DAMS 75 



are also deciding factors, as are also the facilities for spillways to 

 take care of the overflow. 



It is, therefore, evident that the location can only be deter- 

 mined after a careful consideration of all the above facts, and com- 

 parative estimates are often required for a number of sites before 

 the problem can be intelligently solved, both from a technical 

 and economical standpoint. 



Timber Crib Dams. These dams are only used for low heads 

 of about 30 feet and less and in locations where timber is plentiful 

 and cheap. They are generally used for diversion purposes and 



PIG. 28. Timber Crib Dam, Montana Power Company. 



mostly entirely submerged, which gives them a long life. They 

 are, however, often used for temporary structures or when the 

 cost of other types would be prohibitive for the development in 

 question. 



They consist of a crib or framework of logs or sawed timbers 

 bolted or otherwise fastened together, the structure being filled 

 with rock, gravel, earth, etc., and the sloping sides are faced with 

 planks to prevent leakage. 



Almost any kind of foundation may be used if the proper pre- 

 cautions are taken. With solid rock the framework should be 

 securely bolted thereto to obviate any tendency of the dam to slide. 



