COMPARING MEASURED SHORELINE CHANGE USING THE POSITION OF THE LAST HIGH TIDE SWASH AND BLUFF CREST BETWEEN 1952 AND 2014/2016 FOR BLOCK ISLAND, RHODE ISLAND, USA

Recently completed mapping quantified shoreline change using the position of the last high tide wash (LHTS) between 1952 and 2016 for Block Island (BI), a microtidal, mixed-energy wave dominated island 21 km offshore of Rhode Island. The BI shoreline consists of a variety of a mixture of barriers, welded barriers and bluffs composed of a variety of sediment types >50 m in relief. The coastal construction setback in Rhode Island is 30X the annualized rate of change for residential structures. The objective for this project is to compare measured shoreline change using the bluff crest and the last high tide swash (LHTS) over the decadal time scale.

Position of the bluff crest was mapped using 1952 vertical aerial photographs and 2014 USGS orthophotographs for segments of BI with a variety of bluff heights (6.3 to 47 m), orientation and composition. Total bluff retreat and annualized rates of change were measured at transects (n=50) cast perpendicular to the shoreline. Values for total bluff retreat (1952-2014) ranged from near zero (1.7m) to >40 m; LHTS (1952-2016) showed similar values, (6 m to 47 m). There is a slight offset in the timing of the bluff crest (2014) and LHTS (2016) position; however, given the lack of significant storms between 2014 and 2016, this is likely not a substantial source of error. The bluff crest retreated less than LHTS at 60% of the transects, however, there is a wide distribution within these results. No correlation was found between bluff height and shoreline change. Spatially, along the northeast shoreline only 13% of transects showed the bluff retreated more than LHTS; results were split along the southern shoreline.

The measured annualized rates of change produce a difference in construction setback up to 10 m, however, for 60% of the transects the difference in setback would be less than 3.3 m (10 ft), suggesting at least as a first order estimate, rates of change for LHTS and bluff crest are similar for this use. Given the episodic nature of bluff retreat, and risk of storms and sea level rise accelerating erosion in the future, regulators should be cautious when applying setbacks on eroding bluffs. Future work on this project will likely expand the number of measured transects and will examine bluff composition as a factor in determining bluff retreat rates.