160 



THE IRRIGATION AGE. 



maintenance must be considered in de- 

 termining whether a cut should be made 

 or some other method employed. The 

 materials encountered in river beds are 

 commonly of a loose character and will not 

 stand on abrupt slopes. 



It is therefore necessary to make the 

 slope of the sides flat, which increases the 

 quantity of earth to be moved very rapidly 

 as the depth of the cut increases. 



When the limit in depth has been 

 reached at which a tunnel can be con- 

 structed and maintained equally as cheaply 

 as a cut the former is to be preferred, even 

 if a cut will be equally as safe. 



DEVELOPMENT BY MEANS OF TUNNELS. 



Tunneling is undoubtedly the most 

 customary method of making underflow 

 developments. It is always safe provided 

 the proper location is made and the proper 

 method of construction is employed. The 

 mouth of the tunnel should invariably be 

 located at one side of the channel of the 

 stream and above the high water or flood 

 mark. The mouth of the tunnel should 

 also be located far enough down or away 

 from the channel so that sufficient grade 

 will be obtained to preserve a depth when 

 the tunnel penetrates under the channel, 

 which will place its depth at a point safely 

 below the erosions liable to occur from the 

 heaviest floods. At times an open cut and 

 a tunnel can be combined more advan- 

 tageously than the use of either one singly. 

 When for a long distance outside of the 

 regular channel and above the flood line 

 the tunnel would run at a shallow depth 

 below the surface, a cut can be made, 

 thereby causing a considerable saving, 



until a depth is reached at which a tunnel 

 would be more economical. 



SUBMERGED DAMS AS A MEANS OF DEVELOP- 

 MENT. 



When the underflow is near the surface, 

 and a point where bed rock approaches 

 the sides of the canyon and is found at a 

 shallow depth can be located, a submerged 

 dam is possibly the cheapest method of 

 development. This will prove to be true 

 more particularly when the grade of the 

 channel is light, so that a very long tunnel 

 would be required. Before a submerged 

 dam is undertaken it must be known with 

 certainty that the bed rock is continuous 

 and unbroken, so that no water will escape 

 under the dam after it is completed. The 

 object of a submerged dam is to raise the 

 underflow to the surface, and leaks in the 

 foundation, which will grow' and under- 

 mine the structure or allow the water to 

 escape under the dam, must be avoided. 



Submerged dams can sometimes be con- 

 structed with some impervious stratum 

 other than bed rock for a foundation. 

 Such cases, however, are quite rare and 

 undertakings of this sort more often result 

 in failure than in success. 



Tunnels and submerged dams are often 

 combined in the development of underflows. 

 A tunnel may be used in connection with a 

 submerged dam for the purpose of drain- 

 ing the foundation to facilitate con- 

 struction, or a dam may be constructed 

 across the channel at some suitable point 

 on the line of the tunnel for the purpose 

 of collecting the underflow into the tunnel, 

 or for holding it back and regulating its 

 flow when only a part of it is required for 

 use or when it is not to be used at all. 



(To be Continued.) 



A YOUNG ORCHARD IX CALIFORNIA. 



