Effect of Future Sea Level Rise on Barrier Migration and Headland Erosion Along the Southern Coast of Rhode Island

Abstract

The southern coast of Rhode Island consists of a series of barriers and headlands currently retreating landward by inundation and frontal erosion. Over the next century, a projected atmospheric temperature increased caused by the continued emission and buildup of greenhouse gases may elevate eustatic sea level by inducing the thermal expansion of the oceans and the melting of glaciers (Hoffman and others, 1983; Meier, 1989; Houghton and others, 1990). Using the mid-moderate projection for eustatic sea level rise of the Environmental Protection Agency (EPA) (Hoffman, 1984) and the rate of local subsidence (1.5 ± 0.5 mm·yr-1), local projections of sea-level rise were determined for the years 2020, 2050 and 2100. These projections were two to four times greater than the historical sea-level rise rate extrapolated over the same time period. To model the effect of projected sea-level change in Rhode Island, nine coastal profiles were surveyed. Landform changes were modelled using two methods, a historical erosion method (HEM) and a method adopted by National Academy of Sciences (NAS) (NAS, 1990) that incorporates historical erosion rates and projected sea-level rise rates. Frontal erosion ranged from 3 to 102 m for HEM-modelled profiles and from 5 to 363 m for NAS-modelled profiles. Both methods showed barrier profiles had greater frontal erosion than headland profiles. A sediment-budget analysis of modelled profiles gave the ratio of eroded sediment from source areas along the profile to sediment deposited in sink areas. The NAS-modelled profiles generally exhibited a surplus of eroded source material, while the HEM-modelled profiles showed a sediment source deficit. The effect of a 100-year storm was modelled at each site for 2020, 2050 and 2100. Berm and foredune zone erosion averaged 50 m3•m-1 and foredune retreat averaged 36 m. By 2100, a 100-year storm surge will flood most of the first floors of the structures surveyed. During the next 110 years, Federal Emergency Management Agency {FEMA) V and A flood zones will migrate landward with a rising sea level. Analysis of beach profiles showed A-zones extending landward up to 342 m. The combined length of FEMA V- and A-zones along a profile, however, will change little over time as frontal erosion keeps pace with the landward extension of the A-zones. Present coastal legislation in Rhode Island should revise structural setback distances based on erosion hazard zones, and include updated projections for future sea level elevations.