Which of the following is used to contain materials that are lighter than water?

The correct answer is the underflow dam, which is specifically designed to prevent the escape or movement of materials that are less dense than water, allowing them to be contained effectively. This type of dam is used in situations where contaminants or hazardous substances, such as certain oils or chemicals, need to be prevented from floating away or dispersing on the water surface.

An underflow dam manages the flow of water in such a way that lighter materials remain contained at the surface or are directed into a designated containment area, while ensuring that the bulk water flows freely beneath it. This is particularly crucial in environmental protection and hazardous materials management, where controlling pollutants is essential to preventing environmental contamination.

In contrast, other options such as an overflow dam or barrier dam are not specifically designed for containing lighter-than-water materials. An overflow dam primarily manages surface water levels and directs excess water without addressing the containment of lighter substances. Similarly, a barrier dam may serve to redirect water flow but does not specifically target the control of lighter materials, whereas a retention system typically involves capturing and storing water or contaminants, which may not focus solely on the unique characteristics of lighter substances.

Therefore, the function and design of the underflow dam make it the most suitable choice for the effective containment of materials