258 On the Construction of Dams, 



DAM, — a bank or obstructiou built across a river or 

 stream for the purpose of raising the level of the water on 

 the upper side of it. 



There are many objects for the attainment of which it may 

 be necessary thus to check the course of a stream, and gain a 

 head of water. It may be requisite to turn it for a time 

 into another channel — to inundate the ground in front of 

 part of a military position — to make a portion of the stream 

 uufordable- — to secure depth sufficient to enable vessels of 

 a given draught of water to navigate the stream — or to gain 

 a power to be applied to mechanical purposes. The Avorks 

 for the attainment of the first three of these objects belong 

 more particularly to the class which officers may be called 

 upon to execute in the course of a campaign ; and the 

 details here given refer entirely to works composed of such 

 materials as may be expected to be Avithin reach of an 

 officer on service. 



Dams built for the purpose of inland navigation, or for 

 that of securing a water power, may be considered as having 

 a more permanent character, and will be treated of in the 

 second volume of this work. 



The first consideration in forming a dam across a stream 

 is the choice of a proper site. This must, of course, be 

 decided with reference to the objects to be attained by the 

 rise of water ; but there are a few general rules to which 

 attention should be paid. 



In streams liable to sudden floods it would be advisable 

 to carry the dam across the Avidest part of the stream, so as 

 to allow ample space for the water to flow over, and thus to 

 prevent any sudden and great rise above the dam ; or it may 

 even be advisable to carry the dam in an oblique line across 

 the stream. In rivers, where much drift wood is likely to 

 be brought down, the dam should be situated below a bend 

 in the stream where an eddy is formed, by means of which 

 the collection and removal of the timber Avill be facilitated. 



