PRELIMINARY SUB-SURFACE INVESTIGATION AT SANDY HOOK, NEW JERSEY USING GROUND PENETRATING RADAR, FROM DUNE TO SWASH

We have captured the recent evolution of the prograding North Beach site located at the northern end of the Sandy Hook Unit of Gateway National Recreation Area using Ground Penetrating Radar (GPR). At this site, we collected GPR data along a ~350 m cross-shore transect using a 400- and 100-MHz antennas. The cross-section spanned from the base of a foredune to the intersection of the beach and the ocean on the beach face, at approximately mean sea level. The morphology of Sandy Hook, the northernmost 10 km of a barrier spit along located in Highlands, New Jersey documents a story of barrier growth, coastal erosion, storm effects, and anthropological influence. The North Beach site is located just south of the tip of the spit that has grown into the Raritan Bay over the past few hundred years. Although North Beach is not at the tip of the spit, it has been prograding to the east at a relatively rapid rate over the past 20 years and is representative of the spit’s continuous growth, fueled by sediments eroded and transported from the mainland to the south, as well as the northerly alongshore transport of large-scale beach replenishment efforts. This growth and progradation has left its signature not only in the topography, but also in the stratigraphy. We have captured the near-surface stratigraphic architecture of these prograding beach deposits.

The GPR transect collected with the 400-MHz antenna returns clear reflections to ~2.5 m in depth along the majority of the transect. The reflections capture what we interpret to be prograding deposits associated with the development of the beach face, ridges and runnels developed in the swash zone, foredunes, and berm surface deposits. We interpreted the GPR transect with the aim of mapping unconformity bound units to determine the sequence of stratigraphic development. The shoreward-most 100 m of the GPR transect we collected shows as many as 10 distinct progradational units. Tying these data to the lidar topography we determine that these units are likely representative of the past 10 years of progradation. We interpret the boundaries between these units to be generated by periods of erosion. The progradational nature of the deposits we observe fits with the measurements made using the lidar data, which indicate >300 m of progradation at this site since the year 2000.