Page 283 hydrography 3661 



, 3661. Search for Shoals at Low Tide 



All examinations to discover least depths may be conducted more easily and with 

 more certainty as to results at extremely low tides, except that the survey vessel must not 

 be endangered in doing so. Advantage should be taken of the period of spring tides 

 for this purpose and generally only that period from 2 hours before to 2 hours after 

 low tide. 



If there is danger of striking the shoal, the examination should always be made on a 

 rising tide, preferably just after low tide, so that if by accident the survey vessel grounds 

 it can be floated more readily during the rising tide. (See also 361.) But for the sur- 

 vey of extensive shoal areas it may be advantageous to utilize periods of high tide when 

 the survey vessel may run sounding lines over the area with assurance of not striking 

 or stranding. 



3662. Systematic Sounding Lines Over Shoal 



To delimit the area within which the least depth is, and often to determine the 

 least depth, the most practical method is a system of closely spaced sounding lines run 

 over the limited area in which the shoal is known to exist. The closeness of the spacing 

 will depend on the character of the area and the type of danger or shoal. In a search 

 for an isolated rock, lines 10 meters apart may be required, while over large shoals 

 composed of sand in which rocks are unlikely, all that may be required is to split the 

 principal system of lines with one or more lines. 



If splitting the principal system of lines will not give an adequate development, 

 a new system of lines should be run over the shoal, at an angle of 45° or more to the 

 direction of the principal system. 



For a close development the use of ranges is almost essential, and for areas near 

 enough to the coast they should always be used to assist in covering the area closely 

 and economically (see 3142). Distance angles may be used for the same purpose if 

 more convenient (see 3143). 



3663. Examination With the Wire Drag 



When a danger or obstruction is not visible the only positive way of proving the 

 least depth on it is by the use of a drag. In important areas the standard wire drag 

 should be used for such purpose and to disprove the existence of reported dangers and 

 obstructions. 



In many cases time can be saved by developing danger indications with the drag. 

 This is especially true where the shoal area is large and is suspected of containing 

 isolated rocks or pinnacles. The wire drag should also be used where the nature of 

 the bottom and the adjacent shore indicates the probable existence of dangers that 

 may have been missed by ordinary sounding. 



Any important danger discovered must be cleared with the wire drag within 2 

 feet of the least depth obtained on it if this is possible. If it is impracticable to clear 

 it by this small margin the reason should be stated in the Descriptive Report. 



Unless otherwise directed all wire-drag operations shall be executed in accordance 

 with the requirements of Special Publication No. 118, Construction and Operation of 

 the Wire Drag and Sweep. (See 391.) 



