Lacey's Regime Theory MCQ with answer and solution

Concept

Lacey's silt factor 'f' represents the erodibility and size of sediment particles in an alluvial channel.

Formula

f = 1.76 × √(d)

Step-by-step solution

1. Understand Lacey's regime theory parameters. 2. Identify the definition and formula for Lacey's silt factor: f = 1.76√(d), where d is the mean particle size in mm. 3. Conclude that silt factor is directly dependent on the mean particle size of the bed material.

Explanation

According to Gerald Lacey's regime theory for alluvial channels, the silt factor (f) is a parameter that depends primarily on the mean particle size (d) of the silt or bed material. The standard empirical formula given by Lacey is f = 1.76√(d), where d is in mm.

Why the other options are wrong

A: Soil pH has no direct relevance to Lacey's silt factor. C: Flow velocity is determined using Lacey's equations involving the silt factor, not the other way around. D: Bed slope is calculated based on the regime equations, not a direct input for defining the silt factor.

Shortcut

Remember the relation: Silt factor is always related to the size of the soil particles (mean particle size, d).

Common mistake

Confusing silt factor with bed slope or flow velocity, which are calculated using the silt factor rather than being the direct determinants of it.