The Supply and Uses of Water 201 



Reclamation Service has made remarkable advances in con- 

 structing dams for irrigation and power purposes, which are of 

 a size, capacity, and cost that a generation ago would have been 

 regarded as impossible and unnecessary. Again, industrial 

 necessity is pressing into service a small part of the total 

 available water-power possibilities of our country. Running 

 water that heretofore was wasted is now being restrained by dams 

 and made to yield its power in the hydro-electric plants that 

 are rapidly multiplying wherever conditions allow. Instances 

 are numerous where streams have been dammed in order to 

 divert them into a new course. 



80. What a dam must do. In the planning and construction 

 of dams there are many indispensable conditions which must be 

 considered if the structure is to be safe and adequate. There 

 is the pressure to which the dam is to be subjected in all situa- 

 tions. It is a simple matter to ascertain the water pressure from 

 the depth of the water and a unit area of any section of the dam. 

 But there are extraordinary occasions of flood and storm when 

 the incoming water may have a volume and a velocity against 

 which a dam built for " still " water conditions would prove 

 utterly inadequate. The great seawalls at Galveston have to 

 resist enormous force as they meet the incoming waves of the 

 ocean. Moving water has a velocity and energy which must 

 be taken into account wherever such condition is possible. The 

 dikes of Holland must meet more than still or static water 

 pressure ; they must oppose a moving force of tremendous 

 proportions. 



Difficulties in dam construction. The design and con- 

 struction of dams cannot be said to have passed beyond the 

 experimental stage. The conditions which every dam must 

 meet are almost impossible to predict. The strength of the 

 materials is so variable as to defy calculation under extraor- 

 dinary conditions. The foundation is always a problem, for 

 after the construction has continued awhile it is not open to 

 inspection, and it cannot be tested beforehand what effect 



