1. GENERAL REQUIREMENTS 



(Change 1) 17 



the area and the change in time and range 

 of tide from place to place. If, on arrival at 

 the working grounds, the selected sites are 

 found to be impracticable, the Chief of Party 

 may make necessary subsitutions, but he 

 must inform the Office of such changes and 

 the reasons for them (see 2-49 to 52) . 



Instructions for installation, maintenance 

 and removal of tide gages are contained in 

 Special Publication 196, Manual of Tide Ob- 

 servations. When practicable, observations 

 at each secondary station shall be continued 

 over a period of at least 29 days. The hourly 

 heights of the tide required for reduction 

 of soundings shall be tabulated before the 

 marigrams are forwarded to the Washington 

 Office. Hourly heights from standard gage 

 rolls will be furnished by the Office on 

 request. 



The time meridian used should be clearly 

 marked on the first marigram. When the 

 observations at any station are terminated, 

 a notation of the hour and date of discon- 

 tinuance should be entered on the last mari- 

 gram taken from the gage. The exact loca- 

 tion of each tide station shall be shown on 

 hydrographic sheets (see 6-71). 



1—47 Current observations. — A current 

 station is any specific location at which currents 

 are systematically measured. Geographic posi- 

 tions of project current stations will be listed in 

 the project instructions or will be indicated on 

 an accompanying chart of the area. Observa- 

 tions shall be made as close as possible to the 

 station sites specified. 



The project instructions will also specify the 

 depths at which observations are to be made, the 

 number of hours (usually 100) of observations, 

 and the groups of stations which are to be ob- 

 served concurrently. 



Velocity and direction of the current shall be 

 determined with the best available instrumental 

 equipment suited to the particular station. The 

 project instructions will usually specify the 

 equipment to be used, whether current pole and 

 Price meter, the Roberts meter, or other types 

 of meters. General instructions are outlined in 

 the Manual of Current Observations, and in- 



structions for use of the Roberts meter are in 

 the Roberts Radio Current Meter Operating 



Manual. 



To insure satisfactory meter operation, all 

 meters shall be checked by pole observations. 

 At least one series of three such comparisons 

 shall be made at regular half-hourly meter 

 readings, preferably bracketing a strength of 

 current. Meter and pole ob.sei-vations must be 

 simultaneous; special meter readings shall be 

 made and recorded for pole observations which 

 cannot bo made on meter scliedule. Any (nies- 

 tioiiable meter performance indicated by the 

 comparisons shall be investigated and corrected 

 as soon as possible. 



Unleas modified by the project instructions, 

 observations in charted depths of 35 feet or 

 more shall be made at i/o, i/o, and % M,e depth 

 but in no case shall the top meter l)e set deeper 

 than 15 feet. In charted depths of 25 to 35 feet, 

 observations shall be made at % ^^'ic^ V> the 

 depth and at 6 feet less than charted dejjth. 

 (Observations in charted depths less than 25 feet 

 shall be made at 1/3 and % the depth. Tidal 

 changes need not be considered in determining 

 depths of metei-s. 



Current observations shall be made half- 

 hourly for the full period specified in the project 

 instructions except where modified in following 

 jiaragraplis. 



Previous instructions state that a station shall 

 be tenninated when the current does not exceed 

 0.4 knot during the first 25-hour period, and this 

 rule shall still apply when the current is both 

 weak and erratic. However, engineers and 

 other scientists are interested in the water move- 

 ment at low velocities, and much useful data will 

 be lost if a station is automatically terminated 

 when the current does not exceed 0.4 knot. 

 Therefore, when a weak current exhibits a def- 

 inite pattern such as uniform flow in one direc- 

 tion, re]ieating rotary action, or reversal of 

 direction the station shall be observed for the 

 full period stated in tiie project instructions. 



If a series of (>l)servations is broken due to 

 faihue of ecjuipment or to other causes, every 

 reasonable effort shall be made to resume ob- 

 servations as soon as possible. 



81018 O - 63 



