Table 1 



Tidal Datums and Definitions, Yaquina Bay, Oregon 



Tide 

 Staff 



(ml 



Datum and Definition 



4.42 



3.14 

 2.55 



-.88 

 -.96 

 1.07 



Extreme high tide. The highest projected tide that can occur. It is the sum of the 

 highest predicted tide and the highest recorded storm surge. Such an event would be 

 expected to have a very long recurrence interval. In some locations, the effect of a 

 rain-induced freshet must also be taken into consideration. The extreme high tide level 

 is used for the design of harbor structures. 



Highest measured tide. The highest tide actually observed on the tide staff. 



Highest predicted tide. Highest tide predicted by the Tide Tables. 



Mean higher high water. The average height of the higher high tides observed over a 

 specific time interval. The intervals are related to the moon's many cycles, which 

 range from 28 days to 18.6 years. The time length chosen depends upon the refine- 

 ment required. The datum plane of mhhw is used on NOS charts to reference rocks 

 awash and navigational clearances. 



Mean high water. The average of all observed high tides. The average is of both the 

 higher high and of the lower high tide recorded each day over a specific time period. 

 The datum of mhw is the boundary between upland and the tideland. It is used on 

 navigational charts to reference topographic features. 



Mean tide level. Also called half-tide level. A level midway between mhw and mlw. 

 The difference between mean tide level and local mean and sea level reflects the 

 asymmetry between local high and low tides. 



Local mean sea level. The average height of the water surface for all stages of the 

 tide at a particular observation point. The level is usually determined from hourly 

 height readings. 



Mean sea level. A datum based upon observations taken over a number of years at 

 various tide stations along the west coast of the United States and Canada. It is offi- 

 cially known as the Sea Level Datum of 1 929, 1947 adj. The msl is the reference for 

 elevations on USGS quadrangles. The difference between msl and local msl reflects 

 numerous factors ranging from the location of the tide staff within an estuary to global 

 weather patterns. 



Mean low water. The average of all observed low tides. The average is of both the 

 lower low and of the higher low tides recorded each day over a specific time period. 

 The datum of mlw is the boundary line between tideland and submerged land. 



Mean lower low water. The average height of the lower low tides observed over a 

 specific time interval. The datum plane is used on Pacific coast nautical charts to 

 reference soundings. 



Lowest predicted tide. The lowest tide predicted by the Tide Tables. 



Lowest measured tide. The lowest tide actually observed on the tide staff. 



Extreme low tide. The lowest estimated tide that can occur. Used by navigational and 

 harbor interests. 



20 



Chapter 2 Secondary Sources of Coastal Information 



