CHAPTER III 

 SOUNDINGS AND HYDRAULIC SURVEYS 



IN order to estimate, even in an approximate way, the time 

 and cost of a contemplated job of dredging, it is necessary to make 

 an accurate survey of the locality. This should indicate the shore 

 lines, soundings, obstruction to navigation as bars, sunken -ledges, 

 boulders, etc., giving all the data from which can be obtained a thor- 

 ough knowledge of the place of the contemplated improvement. 

 From this survey is deduced the amount of material to be removed 

 from the bottom, in order to obtain the required depth of water; 

 and from the amount of material is calculated also the cost and 

 time required for the proposed work. 



Any hydraulic survey consists of two distinct operations, namely, 

 the measuring of the depth of water at various points and the location 

 of the different points where the soundings are made. 



Soundings. Soundings are made in three different ways: (a) 

 by means of graduated rods, (6) by lead lines, (c) by automatic 

 apparatus. 



(a) Soundings are made by means of graduated rods which 

 while resting on the bottom are kept vertical by the operators, 

 who read directly the depth of water at each point. The operators, in 

 small boats, go all over the part of the harbor or river to be improved, 

 while the different points are located by any one of the various 

 methods to be explained. Owing to the limited length of the grad- 

 uated rods, this manner of making soundings is only applicable 

 to localities with very shallow waters, and it is not accurate. 



(6) Soundings are more commonly made by means of a lead line. 

 This consists in lowering a weight of 10, 15 or 20 Ibs. of lead attached 

 to a graduated rope. The lead is shaped like the frustrum of a cone 

 with the base hollowed out to hold some grease. The soil of the 

 bottom adheres to the grease, and thus shows the nature of the 

 excavation, which information should be entered in the field book 

 and marked upon the map. The sounding line is made of strong 



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