140 



HYDROGRAPHIC MANUAL 



the right and left quarter and center half 

 of the channel or for each quarter of the 

 channel width. When the hydrographic sur- 

 vey reveals that shoaling is taking place in 

 a channel by amounts which may present a 

 hazard to ships using it, the Chief of Party 

 should notify the nearest office of the Corps 

 of Engineers. Copies of all correspondence 

 and sketches shall be forwarded to the 

 Director. 



5—25 Spacing sounding lines. — The prop- 

 er spacing of sounding lines depends on the 

 depth of the water, character of the sub- 

 marine relief, scale of the survey, and the 

 importance of the area being surveyed. The 

 general spacing of lines should give a repre- 

 sentation of the depths and generalized depth 

 curves in the area, and should be sufficiently 

 close to give indications of all submerged 

 dangers. The general spacing must be reduced 

 wherever necessary to fully develop all bot- 

 tom relief and to obtain the least depth on 

 shoals, banks, and pinnacles (see 1-25). 



The project instructions usually specify the 

 maximum spacing which may be used in 

 various depths or areas. The hydrographer, 

 or the chief of party, should reduce the line 

 spacing as necessary in critical areas to ac- 

 complish a complete hydrographic survey, 

 but the general spacing specified in the pro- 

 ject instructions shall not be increased or 

 decreased over large areas without prior ap- 

 proval of the Director. If the Chief of Party 

 is of the opinion that the general spacing 

 should be changed, he should submit a full 

 report in support of his request for an amend- 

 ment to the instructions. 



There is a practical limit to the number of 

 lines of soundings which can be plotted at any 

 given scale. In general, four of five sounding 

 lines to the inch can be plotted and the 

 soundings inked without difficulty. With a 

 little more care in plotting and selection of 

 soundings to be plotted, the number of lines 

 can be increased to 7 or 8 to the inch. Lines 

 plotted in excess of this number seldom con- 

 tribute significantly to the survey. After the 

 bottom configuration has been well developed, 

 the hydrographer may find it desirable to 

 run many more short lines over a shoal in 



an effort to determine the least depth on it. 

 These lines should be plotted on the boat 

 sheet or an overlay, but, unless they furnish 

 new information not supplied by the other 

 lines, the positions on the extra lines should 

 be clearly marked "not to be smooth plotted" 

 and the reasons stated (see 1-28). Such 

 lines are not to be rejected and the fatho- 

 grams and other pertinent data shall be for- 

 warded with the records. The fathograms 

 must be carefully studied first to make cer- 

 tain that the least depth is not overlooked. 



When the bottom consists entirely of mud 

 or sand, its slope will not, as a rule, be very 

 great at any point and a shoal must there- 

 fore occupy a considerable area in porportion 

 to its height. It is unlikely that a shoal in- 

 volving any great change in depth could lie 

 wholly between two adjacent lines and so 

 remain undetected. Conversely, where the 

 bottom is rocky, sharp irregularities must 

 be expected and every shoal indication must 

 be examined. Though a shoal is usually in- 

 dicated by a decrease in depth, it is also neces- 

 sary to regard unexpectedly deep soundings 

 with suspicion since they may mark the scour 

 which is caused by the currents beside a 

 rock or other obstruction rising steeply from 

 the bottom. Side echoes may be recorded on 

 the fathogram in such cases. 



5-26 Line spacing in harbors. — In har- 

 bors, bays, passages, channels, and rivers 

 the general rule is that the maximum spac- 

 ing of sounding lines is: 100 meters in depths 

 less than 11 fathoms; 200 meters in depths 

 from 11 to 30 fathoms; and 400 meters in 

 greater depths. In narrow channels, either 

 dredged or natural, the line spacing shall 

 not exceed 50 meters. Soundings shall be 

 obtained along the faces of all piers and in 

 the berthing areas adjacent thereto (see 

 5-83). 



If the area is of sufficient importance to 

 require a survey at a scale of 1:5,000, the 

 sounding lines shall be spaced at intervals 

 of 50 meters. 



In all cases, the sounding line spacing must 

 be reduced as necessary to develop shoals 

 and ascertain the least depths on them, and 

 to provide soundings in sufficient numbers 



