The correct answer is: D. chimney drain.
A chimney drain is a vertical perforated pipe that is installed in the foundation of an earthen dam to control seepage. The pipe is filled with gravel or crushed stone to provide a drainage path for water that seeps
through the foundation. The water flows down the pipe and out of the dam, preventing it from building up and causing erosion.A rock toe is a layer of large rocks that is placed at the toe of an earthen dam to protect the dam from erosion. The rocks help to dissipate the energy of waves and prevent the dam from being undermined.
A horizontal blanket is a layer of impervious material, such as clay or concrete, that is placed on the upstream face of an earthen dam to prevent seepage. The blanket helps to create a barrier that water cannot pass through.
An impervious cut off is a vertical barrier of impervious material, such as clay or concrete, that is installed in the foundation of an earthen dam to prevent seepage. The barrier helps to create a barrier that water cannot pass through.
Seepage is the flow of water through a porous material, such as soil or rock. Seepage can cause problems in earthen dams, such as erosion and instability. Chimney drains are an effective way to control seepage in earthen dams.