hole is pierced in the dam.

Early embankment dams were often built of a single type of earth, but starting in the mid-16th century, engineers began to use several types of material layered in zones. A typical zone pattern is a clay center (a vertical wall, extending from the riverbed to the crest of the dam), with gradually sloping banks of soil on both upstream and downstream sides, and both faces covered with large rocks. Large rocks on the upstream face protect the structure from wave action. The resistance to water seepage varies