Guide bank design practice: The downstream expansion head (curved end) of a guide bank is commonly extended to subtend which angle at the centre?

Introduction / Context:
Guide banks (training works) at barrages and bridges stabilize river approach flow and control the alignment of the main current. The upstream and downstream curved heads of guide banks are laid out as arcs that subtend standard angles at a common centre to achieve smooth expansion and contraction of flow and to reduce eddies and local scour.

Given Data / Assumptions:

• Standard practice for alluvial rivers with modest skew.
• No special site constraints specified.
• Question refers specifically to the downstream expansion head.

Concept / Approach:
The downstream head provides a controlled expansion to minimize adverse curvature and secondary currents. Empirical guidelines recommend typical subtended angles; a commonly adopted value for the downstream head is about 60°, providing adequate flare without excessive widening that could trigger separation and deep scour.

Step-by-Step Solution:
1) Recognize the head geometry is set as an arc subtending a design angle at its centre.2) For downstream expansion, typical recommended angle is around 60°.3) Hence, select 60° as the standard answer among the given options.

Verification / Alternative check:
Design manuals and standard works on river training recommend downstream heads in the range of about 45° to 90°, with 60° frequently cited as a balanced value for expansion, depending on river regime and width-depth ratio. The chosen value falls within accepted practice.

Why Other Options Are Wrong:

• 30° and 45°: too small for general downstream expansion, risk constriction persisting.
• 90°: larger flare can cause poor guidance and increased turbulence at exit.
• 120°: excessively wide; not a typical standard angle for guide bank heads.

Common Pitfalls:

• Confusing upstream (contracting) head angle with downstream (expanding) head angle.
• Ignoring river-specific studies; actual projects may refine angles based on model studies.

Final Answer:
60°.