2 BULLETIN 831, U. S. DEPARTMENT OP AGRICULTURE. 



and must protect the dam or embankment from being topped if it has 

 not sufficient structural resistance to withstand the resultant shock, 

 pressure, and vibration imposed upon it by overflow of considerable 

 depth. In the case of earth dams or embankments it is necessary to 

 provide a means for conveying the falling water away from the point 

 where the embankment strikes the surface below it. 



An elaborate method resorted to in accomplishing this purpose is 

 shown in figures 1 and 2, Plate I, illustrating a system of drops and 

 a stilling pool at the foot of the Lahontan Dam in Nevada. In any 

 case it is necessary to provide a means of neutralizing the energy de- 

 veloped in the fall of the water over the dam at the point where it 

 strikes the stream below so as not to endanger the structure by under- 

 mining it. 



When the reservoir is not in the stream channel the conditions 

 under which the spillway is to operate are greatly simplified in that 

 the flow of the water is generally regulated either by diversion of all 

 or part of the stream flow into a channel and thence to the place of 

 application or storage (see fig. 1, PL II) ; or it may be supplied 

 by long conduits always controlled by some system of headworks. 

 Under such conditions, when it is necessary to provide spillways, they 

 are designed to pass such excess of water as may reach the reservoir 

 by the failure of the inlet works to function properly, the accumu- 

 lation of surface water due to superdrainage, or a combination of 

 the two. \ 



Examples of spillways where there is regulated flow into the reser- 

 voir are numerous, and figures 1 and 2, Plate III, show a provision 

 for an earthen dam in Mockingbird Canyon near Eiverside, Calif., 

 and the East Park Reservoir Dam of the Orland Project, near Or- 

 land, Calif., where there is a separate spillway and, in addition, a 

 diversion dam and inflow control. The Roosevelt Dam shown in 

 figure 2, Plate II, is an example of the type where the spillway is 

 provided either in a center section or at the end, and provision made 

 for the impact of the falling water where it strikes the stream below. 

 The Holter Dam of the Montana Power Co., near Helena, Mont., a 

 cross-section of which is shown in figure 4, is an example of an 

 " ogee " type of dam provided with baffles and a water cushion to 

 dissipate the energy developed in the overpour. 



With relation to spillways in connection with canals, the same gen- 

 eral characteristics prevail and their discussion may be taken up 

 along the same lines though more properly following the cases where 

 the inflow is regulated. -In discussion of canals, spillways and escapes 

 are generally included in the same class of structures, called waste- 

 ways, and although they are both used as protective agencies in the 

 canal system, they differ somewhat in their functions. They properly 

 should be divided into two classes in that an overflow spillway is 



