SMALL DAMS PETITS BARRAGES

The junction of the downstream slope with the ground surface is protected from seepage
and piping with a downstream toe. It is a small rockfill zone, divided from the main dam by one
or two filter layers and acting as drain. Except of horizontal blanket drain and chimney drain
with blanket, the toe zone of rock material is a good internal drain to control seepage. Dams
without internal drains have seepage problems, because the seepage (phreatic) surface
emerges on the slope surface. The toe dam protects the dam also from tail water if any.
The part of the valley side against which the dam is built is the abutment. Surface runoff
from the abutment could cause rapid erosion on the slope-abutment interface. To avoid this
there must be provided a concrete ditch to collect all rain surface water from the abutment and
the dam slope.
Seepage is also often coming out on the lower part of the slope-abutment interface. This
contact is prone to seepage because the embankment fill near the abutment is less dense and
less watertight. The embankment fill near the abutments is less dense because compaction is
difficult along the interface. A toe drain is to be put on the slope-abutment interface to carry the
internal seepage water away from the dam. It must be met by filter under the slope protection
to avoid any seepage erosion.
5.11 CREST DESIGN
Placing a layer of gravel on the crest will in particular avoid the formation of ruts due to
traffic and desiccation of the last layers of compacted clay materials. If the crest is not
protected, it can experience severe erosion. Crest erosion protection is usually the road
surfacing such as gravel, concrete pavement or asphalt and depends on the amount of the
anticipated traffic. If no traffic is expected, a grass cover could be enough. In designing the
dam crest of a small earthfill dam the following items should be considered:
� Width
� Drainage
� Camber
� Surfacing
� Safety requirements
5.11.1 Width
The crest width of an earthfill dam depends on considerations such as nature of
embankment materials and minimum allowable percolation distance through the embankment
at normal reservoir level, height and importance of structure, possible roadway requirements
and allowable materials at the site.
Because of practical difficulties in determining these factors, the crest width is, as a rule,
determined empirically and largely by precedent experience. According to the Bureau of
Reclamation, in the United States, the following formula is suggested for crest width for small
earthfill dams:
w = z/5 + 10
where: w = width of crest in feet
z = height of dam, in feet, above the streambed.
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