Because drainage galleries cause significant scheduling problems, and weaken the overall strength of dams, they should be used judiciously. Construction can be slowed by as much as 50 percent when constructing the gallery. Very large internal flows necessitate the use of a gallery to effectively route the flows to the appropriate area.

Water seeping through the dam is directed from several smaller collectors into the main drainage gallery. The gallery decreases the dam’s thickness in that area, and thus increases the internal flow. Most dams under 100 feet (30m) high can effectively prevent seepage problems by only utilizing a more dense structure and avoiding the construction of a gallery. Galleries are used in higher dams because an easier access for foundation drains is provided, and a reduction in mass is considered economical. Galleries are also needed in higher dams because of the increased water pressure associated with restraining more water causes increased internal flows through the dam.

Constructing a gallery in a RCC dam is difficult because expensive formwork often interferes with the construction equipment’s access to the dam. Alternative methods include placing a non-cemented fill in the region designated to be the gallery, and then excavating the material after the RCC has hardened. This method allows direct inspection of the concrete, whereas construction with wood formwork hides the RCC from inspection.