SMALL DAMS PETITS BARRAGES

7. REHABILITATION PRACTICES FOR SMALL DAMS
7.1 INTRODUCTION
Aging of embankment dams, updating of design standards and criteria and the
development of conditions affecting the safety of dams have resulted in a need for reevaluation
and, in some instances rehabilitation of dams.
The shortcomings are identified during the execution of safety inspections discussed in
the previous section.
Even if finance is not readily available, rehabilitation or improvement is necessary to
protect the asset of the owner of the dam, but also to protect the owner against claims caused
by dam breaches.
In this section techniques are presented for rehabilitation of embankment dams.
7.2 TECHNIQUES FOR IMPROVING SPILLWAY SAFETY
7.2.1 Introduction
A 1981 survey of non-Federal dams in the United States, concluded that 81 % had dam
safety shortcomings because their spillways were not adequate to pass the estimated
maximum design floods. This often reflects the difference between present-day design flood
and the criteria in vogue at the time the dams were constructed.
Embankment dams are particularly sensitive to failure caused by overtopping, both
during construction and while in service. Overtopping of a dam often causes dam failures.
National statistics show that overtopping due to inadequate spillway design, debris blockage of
spillway, or settlement of the dam embankment crests account for approximately 34 % of all
U.S. dam failures. Embankments compacted to Standard Proctor density standards provide
an elastic structure and show less cracking problems – often embankment dams are not
compacted to this standard. Dam owners sometimes raise the spillway structures temporarily
with say sandbags or definitely with concrete without realizing the effect on the safety of the
structure.
In South West of France, a survey of small embankment dams has been carried out in
1997-1999 on more than 200 dam less than 20 m high [Lautrin, 2003]. 43 % of the spillways
of those dams have been raised, ranging from 0,1 to 1,2 m and thereby reducing significantly
the spillway capacity. The raising is obtained by means of a wooden beam (theoretically
removable) or by means of a concrete beam.
The most frequent type of spillway used with small embankment dams, in South Africa,
is a spillway cut from one of the banks of the river. Usually the material excavated is used for
the fill in the embankment. This type of spillway is known as a by-wash spillway. A spillway
can be located in a saddle adjacent to the reservoir if located at the proposed full supply level.
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