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  Embankment
dams are made mainly from natural materials. The two main
types are earthfill
dams and rockfill
dams. Earthfill dams are made up mostly from compacted
earth, while rockfill dams are made up mainly from dumped
and compacted rockfill. The materials are usually excavated
or quarried from nearby sites, preferably within the reservoir
basin.
 A
cross-section (or slice) through an embankment dam shows that
it is shaped like a bank, or hill. Most embankment dams have
a central section, called the core,
made from an impermeable
material to stop water passing through the dam. Clayey soils,
concrete
or asphaltic
concrete can be used for the core.
 Rockfill
dams are permeable.
They can have a core or an impermeable cover on the upstream
face. Materials used for the cover include reinforced
concrete and asphaltic concrete.
Embankment
dams are usually chosen for sites with wide valleys. They
can be built on hard rock or softer soils, as they do not
exert too much pressure on their foundations.
 The
main type of dam in Britain is the embankment dam, and there
are over 3000 of them. Some of them date back to the 9th Century,
including Fishers Pond Dam, near Winchester, which was built
by monks to provide fish. The average age of our embankment
dams is over 100 years old.
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