DEVELOPMENTS IN TIDAL & TIDAL CURRENT MEASUREMENTS 775 



From this it can be seen that the equipment required for one cur- 

 rent party represents a modest investment. 



TroE AND Current Program 



Early in 1951 the Bureau embarked on a tide and current program 

 in domestic waters on a scale it had never before attempted. Several 

 vessels were assigned this duty exclusively. Two of these were large 

 enough to provide suitable berthing and working spaces for all per- 

 sonnel employed. Others were not, and it became necessary to estab- 

 lish self-contained receiving stations ashore. In protected waters the 

 employment of the shore-station method permits the use of medium- 

 sized launches suitably equipped to handle the buoys, but otherwise re- 

 quiring few men. A shore station can be operated quite satisfactorily 

 by nine men. One of these is the supervisor and relief and the other 

 eight are divided into four watches of two men each. The launch and 

 station combination makes for very economical operation, if or when 

 overhead costs of larger vessel operation are considered. 



Simultaneous observations of tides and currents have been under- 

 taken as regular routine in this program. Normally, about one-third 

 of the special tide stations in each area are established and operated 

 for a period of 12 consecutive months. They are equipped with stand- 

 ard recording gages. The remainder are equipped with portable gages 

 which are operated for 2 consecutive months. While the tides and cur- 

 rents are being observed, all tide station bench marks and tide staffs 

 are interconnected by closed loops of first-order levels. This provides 

 the means of relating the tide planes determined independently at each 

 station. When the observations at the tide stations have been cor- 

 rected to a 19-year mean through comparison with a reference station, 

 the results show the variation of the different tide planes throughout 

 the survey area as referred to a common datum. Also through com- 

 bination of simultaneous tide observations and the interconnecting level 

 lines, the pattern of gradient variation between stations throughout the 

 tidal cycle can be established. 



