Web Exclusive: Lessons Learned
USACE Projects and Vertical DatumsThe Corps of Engineers has over 3,000 projects related to flood control, shore protection, hurricane protection, water supply, hydropower and navigation that rely on knowing the relationship between a water surface or depth and a structure (natural or man-made) or land mass. The Corps of Engineers uses a variety of geodetic, tidal and hydraulic reference datums to define the elevations of project features relative to land or flood profiles. Some coastal protection and navigation projects are referenced to local tidal datums rather than those defined by NOAA.
On the Mississippi River between the mouths of the Missouri and the Ohio rivers (the Middle Mississippi River), depths and improvements are referenced to a Low Water Reference Plane (LWRP). No specific LWRP year is used for the Middle Mississippi north of Cairo, Ill. Below Cairo, Ill., depths and improvements along the Mississippi River are referenced to the 1974 LWRP and, most recently, the 1993 LWRP. These hydraulic-based reference planes were established from long-term observations of the river's stage, discharge rates and flow duration periods developed about the 97-percent flow duration line. Changes in the stage-discharge relationship will influence the theoretical flow line for the LWRP and, indirectly, the protection elevations of flood-control structures.
On controlled portions of the Mississippi and other navigable rivers, lakes and reservoirs maintained by the Corps, references such as “minimum regulated pool,” “flat pool level,” “normal pool,” “full pool level” and other surfaces are used. The relationship between these hydraulic references to an orthometric datum may be dated or, in some cases, unknown. Some navigable waterways maintained by the Corps are still referenced to the 1912 Mean Sea Level Datum, which preceded the NAVD 1929 adjustment. A variety of vertical datums can exist in a given geographical area such as those shown for Southeast Louisiana in the table below.
The IPET StudyIn October 2005, just two months after Hurricane Katrina hit the Gulf Coast, Chief of Engineers Lt. General Strock initiated the IPET to conduct a detailed investigation into the cause of the failure of the hurricane-protection system around the greater New Orleans area. The IPET consisted of more than 150 engineers and scientists from various federal and state government agencies, academic institutions, and private industries. The investigation performed by the IPET members was broken into 10 separate tasks, or teams. One of those tasks was the Geodetic Vertical and Water Level Datums team, which was responsible for evaluating the original constructed elevations for the various hurricane-protection structures in the New Orleans region. The team was also tasked to establish a consistent vertical reference framework, or datum, for hydrodynamic modeling studies and update the relationship between the local geodetic datum and the local mean sea level and evaluated subsidence across the southern Louisiana region.
The Geodetic Vertical and Water Level Datums team (hereafter referred to as the Datum team) was comprised of members from the USACE, NOAA and contractors from 3001 Inc., who were involved in a majority of the GPS and leveling data collection used to establish new bench marks, collect high water marks and other various survey data required by the Datum and other IPET teams. The results from the IPET study were independently reviewed by the American Society of Civil Engineers (ASCE) and the National Academy of Sciences.
In October 2005, the National Geodetic Survey released a new adjustment for 85 bench marks across southern Louisiana as a result of re-observations to update the NAVD 88 heights due to subsidence across the region (see IPET Volume II). These new heights were referred to as NAVD 88(2004.65) and are considered Vertical Time Dependent Positions. In order to determine the relationship of these bench marks to the local mean sea level in the region, direct observations using GPS static measurements were made between several of these bench marks and existing or historical NOAA tidal stations. All static GPS observations followed the NOAA NGS 58 Guidelines for Establishing GPS Derived Ellipsoid Heights 2/5 cm Standards Version 4.3 to insure data quality. Leveling between tidal bench marks was performed to Second Order Class I specifications and blue booked.
NOAA CO-OPS reestablished the tide gauge station at New Canal, which is located at the end of the 17th Street Canal on Lake Pontchartrain, as part of the effort to update the local mean sea level across this region. This station along with other existing and historical tidal stations provided a basis to compute the local mean sea level relative to the NAVD 88 (2004.65) geodetic datum. CO-OPS computed updated local mean sea level values based on the current 1983-2001 National Tidal Datum Epoch (NTDE) published in 2003 and a modified five-year tidal datum (2001-2005) due to rapid sea level rise and subsidence across the IPET study region (see IPET Volume II).
The Datum Team performed a historical review of the original design documents and as-built drawings for the various hurricane protection projects in and around New Orleans. An analysis was made of the specific bench marks that were used in design and construction, how they were established and the elevation at the time of design and construction. The elevations of these bench marks were later used in the analysis to determine if and how the relationship between the local mean sea level and geodetic datum was considered and/or accounted for in the construction of the flood control and hurricane protection structures.
Analysis done by the Datum team found that elevation values used in construction were based on geodetic datums, not the local mean sea level as was the intent. Elevation values were often from older epochs of the existing NGVD 29 geodetic datum instead of the most current published values. 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. Construction projects were tied to/based on only one bench mark and often the datum epoch or date established was not included in construction documents. Subsidence across the region has caused published bench mark elevations to change by more than 2 feet in the past 50 years (see figure 2 below) .
