Which of the following dams in India is a non-overflow, double-curvature concrete arch structure (i.e., an arch dam with curvature in both plan and elevation)? Select the correct example from practice.

Introduction / Context:
Dam engineering employs several structural types: gravity, arch, buttress, and earthfill/rockfill. A double-curvature concrete arch dam is curved in plan and in elevation, transmitting loads to abutments efficiently. Recognizing real-world examples is important for structural type identification.

Given Data / Assumptions:

• We focus on large Indian dams and their structural typology.
• Double-curvature concrete arch (non-overflow) means a thin arch shell without spill flow over the crest by design.

Concept / Approach:
Arch dams rely on arch action to transfer hydrostatic pressure laterally to strong valley abutments. Double curvature provides additional stiffness and distributes stresses efficiently, allowing thinner sections compared with concrete gravity dams.

Step-by-Step Identification:
Bhakra: primarily a straight concrete gravity dam (with a high spillway), not a double-curvature arch.Hirakud: composite earth and masonry/concrete structures; not a double-curvature arch.Nagarjuna Sagar: masonry/gravity type on the Krishna River; not an arch dam.Idukki (Kerala): iconic double-curvature concrete arch dam (central arch between Kuravan and Kurathi hills).

Verification / Alternative Check:
Engineering texts and official project descriptions consistently classify Idukki as a double-curvature arch dam, confirming the selection.

Why Other Options Are Wrong:
Bhakra, Hirakud, Nagarjuna Sagar, Tehri: these are gravity or earth/rockfill or composite types; not a double-curvature arch.

Common Pitfalls:

• Assuming any high concrete dam is an arch; many are gravity dams.
• Confusing plan curvature in spillway layouts with structural arch behavior.

Final Answer:
Idukki Dam