On hill roads, intermediate catch-water drains (in addition to the main catch-water drain) are provided under which circumstances?

Introduction / Context:
Catch-water drains run along the uphill side of hill roads to intercept runoff before it reaches the carriageway and slopes. In severe conditions, a single drain may be insufficient.

Given Data / Assumptions:

• Steep hillside with significant contributing catchment.
• Events of high-intensity rainfall are possible.
• Objective is to prevent erosion, scouring, and flooding of the roadway.

Concept / Approach:
Where inflows are large or concentrated, intermediate drains at different bench levels divide and safely convey runoff, reducing peak flow into any single channel and improving stability.

Step-by-Step Solution:

Verification / Alternative check:
Hydraulic sizing shows that multiple drains lower flow depth/velocity, preventing overtopping and erosion.

Why Other Options Are Wrong:
Any single listed condition can justify extra drains; restricting to only one factor ignores combined hydrologic needs.

Common Pitfalls:
Underestimating peak intensities; neglecting sediment/debris which further reduces capacity of a single drain.

Final Answer:
All the above