Time of overland flow – key controlling factors Which physical basin characteristics significantly influence the time of overland flow (the travel time of sheet runoff to channels)?

Introduction / Context:
The time of overland flow is a key component of the basin time of concentration, shaping the peak and timing of hydrographs. Understanding its controls helps in urban drainage design, flood estimation, and stormwater management.

Given Data / Assumptions:

• Overland flow is shallow sheet flow before water concentrates into rills and small channels.
• Uniform rainfall excess (infiltration already accounted for) is assumed during the effective storm period.
• Hydraulic roughness reflects land use/land cover and surface condition.

Concept / Approach:
Flow velocity in sheet flow is governed by hydraulic slope, roughness, and flow depth. Steeper slopes accelerate runoff, smoother/impervious surfaces increase velocity (reducing travel time), while longer path lengths increase travel time. Empirical relations (e.g., kinematic wave or NRCS methods) encode these dependencies.

Step-by-Step Solution:
Assess slope: greater slope → higher velocity → shorter time of overland flow.Assess ground type: impervious or smooth surfaces → less resistance → shorter time; vegetated/rough surfaces → longer time.Assess path length: longer surface path → greater travel time, all else equal.Therefore all listed factors are controlling variables.

Verification / Alternative check:
Design nomographs and formulas (e.g., kinematic wave time of concentration) include slope S, Manning’s n (roughness), and flow length L, confirming the multi-factor control.

Why Other Options Are Wrong:

• (a), (b), or (c) alone provide only a partial picture.
• (e) contradicts established hydrologic practice.

Common Pitfalls:
Ignoring micro-relief and land use changes that alter roughness; redevelopment can drastically shorten overland flow time and increase peak discharge.

Final Answer:
All the above.