WATER SUPPLIES FOR IRRIGATION. 



159 



Sometimes the underflow of a stream 

 does not occur in the usual form of a con- 

 tinuous sheet of water gradually percolat- 

 ing through the sand and gravel of a river 

 bed. There are instances where the for- 

 mation is partially cemented and obstructs 

 the free percolation of the underflow water. 

 In cases of this kind the passage of the 

 water is obstructed by the solidity of the 

 formation and it breaks through in small 

 streams, which are separated from each 

 other by intervening dry formations. 



It sometimes occurs in instances of the 

 latter class that the formation in the 

 channel of the stream is so much more 

 compact than that adjacent to the stream 

 that the underflow is either partially or 

 wholly deflected laterally and flows in a 

 different direction from the stream itself. 

 This, of course, does not occur in narrow, 

 rocky canyons, but on streams with low 

 banks of permeable material. On account 

 of cases of this nature we often find that 

 streams with a very large and good water- 

 shed possess little or no underflow. The 

 absence of underflow in streams, however, 

 is generally due to other causes. These 

 are the opposite of the characteristics of a 

 watershed and stream channel, which have 

 been previously outlined as essential to the 

 creation of an underflow, and their discus- 

 sion in detail is therefore deemed un- 

 necessary. 



ESTIMATING AND DEVELOPING IRREGULAR UN- 

 DERFLOWS. 



Such underflows as have been discussed 

 in the preceding paragraphs may be said 

 to be irregular both from the fact of their 

 being exceptional and on account of their 

 departure from established rules. Conse- 

 quently their volume cannot be estimated 

 in the ordinary way, nor can they be de- 

 veloped like regular underflows. Esti- 

 mates as to the quantity of an irregular un- 

 derflow must rest largely on the skill and 

 judgment of the engineer who makes the 

 investigations. Sometimes there are cir- 

 cumstances which can make the results 

 nearly certain, even in irregular cases, but 

 usually the conclusions arrived at are 

 merely rough approximations. 



The most satisfactory method of devel- 

 oping an irregular underflow is by means 

 of a system of tunnels and shafts crossing 

 the stream at nearly right angles. The 

 shafts are necessary in order to make the 

 running of drifts on different levels possi- 



ble so as to follow the levels where streams 

 occur in the formation. The first work 

 should always be the sinking of shafts, 

 after which the tunnels can be commenced 

 back on a level to reach the required ele- 

 vations. 



MODES OF UNDERFLOW DEVELOPMENT. 



The different methods of developing 

 underflows are as follows: 



(1.) By cuts. (2) By tunnels. (3) By 

 submerged dams. 



The conditions and circumstances sur- 

 rounding each case must determine which 

 of the above methods is to be employed in 

 making the development. Without enter- 

 ing into a discussion of the principles 

 governing the designing and construction 

 of these structures, which matters will be 

 discussed in succeeding chapters, we will 

 now briefly discuss the ruleg governing 

 the application of the different methods of 

 development already enumerated. 



THE DEVELOPMENT OF UNDERFLOWS BY MEANS 

 OF OPEN CUTS. 



Cases are rare where open cuts can be 

 employed wholly as a means for develop- 

 ing the underflow waters of a stream. 

 The objection to this mode of development 

 in the ordinary run of cases encountered 

 in practice is, that most streams are subject 

 to heavy floods and overflow at certain 

 times of the year. As the development 

 usually has to be made in the bed of a 

 stream, such floods or overflows would 

 cause open cuts to be tilled up and ob- 

 literated, so that the work would have ,to 

 be done over periodically. The following 

 instances often occur, however, in which 

 this objection does not apply: (a) when 

 the outlet of the underflow drain can be 

 located outside of the overflow channel 

 and the drain under it in such a manner 

 that it can be covered over after being 

 supplied with a flume or pipe; (6) when the 

 stream is not subject to greater floods than 

 can be controlled by aprons or bridges 

 across the cut. 



When either of the above conditions ob- 

 tains cuts can be considered as a method of 

 development, provided always that the 

 underflow at the proposed point is not too 

 deep to be reached by means of a cut. 

 Economy and practicability prescribe that 

 a cut for the purpose stated should not 

 exceed a certain depth. Both the cost 

 of construction and the cost of future 



