Zoning 



Zoning is a procedure in which the project area of a hydrographic 

 survey is divided into sections or "zones" to account for differences in 

 tidal ranges and phases within the area. In the hydrographic survey, 

 adjustments or "reducers" are applied to the depth soundings, which are 

 taken at all stages of the tide, to reference them to a common datum. The 

 correction must also allow for the time differences between the survey area 

 and the water-level gauge used for the control. Tidal zones are 

 distinguished by keeping two properties, range of tide and phase of tide, 

 within certain limits, typically 0.2 ft and 0.2 hr, respectively (Hicks 1989). 

 However, as stated by Hicks (1989), "these limits are subject to change, 

 depending upon survey accuracy, location, and tidal characteristics." In 

 the case of Texas inland coastal waters, not only the astronomical tide, but 

 also wind forcing, must be considered in the zoning. 



Character of the Tide Along the Texas Coast 



Characteristics of the tide along much of the Gulf of Mexico are 

 different than those on the Atlantic Ocean and Pacific Ocean coasts. Even 

 within the Gulf of Mexico, tidal characteristics are variable from one 

 locality to another within short distances. Prior to the 1950s, there were 

 only a few locations where long-term tidal measurements had been made. 

 Although diurnal high and low waters occur only during the extreme north 

 and south declinations of the moon, the tides were classified as diurnal in 

 character. The mlw was therefore used as the reference datum for 

 hydrographic surveys. 



As more systematic tidal observations were made in the coastal waters 

 of the Gulf of Mexico, it became evident that the classification of semi- 

 diurnal tides for the entire Gulf was not defensible. Although many areas 

 have two high and two low waters most of the time, there are nearby areas 

 where only one high and one low water occur. This observation led to the 

 creation of the Gulf Coast Low Water Datum (GCLWD) for the NOS 

 Nautical Charts. The GCLWD is defined as: mllw when the type of tide 

 is mixed, and mlw when the type of tide is diurnal. The National Tidal 

 Datum Convention of 1980 (Hicks 1980) established one uniform, 



Chapter 2 Water Level Definition, Measurement, and Properties 1 3 



