86 BULLETIN 126, U. S. DEPARTMENT OF AGRICULTURE. 
in the discussion cited. The greater the surface width of the water 
in the channel at the time the ice is formed the greater will be the 
tendency for the ice sheet to bulge upward and thus relieve the side 
thrust. Again, the flatter the side slopes of the channel the greater 
will be the upward component of force parallel to the lining surface 
and accordingly the less will be the thrust normal to its surface. Of 
course there are many reasons for limiting the flatness of the side 
slopes, but in the design of concrete-lined canals for use in localities 
subject to low temperatures, and particularly where winter operation 
is necessary, the benefits resulting from flattened side slopes should 
not be overlooked. 
ACKNOWLEDGMENT. 
The grateful acknowledgment of the writer is hereby tendered 
to the engineers and managers of irrigation enterprises and to the 
field members of this division who have assisted in collecting reliable 
information pertaining to the subject treated. He is especially 
indebted to S. T. Harding and Justin T. Kingdon for able assistance 
rendered in compiling the data and computing the tables contained 
in this report. 
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