SMALL DAMS PETITS BARRAGES

conduit to protect against possible seepage (or flow) around the periphery of the grouted (or
sealed) pipe/conduit. Some considerations relating to the abandonment of conduits include
(FEMA, 2005):
Complete or partial filling of the conduit with grout: Normally, complete backfilling of
the pipe with grout is recommended. However, it may be difficult to completely fill the pipe full
of grout if the pipe is long and access is limited to one end of the pipe. It may be possible to
access the pipe by drilling through the dam from the crest to intersect the conduit.
There are inherent risks associated with hydraulic fracturing both from drilling into the
dam and injection of grout from the crest of the dam. These potential risks must be weighed
against the desire to completely fill the pipe with grout. If drilling into the dam is necessary to fill
the pipe, the preferred method is dry augering to avoid risk of hydraulic fracturing due to drilling
fluids.
If water is flowing into the conduit and cannot be stopped by lowering the reservoir, it
may be necessary to seal the pipe with a temporary bulkhead or inflatable bladder.
7.3.7 Techniques for Rehabilitation of Hydromechanical Components
All valves and hydromechanical equipment used for releasing water from a dam must be
properly maintained. It is important to remember that under certain circumstances an unsafe
condition could develop at a dam and there may be a requirement to empty the reservoir (or
draw down the reservoir to a “safe level”) as quickly as possible. It is important to service and
exercise the valve and other mechanical components regularly to ensure that they are
operational at all times. For example, it is recommended that for a dam with basic outlet works,
that the control valve be operated at least four times per year to make sure it is functioning
properly.
If a valve is difficult to operate, for example a tight handwheel, or one which requires
excessive force to turn it for its full range, this is a clear indication of a developing problem that
requires attention. It is possible to exercise a valve without losing water stored in the reservoir
when the spillway is flowing because water is being passed downstream anyway. This “mode of
operation” is recommended at the tail end of flow over a spillway because opening a valve at
the beginning of spillway discharge may worsen the natural increase in discharge downstream
of the dam.
Fig. 7.19 – What are the chances that this valve will work in an emergency? (SMEC/2006)
Provision should be made at the discharge end of an outlet pipe to reduce the velocity of
the water and to dissipate energy. Heavy (and suitably dimensioned) rock placed on a layer of
crushed rock will minimize erosion. Construction of an effective concrete outlet structure e.g.
“Impact-Type Stilling Basin” (Design of Small Dams, 1987) will dissipate the energy of the water
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