Regime channels (Lacey): Under regime conditions, which statements correctly describe how sediment and forces act in an alluvial channel?

Introduction / Context:
Lacey’s regime theory explains how alluvial channels (with mobile beds and banks) naturally attain stable shapes. In regime, the channel adjusts its width, depth, and slope so that neither silting nor scouring predominates. This question probes core qualitative statements behind Lacey’s assumptions.

Given Data / Assumptions:

• Alluvial channel with uniform sediment supply and steady flow.
• “Regime” means long-term equilibrium: no net aggradation or degradation.
• We compare two qualitative statements about turbulence and shaping forces.

Concept / Approach:
According to Lacey, turbulence contains vertical components that help keep silt in suspension. Simultaneously, the channel perimeter experiences normal forces that mold it into a self-formed section. When these processes balance transport capacity with sediment load, a stable geometry develops.

Step-by-Step Solution:
Recognize that vertical eddies help maintain fine sediment in motion (supports statement a).Acknowledge that bed and banks are shaped by pressure/normal forces around the wetted perimeter (supports statement b).Therefore, both statements a and b reflect regime behavior.

Verification / Alternative check:
Classic regime literature cites vertical turbulence for suspension and self-formed sections governed by boundary-normal forces. Modern texts echo these qualitative foundations even when using updated resistance or sediment transport formulas.

Why Other Options Are Wrong:

• (a) or (b) alone is incomplete; both processes act together in regime.
• (d) contradicts regime theory’s fundamentals.
• (e) omits the perimeter-shaping aspect.

Common Pitfalls:
Assuming regime implies no turbulence or that only bed shear matters; overlooking the role of bank shaping and three-dimensional turbulence.

Final Answer:
Both (a) and (b).