WORKS FOR CREATING SLACKWATER. 133 



dams may often be varied to suit the conditions of importance, as for instance, in order 

 to place the works on a desirable foundation, or so as not to divide the harbor of a city 

 into two parts, or for other causes. 



In any work for creating slackwater a careful survey of all the available sites 

 should be conducted, and the location made at the place having the greatest advan- 

 tages. The principal considerations, where the lock and dam are both to be built in 

 the open river and adjoining each other, may be summed up as follows: 



1. A good foundation upon which to build the works, deep enough to prevent 

 shoaling below the lock, but not deep enough to make it difficult to reach and expensive 

 to build upon. 



2. A straight channel for some distance above and below the lock, so as to pro- 

 vide an easy entrance and exit for boats. This can best be secured by making the 

 location near the middle of a long stretch of straight or nearly straight river. 



3. Banks of at least the average height of those along that part of the stream and 

 of firm material, such as clay, rock, etc. , which will not be easily overflowed or cut away. 

 This should particularly be the case at the abutment end of the dam. 



4. Sufficiently level, or nearly level, land on the lock side of the river to form a 

 yard for purposes of construction, and which will provide sites for the locktenders' 

 dwellings. 



Should the location be at a ripple or shoal, then the works should be so built as 

 to make deep water on the shallows; that is, the lock should be below the shoal. 



Some engineers prefer a location for a loclj "just under the point," that is, just 

 below a bend on the convex side, on the theory that the upper entrance will not fill 

 with drift. Unless the bend be slight, however, a lock so located is difficult of entrance 

 from above and may prove dangerous, as, should a boat become disabled in rounding 

 the point, she might drift over the dam before a rope could be got ashore. The upper 

 entrance also fills with sediment more readily in the quiet water found below a point. 

 Where the bend is very pronounced and the stream narrow it is quite difficult to enter 

 or depart with a tow from a lock so situated. 



Other engineers prefer a location directly opposite, that is, on the concave side 

 of a river, in order to reduce the risks to the entrance and exit of craft. This natu- 

 rally throws the floating drift into the upper approach, but it is to be preferred in some 

 cases to a location on the convex side. 



The location of a lock at a bend has the advantage that the river is usually wide- 

 at such a point than in a straight reach, and the dam will consequently restrict the 

 waterway to a less degree. Moreover, there is usually deep water along the con- 

 cave bank, which is favorable to maintaining a good channel. Where the bend is 

 slight, so as to give a slightly curved channel above and below the lock for a distance 

 of several hundred feet, there is but little objection to locating a lock in the concave 

 side of a stream, the tendency of drift and floating bodies to obstruct the upper entrance 

 being really the most objectionable feature. 



