CHAPTER XVII 

 STIRRING DREDGES 



STIRRING dredges can be sometimes employed with advantage 

 for the removal of materials from the bottom of rivers and harbors. 

 When the bottom is composed of very finely divided particles it is 

 evident that by stirring up these deposits while the water has a given 

 velocity, the water will carry in suspension the particles to be depos- 

 ited again at a great distance from the original point. Then the 

 dredging operation is simply reduced to stirring up the materials at 

 the bottom, while the water acts as a means of transportation, 

 and the erosive dredges used for such a purpose will be simply 

 provided With a device for the agitation of the materials. 



For the successful employment of the stirring-up process as a 

 means of excavating the bottom of channels, rivers, harbors, etc., 

 two conditions should be satisfied: First, the material should be 

 reduced to very finely divided particles so as almost to float when 

 agitated, and consequently it is only applicable to the finest sands 

 and muds. Second, the water holding these materials in suspension 

 should have such velocity of flow as to carry them to deep water 

 or to localities where they will not interfere with navigation before 

 depositing them. Hence this method of dredging can only be 

 applied under given conditions. Thus in the harbors and tidal 

 rivers it cannot be used, but during the ebb tide and in the 

 ordinary rivers where the velocity of water is such that will permit 

 the materials to remain in suspension until deep waters will have 

 been reached, thus preventing the formation of other obstructions 

 down stream. 



Different devices have been used to agitate the bottom; the most 

 common being the harrow, the revolving drums, the screw propeller 

 and the pump. All those that have been used and have given 

 good results will be described. There are, however, numerous other 

 methods all covered by letters patent. A long list of these devices 

 can be found in the paper on Dredges and Dredging in the Mississippi 



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