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28 BULLETIN 1358, U. S. DEPARTMENT OF AGRICULTURE 
WIDE FLAT SPILLWAYS PREFERABLE 
The capacity of a spillway depends on its width as well as its | 
depth, and usually the wider it is the better. In cross section the | 
floor should be level, so that waste waters may pass through in a _ 
wide, thin sheet. Narrow V- shaped spillways should be avoided | 
(fig. 2). Fencing across a spillway is a risky practice; driftwood — 
and other débris may be caught, the capacity lessened, and the dam | 
overflowed. The gradient or slope of the spillway from the point — 
of entry to the point of discharge is also important. It is better © 
to carry the spillway channel on a gentle slope as far below the © 
dam as the lay of the land and reasonable expenditure will permit; 
otherwise the foundation of the dam itself may be undercut. (Pl. 
COMPARATIVE DIAGRAMS 
CEASST OF: Co ACTUAL SPILLWAY 
SWAY. | St cages cee teNee 
Y : 
ZZ - arcest KNOWN FLOOD 
RESULTS ON DAM 
19.____|__JBROKEN THREE TIMES 
BY FLOODS 
SPILLWAY 
CAPACITY 
SMALLER THAN 
FLOOD VOLUME BROKEN TWICE 
BY FLOODS 
HAVE OVERFLOWED. 
é - EMBANKMENTS DAMAGED 
SPILLWAY IN VARYING DEGREE BUT 
CAPACITY NOT COMPLETELY BROKEN 
AND FLOOD 
VOLUME APPROX- 
IMATELY EQUAL 
Oz == -{HAs NEVER OVERFLOWED 
Se YE ip [LLILEE 
SPILLWAY 
DECIDEDLY 
LARGER THAN HAS NEVER OYERFLOWED 
FLOOD VOLUME 
FIGURE 4 
XII, fig. 2.) A heavily riprapped “ wing” extending out from the 
end of the dam is sometimes necessary to deflect the current from the 
lower toe of the dam. 
Spillways cut through bedrock are resistant to the scouring action 
of flood waters and are preferred. (PI. XIII, fig. 1.) Where exten- 
sive rockwork is involved blasting may be necessary to secure the 
desired size. Concrete spillways have proved practicable at dams 
where the overflow is large. (Pl. XIII, fig. 2.) A beveled top on 
the eee side lessens the risk of damage from floating drift- 
wooc 
Spillways of earth are cheaper and are generally the most feasible. 
They are more resistant to wear in compact clay mixtures than 
in coarser open-textured soils. Where it is possible to use natural 
spulways that are sodded this should be done, since they wear better 
