RESPONSE AND RECOVERY OF WELDED BARRIER FOLLOWING ‘SUPERSTORM’ SANDY MEASURED USING LIDAR AND FIELD SURVEYS: NAPATREE POINT CONSERVATION AREA, WATCH HILL, RHODE ISLAND

Napatree Point is a 2.4 km long headland-bounded barrier spit (i.e. welded barrier) extending partially across the mouth of Little Narragansett Bay estuary in Watch Hill, Rhode Island. The western 1.5 km of the barrier and headland are part of the Napatree Point Conservation Area (NPCA), and much of the barrier is ‘unmanaged,’ particularly in terms of beach replenishment and the use of sand fencing. The morphology of the barrier has been measured at five locations quarterly, as well as after storm events, since July 2013 (n = 33 surveys). ‘Superstorm’ Sandy is the most significant storm (in terms of storm surge elevation) to impact the area since Hurricane Carol in 1954. Profiles extracted (transect spacing 50 m) from digital elevation models derived from pre- (April 2011) and post-storm (November 2012) LiDAR show that the dune crest retreated up to 13 m (mean retreat distance 4 m). The elevation of the dune crest lowered by up to 1.4 m, (mean 0.4 m). Dune volume (measured above 2.5 m NAVD88 decreased by up to 23 m³ m^-1 (mean 11 m³ m^-1). The foredune was largely removed along the western portion of the barrier (overwash regime) and retreated due to frontal erosion at the eastern half (collisional regime), where washover fan deposition was limited to trail crossings, which acted as surge channels.

The foredune crest elevation has increased by an average of 0.4 m following Sandy (2012-2018). The dune crest now exceeds the 2011 (pre-storm) elevation at 68% of LiDAR transects. Dune volume (above 2.5 m NAVD88) increased by an average of 2.75 m³ m^-1 yr^-1 between 2012 and 2018. The dune volume for the barrier in 2018 (5.9 x 10⁴ m³ (± 8.8 x 10³ m³)) exceeds the 2011 volume (within the uncertainty) suggesting the barrier is largely recovered from Sandy, although the dune crest is still lower in some areas. Volume added within the last 3 years has particularly focused on eolian transport to the dune ramp, as incipient dunes formed around reestablished vegetation (largely Ammophila breviligulata). This process was exemplified following the passage of an extra-tropical storm with southwest (onshore oblique) winds >20 m s^-1. Profiles collected the following day show an increase in sediment volume on the dune ramp for 4 of the 5 measured profiles. The results indicate that for moderate storm impacts, the recovery of the NPCA barrier system is approximately 5 years.