UNITED STATES DEPARTMENT OF AGRICULTURE 
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BULLETIN No. 831 
Contribution from the Bureau of Public Roads 
THOMAS H. MacDONALD, Chief 
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Washington, D. C. 
PROFESSIONAL PAPER 
August 25, 1920 
SPILLWAYS FOR RESERVOIRS AND 
CANALS. 
By A. T. Mitchelson, Senior Irrigation Engineer. 
CONTENTS. 
Page. 
Spillways 1 
Overflow spillways 4 
Spillway control 9 
Flashboards 11 
Sliding gates 11 
Tilting center weigh ted gates 12 
Rolling dams or barriers 13 
Page. 
Siphon spillways 15 
U. S. Reclamation Service siphons 28 
European practice 31 
Huntington Lake siphon spillway , — 32 
Conclusions 38 
Acknowledgments 40 
SPILLWAYS. 
In its ordinary use the spillway is a device for removing surplus 
water from a reservoir or canal, in order that the water level within 
the reservoir or canal may not rise above the point considered safe or 
fixed upon as the maximum allowable height. It is distinguished 
from other types of wasteways by the fact that the surplus water 
passes over a crest or "spills" instead of passing through openings in 
the dam or canal bank. 
The conditions necessitating spillways are many and they vary as 
to the requirement for capacity, the degree of safety factor demanded, 
by the extent or importance of the structure they protect, the location 
of the spillway with relation to that of the dam or canal embankment, 
and the functions they must perform in maintaining a more or less 
perfect control of the reservoir or canal in times of maximum inflow 
when a predetermined flowage line or freeboard must not be exceeded. 
This necessitates the provision for passing the highest floods over the 
spillway within the safe limit of maximum rise, and the conveyance 
of this water away without injury to the dam or canal embankments 
or to their appurtenant structures. 
If a reservoir is to be located in a stream channel where the extent 
of inflow is not under human control, the spillway must provide for 
the passage of both normal and flood flow when the reservoir is full 
149907°— 20 1 
