continuous tidal datum system for all marine waters of the United States 

 and replaced the GCLWD by mllw, among several other actions. 



Figures 2-7 are plots of the year-long hourly water-level records at 

 representative tide gauges on the Pacific, Atlantic, and Gulf coasts. These 

 include two inland coastal stations of Texas, the South Padre Island Coast 

 Guard Station located at the southern end of the Laguna Madre, and 

 Packery Channel located at the northern end of the Laguna Madre near its 

 entrance to Corpus Christi Bay. The figures are plotted to the mllw datum 

 of each station, and the respective values of mhw and mlw are given in the 

 upper right corners of the plots. 



Figure 2 for Los Angeles, California, shows a clear signal of the 

 approximate 14-day neap and spring tide cycle corresponding to the 

 phases of the moon. By its definition as an average, there must be times 

 when the water level falls below mllw; for Los Angeles, this occurs at a 

 fairly regular periodic cycle corresponding to the times of spring tide, 

 when the moon is either new or full. Ships waiting to proceed into or 

 leave a port could consult tide predictions with reasonable confidence to 

 determine safe clearance under keel for a given draft according to load. 

 Overall, the water level at Los Angeles is fairly symmetric with respect to 

 the mtl. Little seasonal trend appears in the yearlong record. 



Figures 3 and 4 show yearlong records for Sandy Hook, New Jersey, 

 and Fernandina, Florida. The water level is not as pronounced in spring- 

 neap cycle as is that of Los Angeles, in part due to the mixed character of 

 the tide and also to the action of storms on the gently sloping continental 

 shelf of the Atlantic coast as compared to the Pacific coast. Water level 

 falls below mllw in a somewhat less regular manner than at Los Angeles. 

 A slight seasonal trend of lower waters in winter is observed. For the 

 Atlantic- and Pacific-coast gauge records, the body of measurements stays 

 well and regularly within the mean range of tide defined by the mhw and 

 mlw lines drawn in the figures. At both locations, the deviation below 

 mllw is typically 20 to 25 percent of the range. 



In contrast to the Atlantic- and Pacific-coast water-level records, the 

 record from Bob Hall Pier, Corpus Christi, Texas, on the Gulf of Mexico 

 (Figure 5), is irregular, with the seasonal trend change in water elevation 

 comparable to or exceeding the daily change in tide. Further, the deviation 



14 Chapter 2 Water Level Definition, Measurement, and Properties 



