Reference Engineering Documentation Report EDR-OD-04 “Houma Navigation Canal, Gulf Intracoastal Waterway (GIWW) to the Gulf of Mexico, including Bayou Le Carpe – MLG to MLLW Vertical Datum Conversion”, dated September 2022, stating that the U.S. Army Corps of Engineers (USACE), New Orleans District is converting the vertical datum for all coastal projects from the legacy Mean Low Gulf (MLG) terrestrial datum to the Mean Lower Low Water (MLLW) tidal datum, in accordance with a 2014 HQ-USACE Memorandum which defined policy for Federal Navigation Projects where the decision documents supporting project authorization and the project authorization in law do not reference the MLLW datum.

As of 4 November 2022, the results of EDR-OD-04 will be reflected on current and future hydrographic survey products as well as dredging specifications.  Channel depths formerly displayed in MLG datum, will now be displayed in MLLW datum.  The datum conversion of 0.0’ MLLW = 1.0’ MLG when applied to project authorization yields the following:

HOUMA NAVIGATION CANAL

-15’ MLG = -16’ MLLW from Mile 36.5 to Mile 0.0

-18’ MLG = -19’ MLLW from Mile 0.0 to Mile -6.0

BAYOU LE CARPE

-10’ MLG = -11’ MLLW from GIWW to the Short Cut Canal

This conversion value is considered absolute and documents current practices.  It will not be redefined for the Houma Navigation Canal.  However, the relationship between MLLW and NAVD 88 will need to be periodically updated to incorporate the future sea level rise and local subsidence.

EDR-OD-04 can be viewed online at the following URL:  https://www.mvn.usace.army.mil/Portals/56/docs/Navigation/EDRs/EDR-OD-04%20(9-Sep-2022).pdf