When are river-training works required? Identify the regimes that commonly necessitate groynes, guide bunds, spurs, or similar measures.

Introduction / Context:

River-training works stabilize channels, protect banks, and manage planform. They are applied in varied morphodynamic settings—where a river swings (meanders), builds its bed (aggrades), or erodes its bed (degrades). The type and layout of training measures respond to the specific instability observed.

Given Data / Assumptions:

• General, alluvial river behavior considered.
• Training works include spurs, groynes, guide bunds, revetments, and vanes.
• Objective: channel stabilization, alignment control, and asset protection (bridges, levees).

Concept / Approach:

Meandering threatens banks through lateral migration; aggradation shoals the approach to hydraulic structures reducing conveyance; degradation deepens bed levels undermining foundations. In each case, river-training measures may be required to re-distribute flow, armor banks, or confine alignment.

Step-by-Step Solution:

Verification / Alternative check:

Case histories around bridges show all three regimes can require training to ensure stable approach flow and safeguard abutments/piers.

Why Other Options Are Wrong:

• Only during floods: Training may be required under normal conditions; flooding simply amplifies existing tendencies.
• Limiting to one regime ignores the breadth of applications for river-training works.

Common Pitfalls:

• Prescribing a generic layout without diagnosing whether the reach is aggrading or degrading; the remedy must match the morphodynamics.

Final Answer:

All of the above