Dowla (road crossing bund) sizing: If the width of a dowla is generally kept between 30 cm and 60 cm, its height above the road level should invariably be more than how much?

Introduction / Context:
A “dowla” is a small earthen or masonry bund/hump constructed across rural roads for safely carrying a watercourse or to check overtopping at crossings. Proper sizing ensures safe conveyance and prevents vehicular damage while maintaining drainage continuity.

Given Data / Assumptions:

• Width specified between 30 cm and 60 cm.
• We need the minimum practical height above road level to ensure functionality and visibility.

Concept / Approach:
The height must be sufficient to prevent accidental run-through during low visibility, assist in water control, and avoid frequent damage. A practical minimum of more than 30 cm is often adopted as a field standard in low-volume rural settings to balance hydraulic and traffic considerations.

Step-by-Step Solution:
Consider hydraulics: small rise needed to impound/control shallow flows.Consider safety: too low (10–20 cm) may be ineffective and easily eroded or unnoticed by drivers.Adopt > 30 cm as robust and commonly accepted practice.

Verification / Alternative check:
Local rural road and irrigation departments commonly specify humps/dowlas around this magnitude to ensure durability and visibility while not impeding traffic excessively.

Why Other Options Are Wrong:
10 cm or 20 cm: Too low, prone to wear and insufficient hydraulic control.40 cm: Acceptable but not a minimum; the question asks the least reliable threshold more than which it should be kept.

Common Pitfalls:

• Confusing dowlas with speed breakers having different geometric standards.
• Ignoring erosion protection on upstream/downstream faces.

Final Answer:
30 cm