RE)SERVOIRS AND PONDS. I05 



rise which might have occurred during constru<5lion, 

 and also to provide room for the pipes to the basin 

 below. This arch is built solid, full of concrete near 

 the upper end, at the same time walling in the pipes. 

 From the opening to the arch and extending upward 

 there is a well for the purpose of taking the supply at 

 any desired level, thus relieving any strain at the con- 

 duit, and serving also as a manhole. 



Silting of Reservoirs. — One of the most common 

 drawbacks to water storage in large reservoirs is the 

 constantly accumulating mass of sand and silt settling 

 at the bottom, and modern science is as yet deficient in 

 completely eradicating the trouble. It has been pro- 

 posed to sluice out such material by providing large 

 openings from the reservoir and occasionally allowing 

 a great volume of water to rush out and carry the 

 colledled material with it. This method has been suc- 

 cessfully employed in diverting dams for keeping open 

 the approaches to the headgates of canals. It is also 

 extensively employed in cleansing reservoirs in Spain. 

 Experience has shown, however, that only a compara- 

 tively small area is cleaned by this method, reaching 

 on a steep grade for only a moderate distance above 

 the scouring sluices. For clearing a reservoir several 

 miles long it is manifestly inadequate and must be sup- 

 plemented by something else. Another method of 

 counteradling the tendency of the reservoir to fill is by 

 enlarging its capacity. This method is not always 

 possible. Where feasible it is always expensive, and 

 so far from being a solution of the problem, merely 

 postpones the date when some means must be adopted 

 for cleaning out the impounded silt. It may, however. 



