Water Level and Wave Observations on Dauphin Island, Alabama - Hurricane Nate 2017, in Barrier Island Hydrodynamics and Morphodynamics During an Extreme Event

We rapidly deployed low-cost pressure sensors and tilt current meters (TCM) on Dauphin Island, AL in advance of Hurricane Nate (2017). The deployment goal was to continuously measure storm surge and waves across the barrier island during overtopping. The deployment objectives were to: 1) measure overland wave propagation during an overtopping event; 2) quantify the cross-shore and along-shore variability in water levels and wave characteristics during an overtopping event; and 3) investigate the role of dune morphology and vegetation on overtopping conditions.

Seven cross-shore island transects were established to measure pre- and post-storm survey profiles. Initial beach profile elevation data were collected on 6 October 2017 within the central portion of Dauphin Island using a real-time kinematic (RTK) Trimble R8 global positioning system (GPS). Data were collected in Universal Transverse Mercator (UTM) zone 16 with respect to the North American Vertical Datum of 1988 (NAVD 88). Profiles extended from the subaerial portion of the beach to ~1 m water depth. Two different sensor types, a wave sensor described in Kennedy et al. (2010) and Webb et al. (2012) and a HOBO water level sensor, were deployed across the island on two of the beach profile transects. The wave sensors recorded at 2 Hz and the water level sensors sampled once every 30 s. A total of 17 sensors were deployed (11 wave gauges, 4 HOBO sensors, 2 TCMs), though one was lost and one failed. A sensor in Mississippi Sound measured the bay side water levels continuously during the event. A tilt current meter was added at the midpoint of each measurement transect.

The two measurement transects were selected based on their terrain and vegetation characteristics. One transect was relatively flat and barren while the other contained a primary and secondary frontal dune, hummocky dunes, dune vegetation, and a well-established marsh on the bay side of the island profile. Though no attempt was made to specifically characterize vegetation composition or coverage on either transect, pre- and post-storm low altitude aerial imagery provides an opportunity to assess vegetation characteristics qualitatively.

Gauge data were analyzed in Matlab to derive continuous estimates of water levels and wave characteristics during the event. Absolute pressures were converted to gauge pressure by subtracting atmospheric pressure measured near the study site. The resulting gauge pressures were passed through a high-pass filter to separate the time-varying water levels from the wave band of expected frequencies. Small-amplitude wave theory was used to convert the high-passed data into a free-surface displacement time-series. A zero-crossing routine was subsequently used to identify discrete wave events from which wave heights and periods were determined.

The highest water level and largest wave height were measured by the gauge deployed on the primary frontal dune. The peak water level was +3.17 m NAVD88 and the largest wave height was 1.37 m. The pre-storm and post-storm elevations at this location were approximately +2.25 m NAVD88 and +1.00 m NAVD88, respectively. Wave heights at other sensor locations ranged from 0.1 m to 0.5 m. Peak water levels at other locations varied from +1.7 m NAVD88 to +2.1 m NAVD88. Pre-storm island elevations varied from +0.5 m NAVD88 to +2.5 m NAVD88, while post-storm elevations along the transect were more uniform with little variation between +1.0 m NAVD88 and +1.5 m NAVD88.

The island experienced considerable overwash during the event. Though flooding depths across both measurement profiles were typically small at less than 1.5 m, the event generated a considerable amount of infragravity wave generation. The infragravity wave energy was not present in offshore wave measurements obtained in deep water, so they were most likely generated in the nearshore through feedback between surface gravity wave reflections from the barrier island. There is a strong correlation between the amount of infragravity wave energy and the total volumetric profile change on the two measurement transects.

This dataset contains the pre- and post-storm elevations, the wave and HOBO sensor data, one TCM dataset, photos, videos, and other relevant deployment notes. The processed water level time-series data for the wave and HOBO sensors (that were successfully recovered with good data) are provided in MATLAB (.mat) format. For each time-series, values are provided relative to the sensor (depth) and the NAVD88 datum. The TCM data are provided in CSV format. Other information is provided in text or image format.

These data can be used to better understand surge and wave dynamics on barrier islands during extreme events. Examples of use cases for these data include: characterizing overland wave propagation; calibrating and/or validating numerical models; and describing barrier island overwashing during inundation.