Scope of hydrology in civil engineering:\r For which of the following civil engineering applications is hydrologic knowledge fundamentally necessary?

Introduction / Context:
Hydrology underpins the quantification of precipitation, runoff, infiltration, and streamflow. Civil engineers rely on it to design safe, economical, and resilient water structures. This question highlights the breadth of hydrology’s role in practice.

Given Data / Assumptions:

• Irrigation projects require dependable flow estimates and storage planning.
• Bridges and culverts must pass design floods and avoid scour.
• Flood control works require frequency analysis and hydrograph routing.

Concept / Approach:
Across these applications, engineers must estimate design discharges (return periods), hydrographs, sediment transport tendencies, and backwater effects. Hydrologic and hydraulic models inform sizes, freeboards, and safety factors.

Step-by-Step Reasoning:
Irrigation: canal capacities and diversion structures rely on reliable flow statistics.Cross-drainage works: bridge waterway, afflux, and scour depend on flood estimates.Flood control: levees, reservoirs, and floodways require design storms and routing.

Verification / Alternative check:
Codes and manuals explicitly require hydrologic inputs (IDF curves, PMP/PMF where warranted) for these designs.

Why Other Options Are Wrong:

• Since all listed applications need hydrology, the inclusive choice is correct; choosing any single option would omit key domains.

Common Pitfalls:

• Treating hydrology and hydraulics as interchangeable; hydrology delivers boundary inputs for hydraulic design.
• Ignoring climate variability and land-use change impacts on design discharges.

Final Answer:
All of the above