to mllw and, preferably, to an as-yet-to-be defined navigation low-water 

 datum for portions of the inland coastal waters of Texas. A rational 

 navigation datum for the Texas coast must be developed and implemented 

 that accounts for the extended periods of time during which water level 

 along the Texas coast lies significantly below mllw. 



According to Engineer Manual (EM) 1 1 10-2-1003 (USACE 1991), 

 page 7-1, "when elevations are referred to a tidal reference plane in coastal 

 waters of the United States, mean lower low water (mllw) shall be used as 

 the vertical datum." The manual further states "In coastal areas, every 

 attempt should be made to assure that USACE datums are the same as, or 

 have a direct relationship to (underlining added by present authors), the 

 NOS chart datums. This is essential to ensure consistency for mariners 

 and others using Federal map and chart products. ... In making a 

 determination as to the correct datum, Congressional intent (authorization 

 documents) should be considered." The EM also states "If the tidal datum 

 that is used is different from mllw, a diagram should be included on the 

 drawings to indicate the relationship between the project datum 

 (underlining added by present authors) and mllw. The relationship to 

 NGVD 29 should also be shown." Presumably, based on more recent 

 guidance in ETL 1 1 10-1-152 (USACE 1994), the relationship to 

 NAVD 88 should now be shown. 



It is clear that guidance provided by USACE technical documents 

 holds provision for definition of project navigation datums, where 

 warranted. If it is not feasible to determine mllw or if it is technically 

 unsound to employ mllw as a navigation datum, then USACE Districts 

 have the authority to develop an appropriate navigation datum. This 

 project datum must be related to mllw or NOS chart datum, if available, 

 for consistency between Federal agencies and in publishing Federal maps 

 and charts. 



Since implementation of NGVD 29, the Texas coast has undergone 

 substantial geomorphic change under the influence of natural and human 

 forces that have altered both the relative location of the water level with 

 respect to land and the range of water-level variation. Along the Texas 

 coast, since the beginning of water-level gauging by NOS and its 

 predecessor organizations (Coast Survey, Coast and Geodetic Survey) in 



Chapter 1 Introduction 



