Storm water drain (open channel) freeboard: If the peak discharge of an S.W. (storm water) drain is expected to exceed 150 m^3/s, what freeboard should be provided for safe conveyance?

Introduction / Context:
Freeboard is the vertical distance between the design water surface and the top of the channel bank or wall. In large storm drains handling very high discharges, adequate freeboard is essential to accommodate uncertainties, waves, surges, and debris without overtopping.

Given Data / Assumptions:

• Peak discharge > 150 m^3/s (very large urban drain).
• Open channel conveyance.
• Conventional design recommendations for freeboard selection.

Concept / Approach:

Design guides prescribe increasing freeboard with increasing discharge and velocity. For very large storm drains (Q beyond about 100–150 m^3/s), a freeboard around 0.9 m is commonly adopted to ensure resilience against transient rises and hydraulic jumps in transitions.

Step-by-Step Solution:

Verification / Alternative check:

Field experience and standard tables both indicate larger freeboards for large-capacity drains to cope with debris and flow instabilities.

Why Other Options Are Wrong:

50–80 cm may be insufficient for very large flows; 100 cm is conservative but commonly, 90 cm is the recommended threshold for this discharge class.

Common Pitfalls:

Adopting freeboard for small drains on major outfalls; ignoring super-elevation at bends and standing waves at transitions.

Final Answer:

90 cm