Katrina-inspired Tidal Studies Utilize HOBO Water Level Data Loggers
In late August 2005, Hurricane Katrina struck the U.S. Gulf Coast and became the country's costliest, and one of its deadliest, natural disasters. Nearly 2,000 individuals perished during the hurricane and the flooding that followed, with the highest number of deaths occurring in New Orleans. The storm surge from Katrina caused 53 levee breaches in the New Orleans area, leaving about 80% of the city under water.
Soon after this catastrophic event, the Interagency Performance Evaluation Task Force (IPET) was formed to conduct an investigative study into why the New Orleans area hurricane-protection system had failed.
The IPET group, which involved more than 150 scientists and engineers from the government, academia, and private industry, was divided into a number of sub-groups, including a "Geodetic Vertical and Water Level Datums" team, made up of members of the U.S. Army Corps of Engineers, the National Oceanic and Atmospheric Administration (NOAA), and contractors.
Through its study of the various hurricane-protection structures and analysis of historical information, the team found inconsistencies as well as insufficient documentation with respect to elevation values used during design and construction of the hurricane-protection systems. The team also found a lack of coordination in the New Orleans District in terms of ensuring a common vertical datum (a reference point from which elevation or depth measurements are made), and it was recommended that the Army Corps of Engineers' guidance on elevation datums should be updated.
In response to the IPET report and, specifically, to the Geodetic Vertical and Water Level Datums team's findings, the Army Corps is re-evaluating its projects throughout the country in order to make certain that they are all consistently referenced to current National Spatial Reference System vertical datums, such as North American Vertical Datum 1988 (NAVD88). The effort is known as the Comprehensive Evaluation of Project Datums (CEPD): Guidance for a Comprehensive Evaluation of Vertical Datums on Flood Control, Shore Protection, Hurricane Protection, and Navigation Projects.
Tide-gauge installation, consisting of calibrated tide board, survey marker, and PVC pipe housing a HOBO Water Level Data Logger, located on a pile at the Shrewsbury River Yacht Club."
The Army Corps' New York District has employed Rogers Surveying of Staten Island to perform tidal studies for the CEPD. Bert Wyness, a certified hydrographer and the project manager of the CEPD program at Rogers Surveying, notes that the company is working on seven Army Corps projects in the New York/New Jersey area.
Wyness explains that a national network of benchmarks was established in 1929 to determine mean sea level, with the local benchmark in his area located at Sandy Hook in New Jersey. The purpose of the CEPD, however, is to establish NAVD88 benchmarks within each Army Corps project, and then establish the mean lower low water (MLLW) at each of the benchmarks. Unlike mean sea level, which changes depending on location within a particular water body, NAVD88 is a datum that is unaffected by the water level. NAVD88, a more useful and recognized datum, was established by utilizing the original 1929 benchmarks along with additional benchmarks and recalibrating the resultant network of elevation points.
For the Army Corps projects being addressed by Rogers Surveying, the initial step was to determine benchmark locations, preferably at a bulkhead or a dock, where HOBO® Water Level data loggers (from Massachusetts-based Onset) could be deployed most easily.
For the first and now-completed project, located on the Shrewsbury River in New Jersey, ten water level data loggers were deployed by individually suspending them from wires within PVC pipes at key locations along the river. One HOBO Water Level Logger was also deployed above water in order to record barometric pressure. The barometric pressure readings, in turn, would be deducted from the pressure readings recorded by the ten submerged data loggers, resulting in data sets of water-induced pressure only.
The water level data loggers, recording at six-minute intervals, are calibrated to tide boards (essentially large rulers) at least twice during a 30-day tide cycle – once near the beginning of deployment and again toward the end, prior to retrieval of the loggers. The data are downloaded and the calibration numbers from the tide board readings are applied using Onset's HOBOware® Pro graphing and analysis software, from which the water level data (times and NAVD88 water elevation readings) are exported to Excel for further analysis and referenced to the nearest NOAA Tide Station (allowing for reference to the most recent 19-year tidal epoch), and the subsequent calculation of the MLLW and other relevant datums.
Although Rogers Surveying was able to deploy all ten water level data loggers in the most convenient type of locations, such as bulkheads or docks, more recent projects have required the use of a boat for deployment and retrieval of the loggers. In a few instances, divers have had to install sand screws into the sea floor and attach a small PVC pipe containing the data logger.
Wyness says that the HOBO water level data loggers are particularly useful to the project because they are user-friendly and versatile, and because they are small and therefore can be deployed easily. The affordability of the data loggers is an attractive feature as well, particularly since multiple loggers are required. Wyness also notes that data retrieval is readily accomplished with a pocket-size device called a "data shuttle," which stores the data from the loggers on site for later downloading into a computer.
The field work has been completed for all seven Army Corps sites, including projects at New York's East River and New Jersey's Sandy Hook Bay. Wyness states that it's now a matter of crunching the numbers and creating the reports to redefine the MLLW. Ideally, these corrective actions, taken in response to lessons learned after Hurricane Katrina, will not just put everyone on the same page regarding vertical datums – ideally, this newly updated and accurate information will actually help to minimize destruction from future disasters involving the phenomenal power of water.