Page 301 hydrography 3835 



3835, Broadcasting Stations 



Commercial broadcasting stations near the coast often have practical value in 

 navigation for use as radiobeacons. Those within the project area should be considered, 

 their value to navigation should be investigated, and those of importance should be 

 accurately located and reported as aids to navigation on Form 567. 



3836. Clearances 



It is essential that the actual clearances above mean high water of all bridges, 

 cables, and telephone or telegraph lines be accurately known. The clearances of 

 bridges are generally listed in the "List of Bridges Over the Navigable Waters of the 

 United States" published by the United States Corps of Engineers. The data given 

 in this publication should be verified in the field by actual measurements and the 

 proper reports made — ^if discrepancies are found they shall be fully discussed. The 

 clearances of bridges not listed in the above publication and the clearances of any other 

 overhead obstructions over navigable waters shall be determined by the field party 

 and the data showai on the smooth sheet and included in the Descriptive Report. (See 

 also 7846.) 



The shore ends of all submarine cables shall be located and shown on the sheets 

 (see 7847). 



384. Character of the Bottom 



In all hydrographic surveys, the character of the sea bottom shall be determined 

 at frequent and regular intervals throughout the project area to meet the needs of navi- 

 gation and for other purposes. This applies particularly to harbors and anchorages 

 and in all depths where vessels may anchor. 



Data on the character of the sea bottom are of value to the mariner in choosing 

 the most suitable place to anchor, where he may expect to find the best holding ground, 

 and what areas are free of rocks. In some instances, bottom data may still be help- 

 ful to him in determining his position, although generally their value for this purpose 

 has decreased because of the increasing use of echo-sounding apparatus in navigation. 

 They are of value in connection with dredging operations in harbors and channels and 

 for any underwater construction. Fishermen utilize such information in selecting 

 places where fish are likely to congregate and in avoiding t3^pes of bottom which may 

 damage their nets or equipment. And for the students of the earth sciences, bottom 

 data secured in deep water add to the knowledge of that part of the globe covered by 

 water. 



3841. Methods of Determination 



Knowledge of the character of the bottom is obtained in two ways; first, by bring- 

 ing to the surface for examination a specimen or sample of the bottom material; and 

 second, by determining the consistency of the bottom material by the feel of the 

 leadline. 



There are two general methods for bringing samples to the surface. The more 

 usual of these is by filling a cup-shaped depression in the bottom of the sounding lead 

 with tallow or soap to which a specimen of the bottom material adheres when the lead 

 strikes the bottom (see 4664). For types of bottom material which will not adhere to 

 the lead, a specially devised snapper with two spoon-shaped jaws closed by a spring is 

 used (see 4761). In oceanographic work special instruments which dig out and retain 

 a comparatively large core are used (see 476). 



