In open drains carrying stormwater, a maximum permissible mean velocity of about 1.5 m/s is generally adopted for which types of beds/linings to avoid erosion and structural damage?

Introduction / Context:
Designing safe non-eroding velocities in open drains is central to stormwater engineering. Excess velocity can scour bed and sides; too low a velocity promotes deposition. The permissible velocity depends on material and lining type.

Given Data / Assumptions:

• Drains intended for stormwater or combined flows.
• Materials considered: rock and gravel beds, stone lining.
• Permissible velocity target ≈ 1.5 m/s.

Concept / Approach:

Empirical permissible velocities correlate to material resistance: rock/gravel or stone lining tolerates higher velocities than bare earth. Around 1.0–1.5 m/s is often acceptable for stone-lined or rock/gravel sections, while unlined silty or sandy beds require much lower velocities to prevent scour.

Step-by-Step Solution:

Verification / Alternative check:

Design handbooks provide tables of safe mean velocities: stone-pitched channels allow higher velocities than bare cohesive soils. Many municipal standards accept ≈1.5 m/s for stone-lined sections depending on gradation and jointing.

Why Other Options Are Wrong:

(a) or (b) alone are incomplete. (d) is false because 1.5 m/s is not generally permissible for unlined erodible soils but is permissible for stone/rock lining. (e) Bare cohesive clay often needs lower velocities to avoid erosion.

Common Pitfalls:

Using peak (instantaneous) rather than mean velocity; ignoring side-slope protection and turbulence at bends, junctions, or culvert outlets.

Final Answer:

Both (a) and (b)