b. Persistent strong winds that blow daily or move through an area as 

 a weather front have a much greater influence on the change in the 

 water level on the shallow inland coastal waters of Texas, 

 producing either water setup or setdown, depending on location in 

 a water body. 



c. Storm surges and water-level changes accompanying differences in 

 air pressure can greatly change water level in shallow basins. 



d. Occasional strong river discharges, such as from the Brazos and 

 Colorado Rivers, will temporarily alter the water level more than 

 the astronomic forces. 



All four of these phenomena are not directly related to the 

 astronomical or deterministic tide and tidal datums as presently defined by 

 NOS. 



In addition, recent densification of tide stations on the Texas coast and 

 inspection of the resultant data have shown that, for much of the GIWW 

 and associated channels distant from inlets, an mllw tidal datum cannot be 

 computed following NOS standards and procedures. The mllw datum is 

 obscure because the meteorologically induced water motions that mask the 

 daily range of astronomic highs and lows is too small. An NOS technical 

 report (Gill, Hubbard, and Dingle 1995) documents the nontidal character 

 of portions of the Laguna Madre. This finding applies to much of the 

 GIWW and associated coastal inland channels. A navigation datum must 

 be developed for such areas as well. The establishment of a fixe d low- 

 water datum , such as is used in the Great Lakes and on the Mississippi 

 River for navigational purposes, is the practical solution. Such a datum 

 should be low enough that the water level will seldom, if ever, fall below 

 it. This datum can be readily implemented by specifying the proper design 

 channel depth in terms of the NOS chart datum for the project water body. 



In light of such observations, in the 1950s the CESWG established a 

 navigation datum which it called "mean low tide" (mlt) and defined it in 

 terms of a fixed vertical reference, namely, NGVD 29. For example, in 

 the Corpus Christi Bay area, mlt is 1 ft below NGVD. Accuracy 

 achievable with modern instrumentation for measuring water level, as well 

 as the requirement of reference to chart datum, bring the need to relate mlt 



Chapter 1 Introduction 



