Wave Climate Sampling

Part 1: Data Collection Methodology

HOBO Sensors in Conduit or Stabilization Pole

HOBO Sensor

This study aimed to determine the general wave attenuation capabilities of partially-submerged detached breakwaters located within the Delaware Bay region of New Jersey. To observe wave attenuation capabilities, the Stockton University Coastal Research Center (CRC) conducted two low-cost deployments of HOBO Onset Water Level Loggers to gauge water level differences pre and post-transmission generated by these coastal structures. Two separate deployments were conducted in the winter of 2022 (November 17-18) and spring of 2023 (March 21-22) to measure seasonal variations in a wind-driven system. Deployments consisted of eighteen individual HOBO pressure sensors calibrated prior to installation. Each sensor was adjusted to collect pressure readings (in psi) in one-second increments, allowing for a 12-hr coverage of water levels above the sensors. To ensure 24-hr coverage was obtained for this coastal system (semi-diurnal tide influenced), two individual HOBO sensors were fitted to each stabilization pole and were programmed to collect readings at different time intervals during deployment. One sensor was programmed to collect for the first designated 12-hr period, while the second sensor was programmed to begin roughly 10 minutes before the first sensor concluded data collection. To measure the physical differences in water levels generated by the detached breakwaters, sensors were deployed approximately 20 feet landward and seaward of two structures (breakwaters 3 & 4) and were deployed via Stockton CRC professional swimmers. A sensor set was also positioned within the channel between the identified structures (see figure below) to observe water level differences within the project delineation without direct attenuation influence from the structures. The water levels in the channel were also collected due to the potential for wave diffraction from the structures, which could amplify waves between the channel (thereby causing more erosion in those specific areas). A control site was deployed south of the project delineation (closer to Nantuxent Point) to account for natural differences unaffected by the hard structures and to record water temperature, which is required for data processing. A series of air pressure sensors were also installed near the Money Island entrance on an anthropogenic-generated dune system to gather atmospheric conditions, including air temperature and pressure. Once sensors were installed, global position system (GPS) points were recorded to collect the exact Northing, Easting, and Z-Depth elevation values for each sensor utilizing a triangulation technique with a Total-Station tripod.

Figure 1a: Depicts the HOBO sensor deployment map for the Money Island project. Map generated by Matthew Deibert and the Stockton University Coastal Research Center (Fall 2022).

Picture 1a: Shows the triangulation technique completed to gather the Northing, Easting, and Z-Depth values for this study. CRC professional swimmers drove the stabilization poles around the structures during low tide scenarios. Photo credit: Matthew Deibert.

Part 2: Data Offloading & Pre-Processing

Equations used to get NAVD88 referenced elevations

RBR Promotional Flyer

Once the sensors were retrieved, the internal data stored was fine-tuned utilizing the HOBOware pro application to account for barometric compensations (fluid density of saltwater: 3.989 lb/ft³) and atmospheric conditions (temperature in Fahrenheit). From the HOBOware software, absolute pressures (psi), absolute pressure barometric (psi), and sensor depth (feet) values were offloaded and exported to a Microsoft Excel spreadsheet which consisted of the seven sensor locations within the project delineation (see figure above). From the sensor depth values, a conversion was then conducted to obtain water level values in relation to the North American Vertical Datum of 1988 (NAVD88) by accounting for the physical offsets (conduit & sensor), which were subtracted from the total depth measurements to get a more accurate reading. sensor depth values represented water levels above the sensor, while the NAVD88 depth displays the water level referenced to a vertical datum based on an orthometric height (location of the geoid/mean sea level for that geographical point). Upon converting the seven individual known sensor location depth values into NAVD88 depths, sensor data for the fourteen corresponding sensors were then individually exported to Microsoft Excel spreadsheets. Data included in the export process consisted of date & time, absolute pressure (psi), absolute pressure barometric (psi), and sensor depth (ft). NAVD88 depths were derived from each pressure reading during deployment following the same offset methodology utilized for the known deployment locations.

Since HOBO sensors cannot generate wave-specific data (limitation of HOBO water level loggers), the values derived from each HOBO sensor were then averaged per minute to create an overall wave climate scenario to compare water levels between the sensor locations. Individual wave/water level data on a per-second interval was not accounted for since comparing the two locations at the same second would yield skewed results that do not correspond. For this analysis, the recorded water levels represent a theoretical wave climate within the Money Island project delineation for a given minute. This is plausible since Money Island is a wind-driven system, meaning that besides tidal influences, winds have the most pronounced effect on waves and water levels along its coast, unlike a traditional coastline which may have other factors, including ocean swells and currents. HOBO sensors were chosen for the analysis since we can infer wave attenuation percentages from differences in water levels due to the natural forces (meteorological and oceanic) found at Money Island. The Stockton CRC does plan to utilize wave-specific equipment such as; RBRs and/or Acoustic Doppler Current Profilers (ADCPs) for future research at Money Island. This research was designated a low-cost pre-analysis to compare the capabilities of the HOBO for wave attenuation capabilities.

