5. HYDROGRAPHY 



157 



shoal indications exist which require further 

 investigation, the limits of the area must be 

 defined by running a series of closely spaced 

 sounding lines. A study of the soundings 

 thus obtained should reveal the approximate 

 location of the least depth, but may not 

 establish the least depth on the feature. A 

 more intensive examination of the shoalest 

 part of the feature should be- made in an 

 effort to obtain the least depth. When the 

 bottom is visible, this is an easy task since 

 a lead can be placed on the top of all high 

 points and the depth and position of each 

 one recorded. In other areas the problem is 

 more diflficult. It is best to use a short wire 

 drag, if this is possible with the available 

 equipment. Otherwise, a small buoy should 

 be anchored near the place where the least 

 depth is suspected to exist. The buoy will 

 serve as a reference point while the launch 

 cruises slowly over the area. A second 

 marker buoy may be used to mark the 

 shoalest sounding obtained by the echo 

 sounder. The final examination is then made 

 by drifting over the shoalest part taking 

 leadline soundings. Three or four leadsmen 

 may be distributed along the length of the 

 boat and, as the boat drifts with the wind 

 or current, the bottom is felt and the least 

 depth recorded. The boat should be allowed 

 to drift across the shoal several times, each 

 time overlapping the previous path about 

 half the length of the boat. 



When the existence of a pinnacle rock is 

 suspected from the general nature of the 

 visible terrain, a very patient and exhaustive 

 search must be made or the least depth may 

 not be found. 



5-70 Record of shoal examination. — 



Where a shoal is examined by sounding along 

 a series of closely spaced lines, all data shall 

 be recorded in the sounding record as usual 

 (see 5-25). Where the least depth is found 

 by drift sounding or any other nonsystem- 

 atic procedure, a full report must be entered 

 in the sounding record of the following items 

 when they are not otherwise evident: 



(a) The method of search used. 



(b) The length of time spent in the 

 examination. 



(c) A statement as to whether bottom 

 was visible or not. 



(d) A brief description of the feature in- 

 cluding character of the bottom. 



(e) Whether the shoal is marked by kelp, 

 eddies, or other visible evidence. 



5-71 Detached breakers. — Where the ex- 

 istance of a sunken rock or other obstruction 

 is evidenced by breakers and it is imprac- 

 ticable to locate it by a three-point fix or to 

 obtain a sounding on the spot, it must be 

 located by cuts from nearby positions of the 

 sounding vessel. The cuts should form a 

 good intersection, and the depth over the 

 feature should be estimated and the time 

 noted. The conditions under which the area 

 breaks must be noted, at what stage of the 

 tide, and under what conditions of sea. Where 

 sunken rocks exist inside a generally foul 

 area they may be symbolized without loca- 

 tion provided the outline of the foul area is 

 accurately located. 



Whenever possible such features should be 

 located during periods of minus tide and 

 relatively calm sea when danger of damage 

 to the launch is least. Small areas around 

 rocky points can often be examined and the 

 limits of a foul area determined in a skiff 

 or other small boat. 



5-72 Wrecks and obstructions. — All 



wrecks and obstructions not afloat should be 

 located and as complete information as pos- 

 sible furnished. Whether the wreck is to- 

 tally submerged, visible at all stages of the 

 tide, or visible at some stage of the tide, 

 should be stated and any visible part of the 

 wreck should be described. Sunken wrecks 

 should be treated as dangers or shoals (see 

 5-69) and the same information should be 

 obtained relative to them. The least depth 

 on a wreck is practically impossible to de- 

 termine without dragging the area because 

 of the probable existence of masts or other 

 parts of the wreck which one cannot expect 

 to find by ordinary sounding methods. 



When large pieces of floating wreckage, 

 logs, or other debris, menacing to naviga- 

 tion, are sighted in areas where such ob- 

 structions are not commonly encountered. 



