a 47% incrase in area over the present. Curves of storm-surge elevation versus return period, updated to reflect a 52 cm sea level rise by 2020 and a 235 cm rise by 2100, indicate a return period for a flood with an elevation of the present 100-year event (approximately 3.6 m) to be 26 years by 2020, and 0.6 years by 2100.