Analysis of Datums and Elevations in USACE Projects: FINAL REPORT
The USACE assembled this team as a direct result of the catastrophic losses endured after Hurricane Katrina struck the Gulf Coast in 2005. The emphasis on vertical accuracy came about as a result of an extensive investigation into the causes of levee failures in and around New Orleans. According to report written by USACE engineers entitled “Lessons Learned,” published in the Point of Beginning website, “…elevation values used in construction were based on geodetic datums, not About This Report the local mean sea level as was the intent. Elevation values were often from older epochs of the existing NGVD 29 geodetic datum [National Geodetic Vertical Datum of 1929] instead of the most current published values” (http://www.pobonline. com/Articles/Article_Rotation/BNP_GUID_9-5 2006_A_10000000000000360086). Design engineers assumed that the NGVD 29 datum was equivalent to mean sea level and used NGVD 29 values as such, resulting in 1- to 3-foot differences between the intended design and constructed elevations.” Moreover, the team learned that, “Construction projects were tied to/based on only one benchmark and often the datum epoch or date established was not included in construction documents.” This is particularly troublesome given that “subsidence across the region has caused published benchmark elevations to change by more than 2 feet in the past 50 years.”
In light of these findings, the USACE set out to assess reference datum accuracy requirements that are currently in place, and to establish whether the USACE is able to perform reliable datum uncertainty analyses to ascertain the risks of project failure. To that end, the USACE contracted with the Conrad Blucher Institute for Surveying and Science at Texas A & M University-Corpus Christi to objectively answer these critical questions. Under the leadership of Dr. Gary Jeffress, Executive Director of the Blucher Institute, the Task 11 team, as it is referred to by the USACE, was assembled to collect, analyze, and ultimately present their findings regarding current datum accuracy requirements, and to determine to what extent the USACE is able
to perform reliable uncertainty analyses to ascertain the risks of project failure.
Over the course of a year, the Task 11 Team produced a series of reports. The first report, Phase 1, provided an assessment of the reference datum accuracy requirements that are currently in place in the USACE, and what challenges and obstacles district staff encounter as they carry out needed projects for their Districts. This phase included a thorough literature review of current USACE policy manuals, circulars, and other documents, and an analysis based on numerous interviews with USACE staff from districts around the country. The Phase 1 report concluded that engineers and surveyors from the USACE generally understand the issues related to datum uncertainty and that they welcome reducing this uncertainty as well as establishing a uniform approach to a datum uncertainty analysis.
The report revealed that each District has its own agenda and timeline to achieve implementation, which is largely dependent upon the resources and available personnel with the education and experience to undertake the work. Over the past decade, Districts have reduced personnel and funding for elevation analysis. Some Districts face significant funding, personnel, and resource shortages needed to implement datum uncertainty analyses. Next, the Task 11 team produced a report entitled, “Uncertainty Model for Orthometric, Tidal, and Hydraulic Datums for Use in Risk Assessment Models.” Phase 2 offered a technical discussion of risk assessment, specifically regarding relevant orthometric and water level datums and datum conversion for use in protection grade design, and discussion of a suggested approach to integrating vertical uncertainty into future USACE project risk assessments.
Findings in Phase 2 included an analysis of existing risk assessment guidelines within USACE, as well as a statistical discussion of perceived risk versus actual risk. This statistical discussion goes on to compare and quantify accuracy versus uncertainty. Each datum used by the USACE is analyzed for uncertainty and the accompanying risks, including terrestrial datum and water level datum, and datum conversions, such as converting legacy NGVD 29 measurements to NAVD 88 elevations.
The findings in Phase 2 also revealed that a very limited analysis of risks associated with converting legacy datums to modern datums has been conducted by the USACE, and that these initial studies reveal the complexities involved in the process, as well as a lack of historical data coverage of significant portions of the United States. Both Phase 1 and Phase 2 technical reports can be read in their entirety at: http://www.agc.army. mil/ndsp/index.html.
This report, the last in the series, is designed to clearly summarize the findings of this yearlong study, offer recommendations, and provide an overview of the findings suitable for non-technical readers.