Which type of dam is used to contain materials that are lighter than water?

The type of dam used to contain materials that are lighter than water is an underflow dam. An underflow dam is specifically designed to capture and hold materials such as oils, floating debris, or other lighter substances that can float on top of water. The design allows water to flow underneath while trapping these lighter materials above, preventing them from contaminating larger bodies of water and facilitating easier cleanup.

In contrast, retention dams are generally used to hold back water and manage larger volumes of water rather than specifically targeting lighter materials. Overflow dams are constructed to allow water to flow over the top, which does not serve the purpose of containing lighter materials effectively. Flood control dams focus on managing excess water during flood conditions and do not have the specialized structure to contain materials less dense than water.