44-7 Indications of Storm Energy Recorded by Stratigraphic Deposits Generated During Coastal Erosion Events Preserved within a Progradational Stretch of Barrier Beach

Abstract:

The rapidly prograding beach at North Beach, Sandy Hook, New Jersey, preserves a ~25 year stratigraphic record of coastal storm impacts that we use to relate storm-driven forcing to the sedimentary characteristics of deposits these storms left behind. Interpreting GPR data collected on site, we identify over 30 storm-generated stratigraphic erosion surfaces preserved within the beach, and highly-accurate GNSS data constrain the timing of their formation between 2009 and 2025 CE. We combined hydrodynamic and statistical modeling to identify the storms most likely to have produced the preserved erosion surfaces. More than 1000 48-h XBeach simulations run using tide-gauge and wave buoy data were used to train a statistical emulation of the XBeach output, which was calibrated to reconstruct the stratigraphic observations. The modeling results suggest storms exceeding a water-level threshold of 1.3 m above MSL and significant wave heights of 3.8 m resulted in significant coastal erosion. Pre-, during-, and post-event observations from a January 10, 2024 nor’easter, which exceeded threshold conditions, validate the model-inferred thresholds.

Three-dimensional 400-MHz GPR data (~5 cm vertical resolution) identify 12 erosional surfaces generated over the past 7 years that can be traced ~60 m alongshore. Extensive sediment sampling of several of these storm surfaces indicates that the facies and sediment composition of storm deposits differ based on their elevation along the beach profile, while also reflecting geomorphological context. At ~1 to 1.5 m NAVD88, visually apparent truncation surfaces correspond with coarse sand overlain by a thin layer of highly concentrated dark minerals. Closer to 2 m NAVD88, the same event is represented by finer, medium sand, only slightly coarser than fair-weather deposits typical of North Beach, but still overlain by a dark mineral layer. At higher elevations, likely landward of the contemporaneous berm crest, trenches expose very coarse, landward-dipping beds that do not coincide with dark mineral deposits. We interpret these to be overwash or high-runup deposition onto the back beach. We integrate grain size measurements from multiple locations along the beach profile for individual storm deposits, across a number of storms, to develop a quantitative model aimed at estimating the relative energy associated with the coastal erosion events that formed the deposits. This approach represents a framework for interpreting relative storm intensity from preserved beach stratigraphy.

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