Extreme, Surf-Zone Wave Event Captured by Bottom Mount Acoustic Sensor During Passage of Hurricane Matthew

The United States National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) has been working on the design, development and testing of a real-time oceanographic and meteorological measurement system, “The Hermit,” for use in coastal regions with very limited infrastructure. The system consists of a surface buoy and bottom mounted instrument platform. The surface buoy contains a meteorological station, near surface conductivity and temperature sensor, and real-time communications, and the bottom mount houses a conductivity, temperature, and pressure sensor (CTD), and acoustic wave and current sensor (AWAC), and an acoustic modem.

On September 2016, the latest prototype Hermit system was deployed for a long-term field test, 1.23 km (0.66 nmi) off the Atlantic Ocean coast of Virginia Beach, VA, in 9m of water (39 ft.). On 9 October 2016, the eye of Hurricane Matthew passed approximately 245 km from the Hermit. During the storm passage, the Hermit test region experienced tropical storm conditions for several hours, with sustained winds of 25-27 m/s (50-55 kts) and maximum gusts exceeding 33 m/s (66 kts). Nearby CO-OPS water level stations indicated storm surges up to 1 m (3 ft.). Coastal Data Information Program (CDIP) wave rider buoy 186, located approximately 29 km (15.6 nmi) east of the Hermit test site, reported significant wave heights up to 3.82m and maximum wave height of 6.88m during peak storm conditions.

While the Hermit’s surface buoy was un-moored and forced approximately 4km south of its original deployment location by high winds, currents, and sea-state, the bottom mount remained on station and successfully recorded measurements for the duration of the storm. Time series of the Hermit’s bottom mounted pressure sensor and the AWAC’s tilt and heading sensors indicate the bottom platform experienced some undesired, abrupt movements during periods of high sea state, bringing into question the quality and stability of water level observations derived from the platform’s CTD sensor. However, average AWAC tilt measurements appear to remain within the AWAC’s tolerance required for good current and wave data. During peak storm conditions, the AWAC reported a significant wave height of 4.14 m, maximum wave height of 6.92 m, and alongshore current magnitudes exceeding 1.5 m/s.

An overview of regional conditions during the storm passage from nearby, in-situ, operational NOAA observatory systems will be summarized, along with an overview of CTD, currents, and waves observations captured by sensors of the Hermit test system’s bottom mount. The discussion will include details of measurements collected by the Hermit’s AWAC sensor and an assessment of system performance throughout the storm event. AWAC observations highlighted to assess system performance include: platform motion detected in tilt, heading, and pressure records; acoustic surface tracking beam (AST) bad detection count; and pressure versus AST derived wave measurements. Additionally, a comparison of AWAC wave spectra to that of the nearby CDIP wave rider buoy throughout the storm even will be summarized.