Drains (Relief Wells)
A vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain".
Source http://www.nws.noaa.gov/om/hod/SHManual/SHMan014_glossary.htmЗураг