SMALL DAMS

F ill dams
98
In the case of very wide and symmetrical valleys, the distance will nevertheless be
very long, which gives rise to the idea of placing the spillway on the fill along the
thalweg . For low embankments that are well compacted with a relatively incompressible
foundation, this solution can be suitable. The concrete structure is built with
articulated joints and can absorb the little settlement that is observed with no damage.
Such a design is now classic for dams up to about 20 metres high and even higher as
long as the length of the overspill sill does not exceed approximately 15 metres, to
avoid building construction joints between the two banks. It is not excluded that a
longitudinal joint may be built, but then the structure becomes more complex.
When there is a rock dam abutment or a rock saddle 1 at an altitude close to that of
the reservoir, an economical solution consists in digging out an unlined channel 2 . If
the rock in the channel walls is crumbly or vulnerable to frost, prudence is required
because of the risk of obstruction by rockfall. Given the possibility of slow retrogressive
erosion in the channel, it will also be necessary to place a sill at its upstream end
anchored down to the unaltered rock.
In the case of a classic reinforced concrete channel with Water-Stop joints between
sections, the action of water outside the channel must be taken into account, in particular
by placing:
!"a cut-off to reduce circulation along the channel walls and bottom (risk of internal
erosion);
!" a system to combat uplift that could cause heave in some parts of the channel, in
particular the sill and energy dissipation areas (drain, weepholes, anchor bars, horizontal
footing). Drains or weepholes must be bent towards the downstream to avoid
generating any uplift due to the kinetic term in water pressure (V²/2g).
The channel should be set out as straight as possible and changes in cross-section or
slope should be as regular as possible. If not, stationary waves may develop at such
discontinuities at the upstream part of supercritical flow. They will rebound back
downstream on the side walls of the chute channel. The rise in the water line that
results can cause overspill during floods, which could damage the embankment's
abutments or the embankment itself. To guard against such a problem, the simplest
technique consists in setting out a spillway with a perfectly straight frontal inlet on the
fill or on an abutment, or in setting out a lateral inflow spillway followed by a narrowing
structure with a gentle slope and a straight chute aligned with that structure.
Calculation of the water line (in a steady state for the attenuated design flood flow)
is used to dimension the elevation of the walls, selecting a minimum freeboard of
0.50 metre. It is recommended to do a second calculation for flow with the safety
flood (see Chapter II, p. 25) in order to check that that flood will not cause overspill.
In this way, it can be effectively considered that the safety flood is reached when the
reservoir reaches the crest of the embankment (or if the embankment has a watertight
core, the crest of the latter when the fill above it is permeable).
1. In the case of a saddle at an altitude close to that of the reservoir, see chapter III, Feasibility study,
stage 2, p. 54.
2. Unlined channels require greater surveillance. They are not recommanded if the rock is cracked.