Final answer:
Combination drains are modified subdrains that serve dual purposes: collecting and channeling both surface and subsurface water. They prevent flooding by efficiently removing excess water.
Step-by-step explanation:
Combination drains are usually subdrains that have been modified to serve dual purposes. They are designed to efficiently collect and channel both surface water runoff and subsurface water. This modification allows combination drains to handle larger volumes of water and prevent flooding. The increased flow rate helps to remove excess water quickly and effectively.