9.1.2 CofferdamsCofferdams are temporary structures used to divert water from an area
where a permanent structure has to be constructed. They must be as
watertight as practicable, relatively cheap and, if possible, constructed of
locally available materials.
Diversion facilities such as tunnels or canals, provided to divert the
flow from the site area, are sometimes used as part of the permanent facili-
ties (e.g. penstocks, spillways, sluices, conveyances to turbines, or dis-
charge channels from turbines, etc.). If the construction work proceeds in
two stages, part of the structure completed in the first stage may be used as
a diversion facility (spillway or sluice) during the second stage of construc-
tion (Fig. 9.1) (Linsley and Franzini, 1979; Vischer and Hager, 1997).
The selection of the design flood for these diversion works depends
on the risk that one is prepared to take (see equation (4.1)). For example,
a more conservative design flood has to be considered for situations where
overtopping during construction would have disastrous results. If overtop-
ping is permitted, care must be taken to strengthen the top and the down-
stream slope of the cofferdam to minimize erosion. The overtopping flow
must be spread over the longest possible length of the cofferdam, thus
reducing the concentration of flow. The control of floating debris is
another essential requirement in order to minimize the clogging of diver-
sion tunnels, especially during the high-flow season.
WEIRS AND BARRAGES 365
Fig. 9.1 Streamflow diversion by cofferdams