The Idukki Dam in Kerala, built between two granite hills, is an example of:

The Idukki Dam is a double-curvature concrete arch dam built across the Periyar river, in a narrow gorge between two granite hills, Kuravanmala and Kurathimala, in the Idukki district of Kerala.

Key facts:

• Height: about 168.9 metres, one of the highest arch dams in Asia.
• It was built with Canadian help and commissioned in 1973-1976.
• It is part of the Idukki Hydroelectric Project, which includes the Idukki, Cheruthoni and Kulamavu dams that together form a large reservoir.
• The underground power station at Moolamattom generates about 780 MW, making it one of the largest hydroelectric projects in Kerala.

What is an arch dam:

• An arch dam is a concrete dam that is curved upstream in plan.
• The pressure of the water pushes against the arch, and the arch transfers the load sideways to the rock walls (abutments) of the valley.
• This allows the dam to be relatively thin compared with gravity dams, using less concrete.
• Arch dams need narrow, steep-sided valleys with very strong rock on both sides.
• A double-curvature arch dam is curved both horizontally and vertically.

Other types of dams:

• Gravity dam: holds back water by its own weight; usually massive concrete or masonry, such as the Bhakra Dam.
• Embankment (earth-fill or rock-fill) dam: made of compacted earth and rock, such as the Tehri and Hirakud dams.
• Buttress dam: a dam with a sloping upstream face supported by a series of buttresses on the downstream side.
• Barrage: a low structure with many gates across a river, used to divert water rather than store large amounts, such as the Farakka Barrage.

Why not the other options?

• ✗Earthen dams, buttress dams and barrages are different types of water-retaining structures.