50 BULLETIN 852, IT. S. DEPAKTMENT OF AGRICULTURE. 
passing that accumulations of detritus do not occur at the low point 
in the profile of a siphon pipe, but will pass this point and precipitate 
at the base of the first steep upward pitch. They are thus, as a rule, 
beyond the reach of a flushing stream if the only blow-off is located 
at the extreme low point. For this reason it is advisable to place 
the main blow-off at the steep upward pitch and a small valve at the 
extreme low point in order that the pipe may be completely drained. 
It has become an accepted fact that cast-iron and steel pipes 
carry less water for a given loss of head from year to year. So far as 
evidence is obtainable, wood-pipe interiors remain about the same, 
though occasionally a pipe is found with interior growths of SpongiUa. 
Moritz writes: 
The effect of age on concrete pipe is not known, but it is customary to assume that 
the carrying capacity does not decrease, as there is no reason to suppose that it should. 1 
Bishop states that there appears to be no deterioration of capacity 
in cement-lined pipes laid in 1871-72 for municipal purposes. His 
examination was made 38 years later. 2 
The city authorities of Grenoble, France, comment favorably upon 
the condition of a concrete pipe after 15 years' service, mentioning a 
slight calcareous deposit one-sixteenth inch thick. 3 
At first thought it would appear that cement or concrete, subjected 
to water free from erosive material, should retain the original surface, 
with the possible exception of sliming, which may cut down the 
capacity from 10 to 15 per cent, but from which concrete surfaces are 
comparatively free. 4 
Examination of the interiors of many pipe lines supplemented by 
interviews with managers of water systems and a study of available 
literature show that the capacity may deteriorate, due to the 
following causes: 
(1) The invasion of rootlets from neighboring trees. 
(2) Scour from the water. 
(3) The accumulation of lime or other deposits. 
In southern California experience teaches that pipe lines may be 
laid under citrus groves with no ill effects from roots, but that where 
the line is laid near black acacia, willow, pepper, or eucalyptus trees, 
rootlets will enter the pipe, not necessarily at the joints, if it is poorly 
made and poorly laid, and retard the flow. These trees are named in 
order, the acacia being the most destructive. Roots will not effect 
a dense, well-made pipe, with tight joints. (See Mr. Finkle's dis- 
cussion on p. 95.) 
Literature contains accounts of clay and concrete pipes being 
clogged with roots, particularly of the elm tree. 5 
1 Working Data for Irrigation Engineers, p. 69. 
8 Experience with Cement Mains at Rahway, N. J. By Wm. Bishop, Pro. American Waterworks Assn. 
(1910), p. 217. 
sCan. Engr., Oct. 19, 1911, vol. 21, p. 455. 
* Water Works Handbook, by Flinn, Weston, and Bogert, New York, 1916, p. 290. 
e'Eng. Rec, 1916, vol. 73, p. 460; id., p. 514; id., p. 688; id., vol. 79, p. 36. Irrigation in Southern Cali- 
fornia, W. H. Hall, Sacramento, Calif., 1888, p. 495. 