Plan of ActionAs a result of the IPET study and the Datum Team’s findings and lessons learned published in Volume II of the IPET report, the Corps has launched an initiative to evaluate all of the flood control, navigation, hurricane protection and shore protection projects in every Corps of Engineers district to ensure they are reliably referenced to the current NSRS vertical datum-e.g., NAVD 88, IGLD 85, PRVD02, etc. This initiative, termed the Comprehensive Evaluation of Project Datums-CEPD, is occurring at the same time other federal agencies are updating their products to the current NSRS vertical datum. Most notably is FEMA’s effort to update its flood maps. Each USACE District has appointed a “Datum Coordinator” who is responsible for overseeing the accomplishment of this task.
An interagency team comprised of USACE, NGS and CO-OPS personnel developed guidance documentation and a three-day training/certification class that each district datum coordinator attended and was instructed on how to evaluate each project. An Engineer Circular 1110-2-6065 Comprehensive Evaluation of Project Datums ( www.usace.army.mil/publications/eng-circulars/ec1110-2-6065/toc.htm ) was developed to provide detailed guidance on the CEPD effort.
The Corps is performing this initiative in two phases: an evaluation phase and corrective-action phase. During the first phase the datum coordinators are to evaluate each project, determine if it is tied correctly to the NSRS and/or the current tidal datum, define the corrective action to be performed if not tied to the NSRS and report on the findings to USACE Headquarters through a Web-based database tool. The second phase is to implement the corrective action plan for those projects not adequately referenced to the NSRS. It is expected that the initial project evaluation is to be completed over the next six to eight months. Corrective action is expected to take place over the next two to five years, depending on the priority and urgency of each project.
Project EvaluationsAs part of this effort, each inland flood-control project is to contain a minimum of one primary control permanent bench mark (PBM) that is part of the NSRS database and provide a relationship between the geodetic datum and water level reference plane developed from hydrologic flood profile studies and analyses. These primary project control PBM can be either existing NSRS control within five to 10 miles of the project or can be newly monumented points established by two separate four-hour GPS observations relative to the CORS network.
In the past, adding control data to the NSRS would be a time consuming and expensive process following the “Blue Book” guidelines. In implementing the CEPD project, the Corps worked with NGS to accelerate development of the OPUS Database (OPUD DB), a Web-based procedure that allows for a cost-efficient means of directly adding control points to the NSRS database. If existing NSRS control is used, these points are considered adequate after cursory checks into nearby NSRS control or by using one-hour OPUS sessions.
The nominal vertical accuracy of ±0.25 foot (8 cm) relative to the NRSR network is intended to meet the requirements for the majority of flood-control projects in the U.S. In areas with no existing NSRS control, where OPUS connections cannot meet the expected accuracy requirements or in high subsidence areas, the modified NOAA NGS 58 Guidelines for Establishing GPS Derived Ellipsoid Heights contained within EC 1110-2-6065 are to be followed to establish new primary project control points. Local project control established for construction and maintenance must consist of a minimum of three points and be tied to the primary project control. All water level gages used on inland projects are to have a minimum of three points, one of which shall be published in the NSRS to provide a direct relationship between the water level surface and geodetic datum.
For coastal hurricane protection, shore protection and navigation projects, each project is to contain a direct tie to NOAA tidal stations for MLLW and LMSL relationships on the current tidal epoch. By requiring all primary project control points to be in the NSRS and the NOAA CO-OPS tidal station database, future adjustments or changes in tidal datum epochs to be applied directly to the project.
Over the next few years, the Corps will be working with NOAA to develop guidance documents and training to ensure project datums are keep current-especially in high subsidence/uplift and coastal regions. Existing Corps design guidance documents related to flood control, navigation, hurricane protection and shore protection will be reviewed and revised to address the importance of reference datums and establishing the relationship between the water level datum and geodetic datum on these projects. The final report from IPET is available at www.ipet.wes.army.mil.
NOAA Technical Memorandum NOS NGS-58, Zilkoski, D. B., D’Onofrio, J. D., and Frankes, S. J. (Nov. 1997) “Guidelines for Establishing GPS-Derived Ellipsoid Heights (Standards: 2 cm and 5 cm),” Version 4.1.3. Silver Spring, Maryland
“Guidelines for Establishing GPS Derived Orthometric Heights (Standards: 2cm and 5cm)” version 1.4, National Geodetic Survey 2005 DRAFT NOS NGS 59; D. B. Zilkoski, E. E. Carlson, and C. L. Smith, NOAA, NOS, NGS.
“Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System,” Draft Final Report of the Interagency Performance Evaluation Task Force, U.S. Army Corps of Engineers, 1 June 2006. (Volume II--“Geodetic Vertical and Water Level Datums”) https://ipet.wes.army.mil .
Comprehensive Evaluation of Project Datums, (http://www.usace.army.mil/publications/eng-circulars/ec1110-2-6065/toc.htm )