26 BULLETIN 1358, U. S. DEPARTMENT OF AGRICULTURE 
end of the dam is supported by rock cribs or similar structures, 
but with this exception the top of a settled dam should be level 
throughout its length. ‘“ Cloudburst” waters may overflow any 
dam. At such times the dam may hold if the flood peak is of short 
duration and the water overflows in a thin sheet instead of being 
concentrated in sags or notches. However, it is well to leave the 
middle of a newly completed dam shghtly higher than the ends to 
allow for greater settling in the center. 
A common mistake in constructing earthen dams is to make slopes 
too steep, in an effort to reduce the quantity of material required. 
The angle of repose of very compact and stable materials like clay- 
erit mixtures is steeper than for the less stable sands. Additional |— 
slope allowance must also be made for beating rains, the tendency _ 
of the mass to slump at the base of the water slope, animal trampling, | 
and wave or ice action. In view of these various erosive agents, 
which tend especially to wear away the water slope (PI. XI, fig. 2), 
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et Sea Se po eS 
FOR LARGE DAMS OR LESS COMPACT MATERIALS 
FIGURE 3 
it should be built flatter than the lower or outer slope (9) (10) (24) 
(27). Study of 200 earthern embankments in Arizona and New 
Mexico indicates that outer or downstream slopes in the ratio of 114 
(horizontal) to 1 (vertical) and water slopes of 214 to 1 will be sat- 
isfactory for dams of average size, built of good, compact material. 
Where less stable material must be used or where dams are large 
and considerable wave action is anticipated, outer slopes of 2 tol © 
and water slopes of 3 to 1 are recommended. These slopes are shown _ 
in Figure 3. 
To allow for the narrow ing of the tops which accompanies the 
wearing down of the slopes, the tops should be at least 10 feet wide 
at completion unless the dams are less than 10 feet high and are not 
hable to be damaged. 
Slope angles may be steeper when special facings are used, the 
object of w hich may be to reduce the amount of lis filler necessary, 
to protect the slopes from wave action trampling or rain cutting, 
or to increase the stability of the dam. Rock facings are the most 
common. (PI. XII, fig. 1.) Where large durable timber is at hand 
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