In storm drainage design, the time of concentration is best defined as which of the following?

Introduction:
Time of concentration (Tc) controls design rainfall intensity in rational and unit-hydrograph methods. It represents the longest travel path governing the response time of a catchment.

Given Data / Assumptions:

• We need the operational definition used in hydrologic design.
• Remote point refers to the point with the maximum travel time to the outlet.

Concept / Approach:
Tc is the time for a drop of water to travel from the hydraulically most remote point in the watershed to the outlet. For storms with duration ≥ Tc, the entire catchment contributes to peak flow; for shorter storms, only parts near the outlet contribute at the same instant.

Step-by-Step Solution:
Eliminate definitions about storm onset or generic duration—they do not define Tc. Select the path-based travel-time definition tied to hydraulics and topography. Choose the statement referencing the most remote point to the drain/outlet.

Verification / Alternative check:
Empirical Tc formulas (Kirpich, NRCS) compute travel times from flow paths and slopes, consistent with this definition.

Why Other Options Are Wrong:

• Storm start/duration: meteorological, not hydraulic response definitions.
• All runoff reaching after the storm: not the standard definition of Tc.

Common Pitfalls:

• Confusing basin lag time with Tc; they are related but not identical.

Final Answer:
The travel time for storm water from the hydraulically most remote point of the catchment to the inlet/outlet.