Which statement about drainage systems is true?

• A. Secondary drainage handles normal rainfall while primary handles floods.
• B. Primary drainage handles rainfall after secondary drains fail.
• C. Both primary and secondary drainpipes operate at the same time with no priority.
• D. A secondary drainage system is used to handle rainfall when the primary drainage is blocked or clogged.

Answer: (D)

Redundancy in drainage design is about having a back-up path so water can still drain if the primary route becomes blocked. A secondary drainage system is intended to take over when the primary path is obstructed or clogged, ensuring water doesn’t pond and damage the structure. Under normal rainfall, the primary path handles the bulk of the flow, and the secondary remains available as a safety feature for blockages or overloads. The other ideas imply either normal rainfall is handled by the secondary, or there’s no priority between paths, or the primary only works after the secondary fails—none of which reflect how a backup drainage system is supposed to function. This approach supports continuity of drainage even during debris buildup or partial blockages, which is a key principle in robust roof and surface drainage design.