below mllw is typically on the order of 50 percent of the range and 

 sometimes equals or approaches the range in 1995. Although a spring- 

 neap cycle is evident, local nonastronomical tidal forcing makes the cycle 

 appear irregular. Similar observations hold for the South Padre Island 

 Coast Guard Station gauge, which is located near the Brazos-Santiago 

 Pass and ship channel (Figure 6). 



Figure 7 displays a year-long record for Packery Channel, which is 

 located just off the GIWW in Corpus Christi Bay, on the bay side of Padre 

 Island and nearly opposite to the Bob Hall tide gauge. Strong seasonality 

 in the water level is apparent, and, in 1995, the water level remained below 

 mllw for approximately 1 month starting in mid- January. Further, the 

 deviation in water level below mllw was typically equal to the mean range 

 of tide and even reached twice the mean range below mllw. This record 

 indicates a need for a navigation datum that accounts for seasonal lows 

 and that is not based on a simple average of daily water-level lows. 



Chapter 2 Water Level Definition, Measurement, and Properties 1 5 