Part 3: Post-Processing

Upon offloading the individual sensor data via the HOBOware pro application, the data was then post-processed utilizing two different methodologies to display differences in water levels, which could account for wave attenuation at Money Island. The first portion of the post-processing analysis consisted of calculating the differences in NAVD88 depths between corresponding sensors and running statistical analysis (T-Tests & Anovas) to determine if significant differences exist. Since the sensors were referenced to NAVD88 depths based on mean sea level, they accurately represent the physical differences in water levels. The individual sensor depths (ft) offloaded from the raw sensor data do not account for an accurate portrayal since they were not installed at the same elevation. This elevation difference can cause skewed data if not accounted for. NAVD88 depths were not utilized to find attenuation percentages but were included in determining if significant water level differences were observed, which would theoretically display significant wave attenuation overall. Differences were calculated simply by subtracting the NAVD88 depth value of a seaward sensor from a corresponding NAVD88 depth of a landward sensor and taking the absolute value of that result. Overall, NAVD88 depth difference and statistical analysis were conducted between corresponding breakwaters (1&2 and 3&4, or 5&6 and 7&8), landward breakwaters and channel sensors (1&2, 5&6, and 9&10), control site (11&12 and 13&14), and all landward sensor values (1&2, 5&6, 9&10, and 13&14).

The second portion of the post-processing analysis consisted of generating attenuation percentages based on the differences in water levels between two corresponding sensors. By knowing the senor depth values (ft), percent differences could be calculated and represented as attenuation percentages once differences in sensor Z-Depth elevations were accounted for. By taking the GPS points and collecting each sensor's Z-Depth elevation, the physical difference in elevation heights could be determined simply by subtracting the value of the seaward sensor from the corresponding landward sensor and taking the absolute value of that outcome. Once the physical difference in sensor elevations was determined, that value could then be subtracted from the Sensor Depth (ft) raw data collected from the seaward HOBO sensor to place an equal theoretical plane between two corresponding sensors. With an equal plane generated based on sensor elevations, the Sensor Depths (ft) could be analyzed between two sensors as if installed at an equal elevation during deployment. From there, a percent difference calculation between the sensors could be completed. The result of the percent difference equation would then represent a theoretical attenuation percentage generated by the detached breakwaters at Money Island.

Percent Difference equation used to observe Attenuation Percentages

Part 4: Comparison between Results and Wind/Tide Data

The last and final portion of the wave analysis of the capstone consists of evaluating the attenuation percentages and NAVD88 difference values with the Tidal and Meteorological (wind) observations from the deployment dates. Since Money Island is a wind-driven system, this is a vital step in the methodology of this research, as changes in these observations can explain the seasonal variations between the deployments. On Money Island, the winter season consists of strong Westerly winds, typically eroding the coastline. Whereas in the spring, the winds tend to be dominant in the South or East direction, with minimal wave action on Money Island. Therefore naturally, the attenuation percentage (percent difference) should be significantly higher during winter than in the spring. Also, a beach tends to gain sediment during the spring (and mostly summer) seasons as wave actions are reduced. Thus, allowing for more cross-shore sediment transport, gradually extending a beach. Which would help explain the results from the sediment distribution portion of the project.

For this project, meteorological observation data was derived from the National Oceanic and Atmospheric Administration's (NOAA) Tides & Current website from their meteorological station #8537121 located at Ship John Shoal Lighthouse in the Delaware Bay. Situated approximately 7.52 miles Northwest of Money Island, the observation tower at Ship John Shoal Lighthouse is the closest in proximity to the site location. Although, meteorological observation data was also examined from two other locations, including Millville Municipal Airport (Millville, New Jersey) (10.84 miles Northeast) and the Dover Air Forse Base (Dover, Delaware) (16 miles Southeast). Tidal observations were derived from NOAA's Tides & Currents website for station ID 8537052 at Money Island, New Jersey. Tides were taken from NOAA's tide prediction model, an accumulation of various subordinate stations in the Delaware Bay (Reedy Point, Delaware).

Page updated

Report abuse